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Aquaculture 324–325 (2012) 1–13

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Review

Epidemiology of Renibacterium salmoninarum in Scotland and the potential for compartmentalised management of salmon and trout farming areas Alexander G. Murray a,⁎, Lorna A. Munro a, I. Stuart Wallace a, Charles E.T. Allan a, Edmund J. Peeler b, Mark A. Thrush b a b

Marine Scotland Science, Marine Laboratory, Aberdeen, UK Centre for Environment, Fisheries and Aquaculture Science, Weymouth, UK

a r t i c l e

i n f o

Article history: Received 18 November 2010 Received in revised form 20 September 2011 Accepted 26 September 2011 Available online 1 October 2011 Keywords: BKD Renibacterium salmoninarum Aquaculture Salmonids Compartmentalisation Scotland

a b s t r a c t Bacterial kidney disease (BKD) (caused by Renibacterium salmoninarum) can result in significant mortality in Scottish salmon farms, but is considered to be a minor issue on trout farms. Controlling R. salmoninarum infection in trout to protect farmed salmon would be effective only if the risk posed from trout is significant both in absolute terms and relative to other potential sources of R. salmoninarum. To assess this, three complementary reviews are undertaken: review of data quality on BKD in Scotland and the national level prevalence and dynamics these data imply; case studies of recent BKD outbreaks in Scotland; and an assessment of the epidemiological and management factors that maintain and spread R. salmoninarum within and between the trout and salmon industries. These are then synthesised into a conclusion on the factors required for control of BKD in salmon. Most observed spread of R. salmoninarum occurred within single species or even companies, so the majority of cases in farmed salmon are linked to other salmon (and not to trout) farms. There is substantive geographical separation of areas of production for trout and salmon and transmission between salmon and trout networks is limited. The bacterium does not survive long in water so hydrodynamic transmission is likely to be localised. Currently R. salmoninarum is extremely rare in Scottish wild fish; this has not always been the case. Wild fish therefore probably play a limited role, but might act as reservoirs or vectors. The general conclusion is that to a large extent the transmission of R. salmoninarum in salmon and trout production can be separated and so there is potential to compartmentalise BKD controls, either by host species or geographical area. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The prevalence and dynamics of BKD and R. salmoninarum . . . . . . . . 2.1. Sampling and diagnostic methods used for the surveillance of BKD . 2.2. Prevalence and persistence of BKD estimated from official surveillance Case histories of BKD in Scotland . . . . . . . . . . . . . . . . . . . . 3.1. Persistent infection in table-production rainbow trout farms . . . . 3.2. Dynamic outbreak from a trout hatchery . . . . . . . . . . . . . 3.3. Dynamic outbreaks in marine salmon on the west coast . . . . . . 3.4. BKD in marine salmon from Shetland . . . . . . . . . . . . . . . 3.5. An outbreak in marine trout linking salmon and trout? . . . . . . Epidemiological factors behind the dynamics of BKD in Scotland . . . . . 4.1. Management and eradication of BKD and R. salmoninarum on farms 4.2. Vertical transmission . . . . . . . . . . . . . . . . . . . . . . 4.3. Geographical structure . . . . . . . . . . . . . . . . . . . . . . 4.4. Fish movements network structure . . . . . . . . . . . . . . . . 4.5. Other anthropogenic networks . . . . . . . . . . . . . . . . . . 4.6. Hydrodynamic transport . . . . . . . . . . . . . . . . . . . . .

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⁎ Corresponding author at: Marine Scotland Marine Laboratory, 375 Victoria Road, Aberdeen, AB11 9DB, Scotland, UK. E-mail address: sandy.murray@scotland.gsi.ac.uk (A.G. Murray). 0044-8486/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2011.09.034

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4.7. Wild and escaped fish 5. Synthesis and conclusions . . Acknowledgements . . . . . . . References . . . . . . . . . . .

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1. Introduction Infection with Renibacterium salmoninarum can cause bacterial kidney disease (BKD) in a variety of salmonid species in North America, western Europe, Chile and Japan (Austin and Austin, 2007; Toranzo et al., 2005); BKD has recently also been reported from Turkey (Savas et al., 2006). Bacterial kidney disease was notifiable to the World Organisation for Animal Health (O.I.E., 2006), but this status ceased in 2006, however it remains legally notifiable in the United Kingdom (UK), including Scotland. R. salmoninarum can be transmitted vertically (Evelyn et al., 1986) and horizontally, particularly by the faecal-oral route (Balfry et al., 1996). However the bacterium survives poorly in the aquatic environment, so longer-distance transmission is likely to be associated with movement of fish (Austin and Rayment, 1985) or other anthropogenic routes. Evaluation of the significance of these potential routes of transmission as they apply in Scotland is a major component of this review (Section 4). In Scotland BKD occurs in Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) farms (Bruno, 1986, 2004). Historically it occurred in wild Atlantic salmon as far back as the 1930s (Mackie et al., 1933; Smith, 1964) but BKD has not been reported from wild Scottish fish since the 1960s, recent evidence of R. salmoninarum in wild fish is reported in this document. Impacts of BKD on farmed trout are considered less economically serious than on farmed salmon where higher levels of mortality can occur in valuable market size fish. This disease is a significant contributor to the total mortality of farmed salmon in Scotland although it is less significant than diseases such as pancreas disease and infectious pancreatic necrosis (Soares et al., 2011). Relatively few salmon farms have BKD so the loss-per case can be substantial and there is a potential for the number of infected farms to increase substantially. It is therefore desirable to minimise the exposure of farmed salmon to R. salmoninarum, but control of infection in trout may be less cost effective. It may be possible to create separate compartments (Zepeda et al., 2008) within which to manage infection in salmon and trout independently, but the practicality of this depends on the degree of interaction between farmed trout and salmon. Bacterial kidney disease has been managed in Scotland using a system of Designated Area Orders (DAOs) imposed on farms that have been confirmed to be infected with R. salmoninarum and import restriction under an EU approved Additional Guarantees (AG)

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programme (Munro, 2007). The DAOs imposed movement restrictions, prohibiting the movement of fish from infected farms to other farms with the exception of those of similar health status. These restrictions are now called Confirmed Designation Notices (CDNs) under the Aquatic Animal Health (Scotland) Regulations 2009, but the term DAO is used here as it applied over the period analysed within this paper. The AG programme allowed the UK to ban imports of ova and fish from areas with BKD, but as a requirement had to have a BKD eradication programme (2006/88/EC). This programme was achieved using the movement restrictions of DAOs and a programme of fallowing affected populations. However, the controls caused problems with costs falling disproportionately on trout farmers and the benefits going to the salmon farmers. Furthermore the UK abandoned the AG programme for BKD (EU, 2010) as progress towards eradication could not be confirmed (Fig. 1). Scotland therefore needed to develop new controls on BKD. In order to assess the potential for optimising control of BKD in salmon without imposing unnecessary costs on the trout sector we review the epidemiology of transmission of R. salmoninarum infection to farmed salmon. Transmission to salmon farms might occur from activity within the salmon industry, or from a wild fish reservoir, or it might occur from trout farms (Murray et al., 2011). If the risk from trout is not significant in both absolute terms and relative to infection risk from the other two sources then reduction of the trout-associated risk would not significantly protect salmon. Future changes in the prevalence of R. salmoninarum in the trout farms or the structure of the trout industry could change the risk posed by trout. This review consists of three parts: (1) a review of the quality of data obtained from surveillance for BKD and R. salmoninarum in Scotland and an overview of BKD prevalence and dynamics at the national scale obtained from this; (2) detailed case studies of recent outbreaks in BKD and R. salmoninarum infection in Scotland to identify patterns within the outbreaks; (3) a review of the epidemiological processes and management practices that control these observed distribution of BKD in Scotland. We then combine these reviews to develop (4) a synthesis and conclusion as to the potential for control of BKD in Scotland that minimise impact in salmon while imposing minimal controls on trout to achieve that end. The interactions identified for the spread of R. salmoninarum can be generalised into a template for assessment of the potential for compartmentalised management in multi-species or multi-sector aquaculture industries, adapted for local industry network and the epidemiology of the relevant pathogen. Due to the large number of species produced in aquaculture worldwide the issue of identification of suitable compartments for disease management is likely to be of increasing importance.

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2. The prevalence and dynamics of BKD and R. salmoninarum

5 0 2002

2003

2004

2005

2006

2007

2008

2009

Date Fig. 1. Prevalence of BKD in terms of percentage of farms with DAOs in place over the periods January 2002–June 2008. Thin solid line= trout farms, thin dashed line= salmon farms, thick solid line= both salmon and trout farms.

In order to assess the potential for control of BKD in salmon farms we need to understand the quality of our knowledge on the distribution and dynamics of BKD and R. salmoninarum in Scotland. To do this, we review the surveillance methods that are used for the official imposition of DAOs for BKD. We use the imposition and removal of these DAOs to evaluate the prevalence and dynamics of BKD at the national level.


