Cases of REV detected in commercial poultry in the Netherlands
Tumours with unknown causes/REV
In July 2024, a group of laying hens was submitted to Royal GD for pathological examination to investigate poor performance. Tumours (lymphomas) were found in various organs of these adult laying hens and reticuloendotheliosis virus (REV) RNA was detected in the tumour tissue.
A member of the retrovirus family
There are several REV subtypes that vary in pathogenic potential but all belong to the same serotype. In poultry, REV can cause a variety of syndromes such as lymphoid neoplasia and runting disease syndrome. Transmission of the virus can be vertical as well as horizontal and iatrogenic. It is found worldwide in wild birds, with various avian species acting as natural hosts.
Malignant lymphomas were found in a significant proportion of the examined animals from the infected laying flock. Tumour tissue was tested at GD using PCR for Marek serotype 1, avian leukosis virus and REV; only the REV PCR was positive. It is known that at least part of the REV genome (its hereditary material) is embedded in the genome of certain field strains of fowl pox. This makes it possible for infections with these fowl pox field strains to trigger development of antibodies against REV – and even positive outcomes for REV PCR carried out on pox lesions – without the birds having directly been infected with REV. In this case, however, the possibility of fowl pox playing a role was ruled