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2.1. Sampling and diagnostic methods used for the surveillance of BKD Implementation of the DAO-based control measures on farms depended on the detection and confirmation of the disease (BKD) or pathogen (R. salmoninarum). Scotland's official Fish Health Inspectors (FHI) therefore systematically collected data on these. Our epidemiological understanding of the prevalence of R. salmoninarum is, historically, entirely dependent on this official sampling since there has been no other screening (e.g. for research) of farms that are not already known to be infected, although research screening of wild fish has occurred. It is therefore upon the quality of the official data that any systematic understanding of BKD dynamics in Scotland must be based. Detection of clinical BKD can be relatively straightforward as the presence of disease is apparent (see Richards (2011) for official list of symptoms) and, being notifiable, any suspicions of its presence must be reported to FHI who will take kidney samples from 150 fish grouped in pools of 5. Diagnostic tests using enzyme-linked immuno-sorbent assay (ELISA) followed by bacterial culture are sensitive for confirming the presence of R. salmoninarum from clinically diseased fish (Bruno et al., 2007). So this surveillance that is passive in its first steps is likely to lead to official confirmation at least in the presence of significant levels of BKD. Detecting sub-clinical R. salmoninarum infection can be problematic. If there is no disease to observe then passive surveillance cannot work. Active surveillance programmes have limited ability to target diagnostic testing on individuals if they are asymptomatic, although seasonal sampling may help as infection is more likely to be detected in autumn or early winter (Austin and Austin, 2007). In Scotland inspections undertaken by the FHI are focused when water temperatures of around 12 °C are expected. Even if an infected population is sampled the proportion of fish infected within that population may be low and within infected individuals the bacteria may persist as only a few isolated clumps in the kidney (Austin and Rayment, 1985). Therefore, even if infected populations are sampled there is a good chance that material from infected individuals will not be included in the sample and diagnostic test sensitivity may be low (Hall et al., 2011). Regular background surveillance consisted of FHI taking head kidney samples from 30 fish, in pools of five, from all farms on a biennial basis until 2010. These were screened using ELISA. If ELISA results were positive a confirmatory sample of 150 fish were sampled. Historically this confirmatory sample has been diagnosed using culture on Mueller Hinton Agar plus L-cysteine hydrochloride and antibiotics (MHCA) plates but, since 2005, quantitative real-time polymerase chain reaction (qPCR) has also been an approved method for confirmation. Culture can take up to 12 weeks to confirm and the absence of R. salmoninarum cannot be assumed until the 12 week test period was complete. Diagnostic test methodologies are detailed by Bruno et al. (2007). Both culture and qPCR were acceptable confirmatory tests listed by the OIE when BKD was still internationally notifiable (O.I.E., 2006); the OIE no longer requires notification of BKD. This level of screening is likely to be very insensitive for the detection of sub-clinical infection as prevalence is often low and it appears ELISA is not particularly sensitive especially when samples are pooled (Hall et al., 2011). However, this was the screening protocol laid down by the EU for assurance of disease freedom (EU, 1999) and so it was used for its approved purpose in Scotland. Since the summer of 2010 testing has been undertaken only if there is suspicion of clinical BKD, i.e. screening for subclinical infection has ceased. It is believed that this change will have made very little difference to the probability of detection (Hall et al., 2011), however we only use data from before this date to ensure consistency. Farms holding broodstock were sampled at the same level as other farms, i.e. 30 fish every second year using ELISA diagnostic testing of kidney samples from pools of 5 fish. This means sampling was unlikely

3

to pick up sub-clinical R. salmoninarum (Hall et al., 2011), however vertical transmission is much more likely to occur when infection is clinical and since BKD is notifiable such diseased fish would be more likely to be detected. Broodstock farms exporting to Ireland or Chile were tested at the 150 fish level on an annual basis (ELISA on kidney). Occasionally, wild broodstock fish that were kept alive for stripping were tested for R. salmoninarum using ovarian fluid samples. To confirm eradication of R. salmoninarum from a farm subsequent to fallowing, pooled kidney samples from 150 fish at six monthly intervals over a two year period had to screen negative by ELISA. Wild fish have also been screened by the FHI and research scientists, effort has varied over time. Diagnostic testing was by bacterial culture (or sometimes ELISA) using head kidney, or whole fish if too small for kidney sampling. A total of 4539 wild fish were screened over 1989–2004. After 2004 qPCR screening was used, 1435 samples were taken over 2005–7 (Section 4.7). Detailed data on mortality and its causes are collected by companies involved in fish farming (Soares et al., 2011). The mortality records have been available from a company that was affected by a substantial number of the cases of BKD in salmon in the period 2003–7. These data have been used to analyse patterns in the mortality ascribed to BKD by farm managers of marine farmed salmon. Marine Laboratory researchers have also followed outbreaks in detail, using the more sensitive qPCR for diagnostic purposes. This has allowed the outbreaks and on-farm spread to be followed (Wallace et al., 2011), but researchers have not looked for R. salmoninarum on undesignated farms. In conclusion, surveillance for clinical BKD was likely to be reasonably good, with clinical cases of notifiable disease being reported and diagnostic methods being highly sensitive and specific for clinical disease. However, data on sub-clinical infection are very limited, with diagnostic methods being of low sensitivity and surveillance limited to active biennial visits. Therefore the prevalence and distribution of R. salmoninarum in Scotland is highly uncertain. 2.2. Prevalence and persistence of BKD estimated from official surveillance The number of DAOs that were in force in each month has been calculated for the period January 2002–June 2008 using the net numbers of DAOs issued and revoked. This has been divided by the number of farms in production in the particular year (Walker, 2010) to give the proportion of fish farms that were known to be infected. From this we can see that infection levels have been consistently at about 2.5% of farms over this period (Fig. 1). However, DAOs have applied to approximately 15–20% of trout farms and 0–1% of salmon farms with periods of absence of any DAOs on salmon. Since around 90% of farms rear salmon the industry average 2.5% is much closer to prevalence in salmon than in trout. The potential for subclinical infection means that absence of DAOs does not mean an absence of R. salmoninarum, and so DAOs are likely to underestimate prevalence of infection. However, trends in numbers of DAOs are likely to be a useful measure of the relative prevalence of infection. Assuming onset of disease, and hence detection, has not changed substantially, the proportion of infected farms expressing BKD, and the therefore the proportion which are detected, should remain reasonably constant. The prevalence of farms with DAOs showed little sign of declining hence the eradication policy could not be demonstrated to be working. Since this was a requirement of the AG programme (Munro, 2007) these have ceased for the UK, which is no longer listed as a BKD free territory (EU, 2010). Eradication from salmon may be possible if these can be isolated from trout, as there have been periods with an absence of orders. The time between imposition and lifting of DAOs from farms can be used to give information on the transience or persistence of infection (Fig. 2). Persistence of infection on farms varies depending on the


A.G. Murray et al. / Aquaculture 324–325 (2012) 1–13

average time (years) since DAO imposed

4

20 18 16 14 12

(6)

10 8

(3)

6 4 2

(1) (13)

(2)

(3)

Trout tanks

Trou ponds

0 Salmon

Trout cage

Trout fishery

Trout sea cage

farm type Fig. 2. Persistence of infection with R. salmoninarum (in years) in different types of fish production, as measured between the initial imposition of Designated Area Orders (DAO), and time revoked (or until 1st June 2008 if still in force at that time). The numbers in brackets indicate the number of DAOs imposed.

type of farm. For trout held in tanks, and for salmon farms, infection is dynamic with infection-free status being regained after a relatively short period of months to at most 2 years. For trout in some freshwater cage farm infection has persisted for decades due to the practice of continuous stocking (Wallace et al., 2011). From 1960 until 2003 neither BKD nor R. salmoninarum was reported in wild Scottish fish. In 2003 an ELISA positive screening result was obtained from herring (Clupea harengus) obtained from a cage with infected farmed salmon (unpublished data) and in 2005 and 2007 qPCR evidence of R. salmoninarum was obtained from wild fish in the vicinity of infected trout farms (Wallace et al., 2011). The bacterium has also been cultured from wild English fish in a recent survey (Chambers et al., 2008). We examine the information on wild fish in more detail later when we consider their potential as reservoirs or vectors (Section 4.7), however here we can state the prevalence of infection was very low to negligible and BKD itself has not been detected in Scottish wild fish for half a century. 3. Case histories of BKD in Scotland Scottish BKD outbreaks from the 1990s–2010 are analysed as case studies on the spread of BKD. The data used for these case studies were collected by FHI and researchers from the Marine Scotland Marine Laboratory, Aberdeen and the aquaculture industry. Outbreaks of BKD in the last two decades have occurred in Scotland in freshwater and marine rainbow trout and marine salmon farms (Bland, 2007; Bruno, 2004; Murray et al., 2011). The characteristics of these outbreaks have varied considerably depending on the type of farm involved. We categorise the epidemiological patterns in recent Scottish BKD cases as: 1. Persistent cases in freshwater trout farms; 2. Outbreaks in freshwater trout farms linked to a hatchery in 2005; 3. Outbreaks in marine salmon farms on the west coast of Scotland; 4. Outbreaks in marine salmon farms in Shetland; and 5. Outbreaks in marine trout and salmon farms that are possibly linked. More detailed descriptions of these outbreaks are provided in Murray et al. (2011).

been infected before stocking and this seems a likely source of reintroduction following farm-level fallowing (Wallace et al., 2011). There is no evidence of these farms infecting other farms and all are considered as dead-end nodes in the farm network (fish not moved off except for harvesting, although escapes are possible; see Section 4.4). As these persistently infected farms do not appear to spread infection to other farms, this suggest that R. salmoninarum positive farms can be effectively isolated if there is no off-site movement of fish. However it is likely that the local spread of R. salmoninarum occurs freely between cages and escaped fish may carry infection. 3.2. Dynamic outbreak from a trout hatchery In 2005 BKD was confirmed (2/150 fish by culture) at a freshwater trout farm in the Scottish Highlands. This infection was traced to the supplying hatchery in central Scotland. Contact tracing showed recent fish movements to seven farms (Bland, 2007), two other Scottish farms (1/150 and 3/150 by culture) and one English farm were confirmed R. salmoninarum positive. One Scottish farm was placed under suspicion due to an ELISA positive result (1/91) but was not confirmed as culture tests were negative (0/150 fish). Full diagnostic results are available in Bland (2007). The hatchery was cleared of R. salmoninarum by fallowing on a tank-by-tank basis; this approach was effective as this was an indoor facility with good internal biosecurity which could be maintained. As a result the DAO was revoked in November 2005. One of the Scottish farms was fallowed and the infection cleared, while the other was fallowed first on a pond-by-pond basis and infection was either not removed or recurred. Subsequently this farm fallowed at the farm level and infection was removed. The spread of the epidemic reflected the network of movements of fish transported between farms (see Section 4.4) and cases were spatially separated (see Section 4.3). This outbreak illustrates the capacity of highly connected nodes in a farm fish movement network to spread R. salmoninarum (and potentially other pathogens) between individual farms and across political boundaries.

3.1. Persistent infection in table-production rainbow trout farms 3.3. Dynamic outbreaks in marine salmon on the west coast Several trout farms in Scotland have been infected with R. salmoninarum, sometimes leading to clinical BKD, over many years (Fig. 2). Four of these farms have been continuously under DAOs since 1981 or 1982. These are freshwater cage farms where continuous stocking was practised and therefore they were never fallowed. Recently this management practice has changed with some fallowing being introduced (Wallace et al., 2011). It is likely that the source of infection to naïve cohorts placed on farms was by horizontal transmission from established stocks on the farm, although it is possible that some cohorts could have

An outbreak affecting six west coast salmon farms (WS1–WS6) occurred in 2003 (Fig. 6; Table 1). Five of these sites (WS1–WS4 and WS6) returned culture positive results (Table 1). The site WS5 did not test positive by culture, but it did test positive by ELISA. The company ascribed 13.5 tonnes of losses on this farm to BKD, so although it officially only had a temporary thirty day notice (TDN) imposed we treated this site as BKD positive for epidemiological analysis. During this outbreak a herring (one of nine) taken from


A.G. Murray et al. / Aquaculture 324–325 (2012) 1–13

5

Table 1 Detection of R. salmoninarum from salmon farms on Scottish west coast 2003–7. The ELISA results refer to individual fish except WS7 and WS10 which are for pools of five. Designated Area Orders were imposed, except on WS5 and WS7 where only TDNs were imposed. Mortality was ascribed to BKD by the company using data collected from 2001 to 2008. Farm

Date

Culture

ELISA

Mortality (tonnes)

Comment

WS1 WS2 WS2 WS3 WS4 WS5 WS6 WS7

Apr 2003 May 2003 Mar 2007 Jun 2003 Jun 2003 Jun 2003 May 2003 Dec 2004

1/2 1/3 32/51 2/3 5/6

116/155 1/3 8/11 3/150 6/150 6/149 1/1 1/1

20.3 0.1 70.9 11.4 133.3 13.5 75.7 0

Also 1/9 herring

WS8 WS9

Apr 2005

1/2

1/2

6.8 0

WS10

Apr 2007

1/150

2/30

0

1/1

within a cage on farm WS1 in April 2003 tested positive by ELISA, a further 22 herring sampled in May tested negative. Further positive diagnostic results were obtained in 2005 (Table 1) with WS7 only ELISA positive, while WS8 and WS2 (for the second time) were confirmed positive. Another farm WS8 was in the western isles, outside the area of the earlier outbreaks (Fig. 3), indicating the geographically widespread nature of the BKD cases. All DAOs from this and the previous outbreak were revoked by mid 2006 (Fig. 3). In 2007 site WS10 was confirmed, with WS2 reconfirmed for the third time, by culture. Farm WS9 did not test positive but had a DAO placed on it because fish were moved onto it from WS10. These farms lie to the north of the area covered by the map (Fig. 3). None of the farms listed here were among the five salmon farms affected by BKD between 1976 and 1985 (Bruno, 1986). Data on the mortality ascribed to BKD, and for tracing sources of smolts, were available for 2004 to 2007. Mortality varied substantially between farms, although they appear to have been infected at the same time (Fig. 4). Some farms infected with R. salmoninarum had

Also positive in November 2003 TDN only Also positive June 2004 Also histology positive in March 2005. TDN only Also positive in May 2005 DAO imposed because fish received from WS10

no mortality ascribed to BKD, while others were severely affected and losses varied from 0 to 133 tonnes per farm with 83% of mortality accounted for by three farms (WS2, WS4 and WS6). These losses were divided by biomass, obtained using records of biomass collated monthly by the Scottish Environmental Protection Agency (SEPA), to show that the mean of the loss ascribed to BKD for these farms was 3.3%, and the worst affected lost 12.6%, of maximum biomass. These outbreaks only affected one company, suggesting anthropogenic spread. Outbreaks were temporally and spatially clustered, with three simultaneous cases in Loch Sunart, however BKD was also detected on Skye as part of the same outbreak; and a neighbouring farm within Loch Sunart remained uninfected. Later cases were even more widespread (WS8, and WS10) While it is possible that farm-to-farm spread had occurred within the clusters, anthropogenic spread seems the most likely explanation for this pattern. Generally BKD did not recur on salmon farms subsequent to fallowing (hence short duration of BKD DAOs in salmon, Figs. 2 and 4). The farm WS2 is an unusual case being R. salmoninarum positive in

Fig. 3. Location of the Loch Sunart, Loch Linnhe (Section 3.3) and Loch Etive (3.5) clustered outbreaks of BKD. FT2 is the location of a persistently infected freshwater trout farm that drains into Loch Etive; FS1 is a putatively infected freshwater salmon farm (3.3) (located off the top right of the map); black circles represent towns and numbers refer to Management Areas (MA).


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(Hall et al., 2011) means that a lack of detection of any such infection would not be at all unlikely. Farm FS1 was potentially exposed to a neighbouring trout farm (again negative but a possible link to the trout network), to wild char (Salvinus alpinus) (a species known to carry R. salmoninarum in other countries (Jonsdottir et al., 1998) although data from Scotland are lacking), or to migratory salmonids possibly transporting infection from the marine salmon farms in Loch Linnhe. It is also possible that the fish could have been infected prior to stocking to these freshwater farms. Any of the risk routes could have been the source of infection to FS1.

WS1 WS2 WS8

Sunart cluster

Linnhe cluster

6

WS3 WS4 WS5

northwest

Skye WS6 WS7

3.4. BKD in marine salmon from Shetland

Western Isles

WS9 WS10 2003

2004

2005 2007 2006 3-30 30-300 300-3k 3k-30k 30k-300k

Fig. 4. Outbreaks of BKD on west coast salmon farms 2003–7. Symbols represent: = quarterly mortality attributed to BKD in industry records (symbol size is proportional to level of mortality);▲ = DAOs imposed ( = TDN only);▽ = DAOs revoked.

three consecutive production cycles. In the first a DAO was imposed, but no mortality was attributed to BKD, limited mortality occurred in the second production cycle and the DAO was re-imposed in association with substantial BKD attributed mortality in the third production cycle. This farm is close to a processing plant, so there is a possibility that it might have been exposed to infected material handled at that plant depending on the biosecurity in place at the time. The most significant association with all the BKD outbreaks is the use of smolts from specific freshwater farms. The 2003–5 outbreaks showed very significant links to two freshwater smolt producing farms FS1 and FS2 (not shown), both through contact tracing and analysis of numbers of movements onto the farms. Overall the industry database records 34 deliveries of smolts to sites that developed BKD and 658 deliveries to farms that did not, giving odds of 34:658 of a delivery being associated with BKD (many sites received multiple smolt deliveries). However, farms receiving smolts from FS1 and FS2 had higher odds for developing BKD, 19:94 and 10:43 respectively. These give highly significant Odds Ratio (Kelsey et al., 1996) of BKD in farms receiving smolts from FS1 and FS2 of 3.9 (p = 2.9E−5) and 4.5 (p = 0.00054) respectively, relative to averaged odds for the whole database using the Fisher's Exact test. It therefore seems likely that there was a link to infection via fresh water farms. Although the freshwater farms FS1 and FS2 are linked epidemiologically with cases of BKD, they never tested positive. It is possible that a low-level undetected sub-clinical infection may have existed on these farms. The weakness of surveillance for subclinical infection

In recent years R. salmoninarum has been detected by ELISA on six farms throughout the Shetland Islands although only two have been confirmed by culture (Table 2). All six farms affected were owned by company Z at the time R. salmoninarum was detected, although ownership of some farms has since changed. Farms SS1 and SS2 were positive by ELISA but were not confirmed by culture (Table 2); although this is insufficient to impose DAOs we considered ELISA good evidence that these farms had been exposed to R. salmoninarum infection (Hall et al., 2011). The affected farms were in different regions of Shetland, and farms belonging to other companies in these regions were not affected. The affected fish from these different farms were all sourced from different freshwater smolt farms throughout Scotland. Farms SS5 and SS6 generated positive test results but were fallowed before confirmatory testing occurred (Table 2). The R. salmoninarum positive from SS6 was obtained as a result of pathogen screening during an outbreak of infectious salmon anaemia (ISA) and the farm was depopulated due to confirmation of ISA disease. Another farm, belonging to the same company, had a DAO imposed in May 2008 because of epidemiological links to an infected farm, but the fish tested negative for R. salmoninarum so this farm was not included in the analysis. There is a significant association between Scottish farms belonging to the company and them becoming infected with R. salmoninarum sometimes leading to BKD. With a total of 107 farms in Shetland and with 31 farms belonging to company Z then the probability using the Fisher's exact test of all six of the positive farms belonging to company Z is p = 0.0012, i.e. ownership is significantly associated with infection. With no geographical association, so no evidence for hydrodynamic or other environmental contact, on the one hand, and a lack of common smolt origin on the other, the intra-company association strongly suggests anthropogenic spread of infection. This pattern of spread is different to the pattern observed for infectious salmon anaemia virus (ISAV) in Shetland which was transmitted through the environment, probably hydrodynamically, and showed little or no relationship with company ownership (Murray et al., 2010). Intra-company spread of R. salmoninarum might be via a processing plant, although other companies used this plant and their

Table 2 Detection of R. salmoninarum from salmon farms in Shetland 2006–2009. Result refers to ELISA screening expressed as positive pools/total number of pools except last case* where individuals were analysed using light microscopy. Farm

Result

Restriction

Date restriction imposed

SS1 SS2 SS3 SS4

1/1 1/6 6/7 4/38

28/06/06 26/06/07 30/01/08 27/05/08

SS5

2/6 2/32 1/11*

TDN TDN DAO DAO (17/07/08) Fallowed

SS6

Fallowed

04/06/08 12/06/08 14/01/09

Smolt source

Management area

Highlands Dumfries Argyll

4a 3a 2c 3a

Jura

3a 3a


A.G. Murray et al. / Aquaculture 324–325 (2012) 1–13

fish did not develop BKD. The use of the processing plant might be confounded with other unmeasured company-based management practices (e.g. shared personnel, equipment etc.). A total of 31 farms from company Z used this processing plant and in contrast 24 farms from eight other companies used this plant and had no positive farms. Although this is statistically consistent with contact with the processing plant being the sole risk factor for infection association of risk with farm ownership is much stronger. In some instances well boats were used for ‘bus-stop’ deliveries (picking up fish from a number of farms on the same boat journey to the processing plant (Murray et al., 2010)). These ‘bus stop’ harvests are more likely to result in contact within a company than contact via a processing plant, which is shared by several companies.

3.5. An outbreak in marine trout linking salmon and trout? Marine rainbow trout farms in Loch Etive (Fig. 3) have a history of R. salmoninarum infection with DAOs imposed on two trout farms in the loch in 2002. This sea loch is located near the outfall of the River Awe which drains Loch Awe, the location of two persistently infected freshwater rainbow trout farms (two of those described in Section 3.1). It discharges into the lower reaches of the Loch Linnhe system, within which there have been BKD outbreaks in marine salmon farms (Section 3.3). The five trout farms in Loch Etive have been supplied with rainbow trout from FT1 (Section 3.3) which has been suggested, but not shown, to play a role in the infection of FS1 which in turn has shown a highly statistically significant link to the outbreaks in Loch Linnhe. Loch Etive therefore might be an area in which BKD outbreaks in salmon and trout are linked. In April 2009 suspicion of BKD was reported at two marine trout farms operated by one company and in close geographical proximity (Fig. 3). This was confirmed by culture in May 2009 at MT1, MT3 and MT4 (Table 3), but not at MT5, while MT2 was fallow. In May 2009 suspicion of BKD was reported from two Atlantic salmon farms (AS1 and AS2) located near the mouth of Loch Etive. One of these (AS1) was confirmed, while the other (AS2) tested negative (Table 3). In April 2010 BKD was confirmed at the previously negative trout farm MT5 and in November 2010 at the previously fallow MT2. Additionally, MT4 screened culture positive in April 2010. Spread among the trout farms reflected inter farm anthropogenic activity. Fish were moved between MT3 and MT4 (both directions) and from MT3 to MT1 in the preceding months. Farms MT1, MT3 and MT4 also share a shore base. The farms MT1 and MT3 are also located within 1 km of each other so contact through the environment, perhaps with water movement, may be possible, although MT4 is more distant (3.3 km). It was therefore hardly surprising that R. salmoninarum was spread among these farms, potentially by any of these routes. The infection at AS1 at the same time is suggestive of a link between salmon and trout through the environment, but coincidental infection is entirely possible.

Table 3 Detection of R. salmoninarum from marine trout (MT) and marine Atlantic salmon (AS) farms in the Loch Etive area 2009–10. Farm

Date

ELISA (pools of 5)

Culture

qPCR

MT1 MT2 MT3 MT4 MT4 MT5 MT5 AS1 AS2

May 2009 Nov 2010 May 2009 May 2009 Apr 2010 May 2009 Apr 2010 May 2009 May 2009

2/30 2/30 2/2 2/2 1/1 0/30 1/6 1/30 0/30

3/150 24/150 10/10 8/10 6/150 N/A 2/2 1/1 0/5

2/2 2/30 2/2 2/2 1/1 N/A 1/1 1/1 N/A

Comment

Fallow 2009

Negative

Negative

7

4. Epidemiological factors behind the dynamics of BKD in Scotland Data on the epidemiological factors behind the prevalence of BKD in Scotland was reviewed, considering both the persistence of R. salmoninarum on infected farms and its spread between farms. These factors are the two components of R0, the rate of spread divided by the rate of removal of infection, that defines the potential for control of a disease (Reno, 1998); for eradication R0 must be less than 1. Disease can thus be eradicated either by increasing removal rate or by reducing its spread. Spread includes vertical transmission through ova or smolts (including ova imports) and horizontal transmission between farms. Horizontal transmission depends on some form of contact between farms, and so we reviewed the geographical location of farms before reviewing potential contact process of: networks of fish movements, other anthropogenic activity, hydrodynamic transmission and the role of wild fish as vectors or reservoirs. 4.1. Management and eradication of BKD and R. salmoninarum on farms Fallowing is used to remove R. salmoninarum from infected cages or farms, however cage-level fallowing in loch-based farms proved ineffective because of horizontal transmission between cages (Wallace et al., 2011) and infection persisted on some such farms for decades (Section 3.1). Fallowing a pond-based farm on a pond-by-pond basis has also proved ineffective (Section 3.2), but a tank-based hatchery with high levels of biosecurity was successfully fallowed on a tank-by-tank basis (Section 3.2). Fallowing at the farm-level has a mixed history, it appears to be successful on most salmon farms as BKD mostly does not recur after fallowing (Sections 3.3 and 3.4). For trout farms there has been post-fallowing recurrence on some farms (Wallace et al., 2011), but other farms appear to have been cleared by farm-level fallowing (Section 3.2). An environmental reservoir such as water or sediment is considered unlikely (Austin and Rayment, 1985) but low levels of infection have been found in wild and escaped fish near the farm (Wallace et al., 2011) so these may undermine the effectiveness of fallowing. Alternatively infected fish might have been re-introduced when the farm was restocked; only one batch needs to be infected due to the potential for horizontal transmission between cages (Wallace et al., 2011). This would imply a more careful sourcing of fish by increasing the sensitivity of screening or/and limiting the number of source farms to reduce the likelihood of re-infection. Vaccination for BKD was not practicable in Scotland under the eradication scheme as it is difficult to distinguish vaccinated fish from infected fish, and some fish in BKD vaccinated populations remained infected (Griffiths et al., 1998). In any case BKD vaccines appear fairly ineffective relative to vaccines for other bacterial diseases (Newman, 1993; Toranzo et al., 2005), although possible new developments with live vaccination with a R. salmoninarum related bacterium, Arthrobacter davidanieli, may be effective in the future (Toranzo et al., 2005). Recent vaccination trials involving this bacterium showed significantly reduced mortality of salmon relative to salmon in the same cage only vaccinated against other diseases during a BKD outbreak (Burnley et al., 2010). However, R. salmoninarum was still present and culturable, if at a significantly lower prevalence (Burnley et al., 2010), so while the vaccine controlled mortality the vaccinated fish remained carriers. To date, vaccines have not been approved for the control of BKD in Scotland. The disease can be treated with antibiotics (Austin, 1985) although such treatment does not necessarily eradicate the bacterium from the infected fish as the antibiotic may not always reach the target bacterium within the host (Austin and Austin, 2007; Bruno and Munro, 1986). Antibiotics can be useful for treating broodstock (Brown et al., 1990; Lee and Evelyn, 1994), since the bacterium can be vertically transmitted. Disease can also be controlled with dietary supplements (Austin, 1985; Lall et al., 1985).


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50000 45000

Imported ova

40000 35000

USA

30000

Australia

25000

Norway

20000

Iceland

15000

EU

10000 5000 2008

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998

1997

1996

1995

0

Date Fig. 5. Numbers and sources of Atlantic salmon ova imported into Scotland from outside the UK (Walker, 2009).

It is possible that R. salmoninarum infection could clear from a population without any specific management action. However R. salmoninarum causes persistent chronic infection and can persist at low prevalence for long periods (Wallace et al., 2011). The long-term persistence on freshwater cage-based sites that do not fallow at the farm level (Fig. 2) suggests that a natural clearance of infection from a continuously stocked site without any intervention is likely to be a rare event. In summary, vaccines and treatments might help control BKD at the farm level, but they do not eradicate R. salmoninarum and so the farms remain potential infection sources. Fallowing is not effective

at eradicating R. salmoninarum if carried out at the cage level except in tank-based farms. At the farm level fallowing is usually effective (especially in salmon) and where infection does occur after farmlevel fallowing then reintroduction of infection with the input of fish may be an explanation, although reservoirs of infection in wild or escaped fish cannot be ruled out (Section 4.7). 4.2. Vertical transmission R. salmoninarum can be vertically transmitted (Evelyn et al., 1986; Paterson et al., 1981) and so could be introduced with imported ova. The AG programme allowed the UK to restrict imports of ova and smolts from areas where BKD was present. However substantial imports of salmon and trout ova occurred with some 700,000 to 30 million salmon ova are imported each year (Fig. 5). The number of these imports has increased with some coming from countries such as Iceland, Norway and the USA where R. salmoninarum is present. Infection was reported in Atlantic salmon broodstock in Norway in 2008 (Nilsen and Sunde, 2008) illustrating the possibility for vertical transmission within the salmon industry. Although imports are certified as R. salmoninarum free any failure in this process could risk the import of infection. In addition, 519,000 smolts were imported in 2008, but only from R. salmoninarum free areas within the EU; these numbers have been falling (Walker, 2010). In 2008 25 million trout ova were imported into Scotland, including 1.49 million from the US and imports also occur from Australia and South Africa (although not in 2008) making trout ova imports truly global (Walker, 2010). Imports of trout ova have the potential to increase the prevalence of disease agents and impede eradication, as shown for infectious pancreatic necrosis virus (IPNV) in Ireland,

Fig. 6. Map of locations of freshwater trout (dark); salmon (moderate); and mixed (light) farms. The symbol size is proportional to the number of contacts. Arrows mark the spread associated with a hatchery outbreak (Section 3.2). Inset map I is the Shetland outbreak, Section 3.4, circles mark: II. An area of mixed salmon and trout farming not associated with BKD; III. The main west coast outbreaks and FS1, Sections 3.3 and 3.5; IV. An area of persistent R. salmoninarum infection in rainbow trout, Sections 3.1 and 5. A drainage basin shared by salmon and trout and to date not associated with BKD. The dashed line represents an arbitrary divide between areas of predominantly salmon (northwest including Shetland) and trout (south east) farming.


A.G. Murray et al. / Aquaculture 324–325 (2012) 1–13

(Ruane et al., 2009), thereby indirectly increasing the risk from trout. None of the available evidence is suggestive of the vertical transmission of R. salmoninarum in Scotland whether from domestic or imported broodstock, but the route is a biologically demonstrable risk. 4.3. Geographical structure The location of trout and salmon farms in Scotland was mapped using geographic information systems (GIS) software (ESRI (UK) Ltd.). There is an apparent spatial separation of the salmon and trout industries, with the majority of trout farms in the south and east, while the majority of salmon farms are in the west or the Northern Isles (Fig. 6). However, there is some overlap with trout and salmon farms sharing a number of small areas (I to V). In total 27 Scottish catchments, consisting of main river and small coastal catchments, contain both farmed salmon and trout (2003 data used to be consistent with the contact network analysis). This list includes 17 catchments with single mixed species farms so the risk here is associated with intra-farm as opposed to inter-farm interactions. In addition, some of these facilities are operated by fishery trusts or District Salmon Fishery Boards so represent fish of wild origin, hence these will not be dealt with in this section. Of the 10 remaining catchments, some of which have multiple salmon farms located within, there were 14 salmon farms which share a water course with trout farms. Five of these salmon farms are separated from the nearest trout farm by 5 km or less at 0.11, 0.34, 0.84, 4.1 and 5.0 km respectively. In 2003 there were 176 smolt production farms in operation (Walker, 2010), therefore the great majority of Scottish freshwater salmon production occurs at farms that do not share catchments with trout farms. Stocked trout fisheries are distributed throughout Scotland, however most are located near large population centres, and therefore within the ‘mainly trout’ zone of south-eastern Scotland. Many fisheries are hydrographically isolated from fish farms but others may have potential hydrodynamic contact e.g. fisheries in upstream reservoirs. The locations of these trout fisheries were not documented by the

9

FHI and so numbers within the ‘salmon’ region (Fig. 6), and numbers of movements where transport was between the regions, have not been quantified. Subsequently these fisheries are required to be registered under the Aquatic Animal health (Scotland) regulations 2009, so this knowledge gap is being addressed. Transmission of R. salmoninarum between the trout and salmon sectors requires that the geographical separation be overcome. In the next sections we examine the routes of transmission that might apply, these being anthropogenic movement of fish (Section 4.4) and other industry related contacts (Section 4.5) and transmission through the environment with water movement (Section 4.6) or by wild/escaped fish (Section 4.7). 4.4. Fish movements network structure A plot has been derived for the farmed fish movement network in Scotland using data from transports authorised in 2003 in which nodes (farms) were divided between salmon, trout and those producing both (Green et al., 2009; Murray et al., 2011). This network showed that there were very few links between trout and salmon farms and in addition the pure trout farms have few links to the mixed species nodes. There is wide interaction between salmon farms and the salmon network (not shown) links most salmon farms together into a single large network or epidemiological compartment (Green et al., 2009; Zepeda et al., 2008). This means disease may have the potential to become widespread with movements in this network, although this may be less severe than it appears because of unidirectional movements and communities within the network which may localise spread (Green et al., 2009). There are a number of mixed species elements within this salmon cluster but these are isolated from the main trout farming network. If farms producing trout only are plotted (Fig. 7) it can be seen that most form a network that is entirely separate from the salmon network. Two smaller sub-components connect into the salmon network, each by single contact routes, therefore the farmed trout movement network is to a very large degree separate from the

Fig. 7. The network for movement of farmed trout in Scotland, and its connections to the salmon farm network. Symbol size is proportional to the number of contacts. Connections to the salmon farming network are indicated by thick arrows connecting a fish symbol (via a square mixed species farm). Note that the main trout cluster and an isolated pair of trout farms have no connections to the salmon network and two minor clusters have only one connection each to the network. Also shown are Scottish farms with persistent R. salmoninarum infections (white) and Scottish farms involved in a hatchery based outbreak (black) (Section 3.2).


10

A.G. Murray et al. / Aquaculture 324–325 (2012) 1–13

salmon network. Outbreaks of BKD in Scottish trout have followed the structure predicted by the network. Persistently infected farms (white symbols) which did not spread R. salmoninarum within this network are considered to be dead-end nodes (Section 3.1). Farms that were positive during the hatchery-based outbreak (black symbols) (Section 3.2) reflect movements from this hatchery. This match between the network and outbreaks indicates that it is reasonable to assume that the lack of links between the trout and salmon industries represents a substantial barrier to disease transmission, and hence contributes greatly to risk reduction. Stocked trout fisheries are not included in the network (Munro and Gregory, 2009) although these may use trout from source farms throughout the UK. Fisheries were not officially registered at the time for which the network was constructed, so the exact identity of individual fisheries could not be verified. However, although fish are moved onto such fisheries, they are not moved off so the fisheries do not form links between nodes in the farmed fish movement networks (i.e. they are dead-end nodes) and therefore do not link the salmon and trout movement networks. Furthermore, movement regulations to fisheries in catchments containing salmon farms are being strengthened and these fishery operators will not be allowed to source fish from farms under movement restriction (Richards, 2011). Fish movements appear to be directly associated with many of the observed cases of the spread of BKD in both trout (Section 3.2) and salmon (Section 3.4) outbreaks. The lack of spread from “dead-end” trout farms (Section 3.1) is associated with lack of fish movements. Fish movements may occur over 100s of km so this is a critical component of BKD epidemiology. 4.5. Other anthropogenic networks The movement of contaminated equipment poses the risk of importing or moving pathogens. The well boat network was shown to play a key role in transmitting ISAV during the Scottish 1998/9 epidemic (Murray et al., 2002) and fish transport lorries have been identified as a potential risk factor for spreading fish diseases when transporting fish from an infected farm if disinfection fails. However, as no UK based lorries transport fish between farms on the continent they are not a risk factor for import of disease (Peeler and Thrush, 2009). The Shetland outbreak (Section 3.3) was confined to company Z and spatially scattered in a pattern that indicated anthropogenic spread. It did not, however, appear related to farmed fish movements and therefore represents a different anthropogenic network with well boats possibly playing a role. Bus-stop deliveries might be particularly associated with the spread of infection. Within the marine trout industry in Loch Etive, farms with shared facilities became infected, although here spatial clustering is also a possible partial explanation. These anthropogenic networks appear to be mostly regional or small scale in structure and so probably spread pathogens over 10s of kilometres at most. 4.6. Hydrodynamic transport Horizontal transmission of R. salmoninarum occurs via contact with skin or eyes (Hoffman et al., 1984) or consumption of faecal material (Balfry et al., 1996), so hydrodynamic transmission is possible. However R. salmoninarum decays rapidly in water, Austin and Rayment (1985) found a decay of approximately 4 orders of magnitude in 4 days in unsterilised river water (equivalent to 10% h− 1), while Balfry et al. (1996) found 60% decay after 8 h in raw seawater giving an hourly decay rate of 11% h− 1. Decay rates in sterile or filtered water are much longer. This is a rapid rate of decay compared to pathogens such as IPNV (Toranzo and Hetrick, 1982). Many pathogens decay more slowly when bound to particles than when free in the water (Sinton, 2005) so R. salmoninarum bound to particles such as faecal material

might survive longer. However, faeces have been calculated to have a settling velocity of 0.017–0.06 ms− 1 and using this estimate Gowen and Bradbury (1987) calculated that faeces are likely to sink out within 200 m from a typical salmon farm. Hydrodynamic distribution depends on the faecal material breaking up into fine slowly sinking particles. Transport risks depend not only on the survival of R. salmoninarum in the water, but also the rate of shedding from infected fish and minimum infectious dose. These have been investigated for Chinook salmon (Oncorhynchus tshawytscha) (McKibben and Pascho, 1999) and a minimum exposure of 7 × 10 2 R. salmoninarum ml − 1 for 24 h has been established, however data are lacking for trout and Atlantic salmon. Shedding would likely be elevated during clinical disease, as is the case with ISAV (Gregory et al., 2009). If this is the case then transmission would be far more likely from a farm with BKD than from one with sub-clinical R. salmoninarum infection. Renibacterium salmoninarum appears to spread via the water between cages in a farm (Wallace et al., 2011) and possibly between very closely located farms (Sections 3.3 and 3.5); however waterborne spread did not appear to occur in Shetland (Section 3.4), although the also very labile ISAV did spread via the water between salmon farms separated by a few km in Shetland (Murray et al., 2010). The risk of spread of R. salmoninarum via this route cannot be ruled out, but is likely to be limited to very short distances (100s of metres or a few kilometres) or special conditions that allow the transport of faecal material. 4.7. Wild and escaped fish Historically in Scotland BKD has been a disease of wild Atlantic salmon (Mackie et al., 1933; Smith, 1964) however R. salmoninarum has not been detected in wild fish since 1960 except in close association with infected Atlantic salmon (a single herring) and rainbow trout farms (Wallace et al., 2011). The estimated prevalence of R. salmoninarum in Scottish wild and escaped fish is extremely low (0.22%) (Wallace et al., 2011). Extensive background (not farm associated) surveillance, carried out by researchers from the Marine Laboratory, of 4520 wild freshwater salmonid fish, from 1989 to 2004 returned no R. salmoninarum positive results. In 2005–7 1143 wild fish were caught from waters distant to aquaculture (so are not farm associated) and the screening results were negative. Over the same time period 1292 wild fish were caught adjacent to infected farms, giving positive pools of 3spined-stickleback (Gasterosteus aculeatus) (×2) and minnow (Phoxinus phoxinus) (×1) using qPCR (Wallace et al., 2011). During this survey 268 escaped rainbow trout were also screened (232 adjacent to and 36 distant from aquaculture) giving three R. salmoninarum positive qPCR pools from the adjacent location. However, as the origin of these fish is not clear these results are difficult to interpret, other than to illustrate a possible R. salmoninarum transmission mechanism. Other researchers report a possible association with R. salmoninarum infection between wild fish and rainbow trout farms from English and Welsh rivers (Chambers et al., 2008). Wild Arctic char were reported as high prevalence R. salmoninarum reservoirs in Iceland (Jonsdottir et al., 1998). Local populations of Arctic char are found in many Scottish fresh water lochs and may form large shoals. One such population is in Loch Arkaig and salmon smolts sourced from a farm (FS1) located in these waters showed a strong statistical association with outbreaks in marine farms. However, the R. salmoninarum infection status of Arctic char in Scottish lochs is unknown. In marine waters there is evidence for the presence of R. salmoninarum from ELISA screening of Pacific hake (Merluccius productus) (Kent et al., 1998) within a Chinook salmon farm and herring (unpublished data) from within a Scottish salmon farm undergoing a BKD outbreak (Section 3.3). Wild fish may spend considerable amounts of time at one farm before moving to another, possibly over distances of several km (Uglem


A.G. Murray et al. / Aquaculture 324–325 (2012) 1–13

et al., 2009) and escaped fish may behave in the same manner. This could enhance the risk of transport of pathogens between farms. The use of hatcheries to enhance wild fish stocks may magnify the prevalence of any low-level R. salmoninarum infection (Fenichel et al., 2009). This would be of particular concern in situations where salmonid fish reared from wild parents were held in cages adjacent to farmed salmonids. In such cases there is a risk of transmission if infection were present in either population. The very low reported prevalence of R. salmoninarum in Scottish wild fish suggests they are not an important reservoir or vector; however, they have historically been important. Escaped rainbow trout may pose a slightly higher risk and currently the status of Arctic char is unknown. 5. Synthesis and conclusions The risks associated with the different routes of R. salmoninarum spread are summarised using the information provided from the historical outbreaks and the analysis of contact structures within the aquaculture industry. The hazard of interest is BKD in marine salmon farms, however since the introduction of R. salmoninarum to freshwater salmon farms will be likely to spread to marine farms when fish are moved (probably to multiple farms) the hazard for our purposes is the introduction of R. salmoninarum to salmon farms. We have identified that R. salmoninarum on salmon farms might come from activities within the salmon industry, from exposure to wild reservoirs (although currently these appear small) or exposure to infection in the trout industry (Table 4). We have established that the majority of cases can be traced back to movements within an industry or even within a company, mostly movements of fish, in support of the conclusion of Austin and Rayment (1985). In the trout hatchery outbreak (Section 3.2) three of the four confirmed or suspect cases can be traced to movements from infected farms. In the west coast outbreaks within company Y (Section 3.3) there are strong associations with movements of smolts from particular freshwater farms. Therefore the farmed fish movement network (Green et al., 2009; Munro and Gregory, 2009) is critical for the spread of R. salmoninarum. Patterns of infection within Shetland suggest anthropogenic movements related to industry activity but not to fish movements (Section 3.4). This might be in association with well boats visiting a processing plant (a similar route drove the spread of an outbreak of ISA in 1998/9 (Murray et al., 2002)) or the practice of ‘bus stop’ deliveries to the plant.

11

Transmission of R. salmoninarum through the environment almost certainly occurs between individual cages in loch based trout farms. It is possible that it could have occurred between neighbouring marine salmon farms on the west coast but did not happen in Shetland as there was no spatial clustering. The bacterium does not survive for long periods in an unprotected (unbound) state (Austin and Rayment, 1985), so transmission requires attachment to non-sinking particles. Wild and/or escaped fish might act as vectors for the transfer of infection from trout farms to salmon farms or between salmon farms. A few R. salmoninarum qPCR positive escaped rainbow trout have been found around infected trout farms and while no wild/escaped positive salmon have been found in Scotland since 1960 (Smith, 1964) a culture positive wild salmon has been found in England (Chambers et al., 2008). There is no conclusive evidence that transmission has occurred by this route, but it is a possible risk. Wild fish might also act as infection reservoirs, this being a possible explanation for the failure of fallowing at a trout farm (Wallace et al., 2011); however it is more likely that this farm was re-infected from stocked farmed trout. Currently there is no evidence for the presence of R. salmoninarum in non-farm associated Scottish wild fish and no cases of BKD in wild fish have been reported for decades. Due to the transmission of infection between cages in loch based farms, and probably between ponds in land-based farms, eradication of R. salmoninarum, as a minimum, should be based on fallowing at the farm level (Wallace et al., 2011). The exception is a highly biosecure tank based farm which has been cleared on a tank-by-tank basis. Generally R. salmoninarum is cleared from marine salmon farms which undergo fallowing at the farm-level between production cycles. However, even farm level fallowing has not been effective with trout (Wallace et al., 2011), likely via the reintroduction with the input of fish or from wild and escaped reservoirs. The farmed fish movement networks for salmon and trout are almost entirely separate. There is a class of mixed species farms, probably in connection with sea-based trout farming, however these appear mostly part of the salmon network and quite separate from the pure trout farm network. There are a few areas in Scotland where salmon and trout farms share drainage basins however only five salmon farms are within 5 km of a trout farm in the same catchment. There is no conclusive evidence for the transmission of infection between farms, but there is almost certainly within-farm transmission (Wallace et al., 2011) and escaped fish (and to a lesser extent wild fish) may carry infection. Marine farms in adjacent areas of Loch Linnhe may have been exposed to R. salmoninarum from trout farms in Loch Etive.

Table 4 Empirically derived risk associated with different forms of transmission from observed Scottish BKD outbreaks. Transmission route

Association of route with existing BKD cases

Trout to salmon transmission risk

Salmon to salmon transmission risk

Movement of farmed fish

Occurred repeatedly

Few risk contacts

Well boats

Likely occurred in Shetland Possible in several cases, certain between cages Arctic char possible reservoir

Very unlikely due to lack of contact Possible but not proven Escaped infected rainbow trout possible vector Indirect effects

Most cases in salmon explained by this route Likely association with some cases Possible but not proven Possible, no evidence from Scotland, Potential risk

Indirect effects

Potential risk

Movement of water or wild fish Wild and escaped fish reservoirs Imported ova

Imported parr and smolts

Potential risk, no evidence from existing cases Potential risk, no evidence from existing cases (EU only)


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Put and take trout fisheries do not increase numbers of links in the farmed fish movement network, but fisheries close to farms might put these farms (whether trout or salmon) at risk of infection. Information on the location of fisheries is now being collected which will allow their distribution to be assessed. Recent controls have been introduced on movement of fish into put and take fisheries that share catchments with salmon farms (Richards, 2011). If the R. salmoninarum prevalence in trout farms were to substantially increase this could increase the risk of transmission from these farms and thereby increase risk to salmon. Prevalence could be altered by imports or by changes in internal biosecurity practices and network structure of the trout industry. As we have discussed the trout and salmon industries do appear to exhibit a fairly high degree of separation, however if the prevalence on trout farms was to change radically then it is possible that these trout farms could become a relatively more important source of R. salmoninarum potentially causing substantially more outbreaks in salmon farms. The same applies if the geographic structure of the trout industry were to change, perhaps with more trout farming in northwest Scotland. If the prevalence of R. salmoninarum in salmon were to decrease to the point of eradication then re-infection from trout, wild fish or imports could prevent final eradication or make any achievement temporary even if most infection is currently due to internal processes within the salmon industry. This may have already occurred as BKD has disappeared from farmed salmon on occasions (although subclinical infection may well have persisted). The increasing use of foreign imported salmon ova may increase the risk of introductions of this vertically transmitted pathogen and hence decrease the relative risk from farmed trout. Imports of trout ova might increase prevalence of R. salmoninarum in the trout industry thereby increasing risk from trout; this would only be significant if imports substantially increased prevalence in trout. There is no evidence supporting a role for vertical transmission in the epidemiology of BKD in Scotland, but the pathogen is truly vertically transmitted and with the lifting of AGs it is possible the risk from imports via this route will increase. The detection of R. salmoninarum in populations that are not expressing clinical BKD may be problematic. Recent results suggest the sensitivity of existing diagnostic surveillance testing may be poor in cases of sub-clinical infection (Hall et al., 2011). Undetected sources of infection may explain re-infection of fallowed trout farms and infection of marine salmon farms on the Scottish west coast. Alternative sampling regimes, perhaps using qPCR as the primary screening test (Hall et al., 2011), could improve detection of subclinical R. salmoninarum. However, if R. salmoninarum is widespread in the network the consequence of such a rigorous policy could be costly, especially in the short term, as previously unknown cases of sub-clinical infection may be detected. These costs might result in limited benefits if these latent cases were in farms without off-site movements, especially in trout farms, but could be more useful under a risk-based approach targeted at farms with many off-site movements. A policy of only screening farms with clinical signs of BKD, such as mortality or morbidity and listed diagnostic symptoms (Richards, 2011), and using qPCR as the diagnostic test has been adopted. It is believed that this will achieve similar detection power to the previous active surveillance regime using ELISA (Hall et al., 2011) while reducing diagnostic and sampling costs. All the costs and benefits of controls need to be considered when making decisions about BKD policy. The costs to industry of an eradication programme can include culling and fallowing at infected farms. The eradication policy increased the financial risks associated with running a trout hatchery and thus decreases investment in this sector of the industry; similar costs could apply to salmon hatcheries and smolt producers if surveillance were improved, as would probably be required for eradication. Under compartmentalisation (with controls varied either between species or within geographically defined areas i.e.

compartments) BKD controls may be more effectively targeted. However, compartmentalisation itself depends on implementation of strong movement controls and surveillance within the pathogen-free zone. Acknowledgements Data for this paper was obtained from Marine Scotland Fish Health Inspectors and scientists, from industry sources and the Scottish Environmental Protection Agency (SEPA). It was financially supported by the Scottish Government through the project ROAME FC11105 and in collaboration with the Centre for Environment, Fisheries and Aquaculture Science (Cefas) in review of UK BKD control policy. References Austin, B., 1985. Evaluation of antimicrobial compounds for the control of bacterial kidney disease in rainbow trout, Salmo gairdneri Richardson. Journal of Fish Diseases 8, 209–220. Austin, B., Austin, D.A., 2007. Bacterial Fish Pathogens: Disease of Farmed and Wild Fish, 4th edition. Praxis Publishing, Chichester. Austin, B., Rayment, J.N., 1985. Epizootiology of Renibacterium salmoninarum, the causal agent of bacterial kidney disease in salmonid fish. Journal of Fish Diseases 8, 505–509. Balfry, S.K., Albright, L.J., Evelyn, T.P.T., 1996. Horizontal transfer of Renibacterium salmoninarum among farmed salmonids via the fecal-oral route. Diseases of Aquatic Organisms 25, 63–69. Bland, M., 2007. Epizootic investigation into the presence of bacterial kidney disease (BKD) in rainbow trout farms in Scotland 2005. Fisheries Research Services Internal Report No.14/07 2007 http://www.scotland.gov.uk/Uploads/Documents/ 1407.pdf. Aberdeen. Brown, L.L., Albright, L.J., Evelyn, T.P.T., 1990. Control of vertical transmission of Renibacterium salmoninarum by injection of antibiotics into maturing female coho salmon Oncorhynchus kisutch. Diseases of Aquatic Organisms 9, 127–131. Bruno, D.W., 1986. Scottish experience with bacterial kidney disease in farmed salmonids between 1976 and 1985. Aquaculture and Fisheries Management 17, 185–190. Bruno, D.W., 2004. Prevalence and diagnosis of bacterial kidney disease (BKD) in Scotland between 1990 and 2002. Diseases of Aquatic Organisms 59, 125–130. Bruno, D., Munro, A.L.S., 1986. Observations of Renibacterium salmoninarum and the salmonid egg. Diseases of Aquatic Organisms 1, 83–87. Bruno, D.W., Collet, B., Turnbull, A.M., Kilburn, R., Walker, A., Pendrey, D., McIntosh, A., Urquhart, K.L., Taylor, G., 2007. Evaluation and development of diagnostic methods for Renibacterium salmoninarum causing bacterial kidney disease (BKD) in the UK. Aquaculture 269, 114–122. Burnley, T.A., Stryhn, H., Burnley, H.J., Hammell, K.L., 2010. Randomized clinical field trial of a bacterial kidney disease vaccine in Atlantic salmon, Salmo salar L. Journal of Fish Diseases 33, 545–557. Chambers, E., Gardiner, R., Peeler, E.J., 2008. An investigation into the prevalence of Renibacterium salmoninarum in farmed rainbow trout, Oncorhynchus mykiss (Walbaum), and wild fish populations in selected river catchments in England and Wales between 1998 and 2000. Journal of Fish Diseases 31, 89–96. EU, 1999. Bacterial Kidney Disease. European Union Sanco/B3/AH/R14/1999 http://ec. europa.eu/food/fs/sc/scah/out36_en.pdf. EU, 2010. Commission decision of 15 April 2010. Official Journal of the European Union 98, 7–11 http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2010:098:0007:0011:EN:PDF. Evelyn, T.P.T., Prosperi-Porta, L., Ketcheson, J.E., 1986. Experimental intra-ovum infection of salmonid eggs with Renibacterium salmoninarum and vertical transmission of the pathogen with such eggs despite their treatment with erythromycin. Diseases of Aquatic Organisms 1, 197–202. Fenichel, E.P., Tsao, J.I., Jones, M.L., 2009. Modeling fish health to inform research and management: Renibacterium salmoninarum dynamics in Lake Michigan. Ecological Applications 19, 747–760. Gowen, R.J., Bradbury, N.B., 1987. The ecological impact of salmonid farming in coastal waters: a review. Oceanography and Marine Biology: an Annual Review 25, 563–575. Green, D.M., Gregory, A., Munro, L.A., 2009. Small- and large-scale network structure of live fish movements in Scotland. Preventive Veterinary Medicine 91, 261–269. Gregory, A., Munro, L.A., Snow, M., Urquhart, K.L., Murray, A.G., Raynard, R.S., 2009. An experimental investigation on aspects of infectious salmon anaemia virus (ISAV) dynamics in seawater Atlantic salmon, Salmo salar L. Journal of Fish Diseases 32, 481–489. Griffiths, S.G., Melville, K.J., Salonius, K., 1998. Reduction of Renibacterium salmoninarum culture activity in Atlantic salmon following vaccination with avirulent strains. Fish & Shellfish Immunology 8, 607–619. Hall, L.M., Wallace, I.S., Munro, L.A., Murray, A.G., 2011. Epidemiology informs policy regarding surveillance of a notifiable disease of salmonids. Epidemiolgie et Santé Animale (Proceedings of the International Conference on Animal Health Surveillance) 59–60, 392–394. Hoffman, R., Popp, W., Van de Graaff, S., 1984. Atypical BKD predominately causing ocular and skin lesions. Bulletin of the European Association of Fish Pathologists 4, 7–9.


A.G. Murray et al. / Aquaculture 324–325 (2012) 1–13 Jonsdottir, H., Malmquist, H.J., Snorrason, S.S., Gudbergsson, G., Gudmundsdottir, S., 1998. Epidemiology of Renibacterium salmoninarum in wild Arctic charr and brown trout in Iceland. Journal of Fish Biology 53, 322–339. Kelsey, J.L., Whittemore, A.S., Evans, A.S., Thompson, W.D., 1996. Methods in Observational Ecology, 2nd edition. Oxford University Press, New York. Kent, M.L., Traxler, G.S., Kieser, D., Richard, J., Dawe, S.C., Shaw, R.W., Prosperi-Porta, G., Ketcheson, J., Evelyn, T.P.T., 1998. Survey of salmonid pathogens in ocean-caught fishes in British Columbia, Canada. Journal of Aquatic Animal Health 10, 211–219. Lall, S.P., Patterson, W.D., Hines, J.A., Adams, N.J., 1985. Control of bacterial kidney disease in Atlantic salmon, Salmo salar L., by dietary modifications. Journal of Fish Diseases 8, 113–124. Lee, E.G.-H., Evelyn, T.P.T., 1994. Prevention of vertical transmission of the bacterial kidney disease agent Renibacterium salmoninarum by broodstock injection with erythromycin. Diseases of Aquatic Organisms 18, 1–4. Mackie, T.J., Arkwright, J.A., Pryce-Tannatt, T.E., Mottram, J.C., Douglas Johnston, W.D., Menzies, W.J.M., Martin, W., 1933. Second interim report of the Furunculosis Committee. Ministry of Agriculture and Fisheries, London. McKibben, C.L., Pascho, R.J., 1999. Shedding of Renibacterium salmoninarum by infected chinook salmon Oncorhynchus tschawytscha. Diseases of Aquatic Organisms 38, 75–79. Munro, P.D., 2007. Implementation of additional guarantees and results of an epizootic investigation for BKD in Scotland. Fish Veterinary Journal 2007 (9), 56–62. Munro, L.A., Gregory, A., 2009. Application of network analysis to farmed salmonid movement data from Scotland. Journal of Fish Diseases 32, 641–644. Murray, A.G., Smith, R.J., Stagg, R.M., 2002. Shipping and the spread of infectious salmon anemia in Scottish aquaculture. Emerging Infectious Diseases 8, 1–5. Murray, A.G., Munro, L.A., Wallace, I.S., Berx, B., Pendrey, D., Fraser, D., Raynard, R.S., 2010. Epidemiological investigations into the re-emergence and control of an outbreak of infectious salmon anaemia in the Shetland Islands, Scotland. Diseases of Aquatic Organisms 91, 189–200. Murray, A.G., Munro, L.A., Wallace, I.S., Peeler, E.J., Thrush, M.A., 2011. Bacterial kidney disease: assessment of risk to Atlantic salmon farms from infection in trout farms and other sources. Scottish Marine and Freshwater Science 2 (3). http://www. scotland.gov.uk/Publications/2011/04/21144833/0 Edinburgh. Newman, S.G., 1993. Bacterial vaccines for fish. Annual Review of Fish Diseases 3, 145–185. Nilsen, H., Sunde, E.B., 2008. The surveillance and control programme for bacterial kidney disease (BKD) in Norway. Annual Report 2008 Norwegian Veterinary Institute, Oslo. O.I.E., 2006. Aquatic Animal Health Code. Office International des Epizooties, Paris. Paterson, W.D., Lall, S.P., Desautels, D., 1981. Studies on bacterial kidney disease in Atlantic Salmon (Salmo salar) in Canada. Fish Pathology 15, 283–292.

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Peeler, E.J., Thrush, M.A., 2009. Assessment of exotic fish disease introduction and establishment in the United Kingdom via live fish transporters. Diseases of Aquatic Organisms 83, 85–95. Reno, P.W., 1998. Factors involved in the dissemination of disease in fish populations. Journal of Aquatic Animal Health 10, 160–171. Richards, R., 2011. Bacterial kidney disease paper 2011. Ministerial group on aquaculture. http://www.scotland.gov.uk/Resource/Doc/1062/0114801.pdf. Ruane, N.M., Murray, A.G., Geoghegan, F., Raynard, R.S., 2009. Modelling the initiation and spread of Infectious Pancreatic Necrosis Virus (IPNV) in the Irish salmon farming industry: the role of inputs. Ecological Modelling 220, 1369–1374. Savas, H., Altinok, I., Cakmak, E., Firidin, S., 2006. Isolation of Renibacterium salmoninarum from cultured Black Sea salmon (Salmo trutta labrax): first report in Turkey. Bulletin of the European Association of Fish Pathologists 26, 238–246. Sinton, L.W., 2005. Biotic and abiotic effects. In: Belkin, S., Colwell, R.R. (Eds.), Oceans and Health: Pathogens in the Marine Environment. Springer, New York, pp. 69–92. Smith, I.W., 1964. The occurrence and pathology of Dee disease. Freshwater and Salmon Fishery Research 34, 1–12. Soares, S., Green, D.M., Turnbull, J.F., Crumlish, M., Murray, A.G., 2011. A baseline method for benchmarking mortality losses in Atlantic salmon (Salmo salar) production. Aquaculture 314, 7–12. Toranzo, A.E., Hetrick, F.M., 1982. Comparative stability of two salmonid viruses and poliovirus in fresh, estuarine and marine waters. Journal of Fish Diseases 5, 223–231. Toranzo, A.E., Magarriños, B., Romalde, J.L., 2005. A review of the main bacterial fish diseases in mariculture systems. Aquaculture 246, 37–61. Uglem, I., Dempster, T., Bjørn, P.-A., Sanchez-Jerez, P., Økland, F., 2009. High connectivity of salmon farms revealed by aggregation, residence and repeated movements of wild fish among farms. Marine Ecology Progress Series 384, 251–260. Walker, A.J., 2009. Scottish Fish Farms Annual Production Survey 2008. Marine Scotland Science, Aberdeen. Walker, A.J., 2010. Scottish Fish Farms Annual Production Survey 2009. Marine Scotland Science, Aberdeen. http://www.scotland.gov.uk/Resource/Doc/295194/0106192.pdf. Wallace, I.S., Munro, L.A., Kilburn, R., Hall, M., Black, J., Raynard, R.S., Murray, A.G., 2011. A report on the effectiveness of cage and farm-level fallowing for the control of bacterial kidney disease and sleeping disease on large cage-based trout farms in Scotland. Marine Scotland Science Report 05/11. http://www.scotland.gov.uk/ Resource/Doc/356407/0120447.pdf. Zepeda, C., Jones, J.B., Zagmutt, F.J., 2008. Compartmentalisation in aquaculture production systems. Revue Scientifique et Technique 27, 229–241.

Murray 2012  

Epidemiologia de Renibacterium salmoninarum

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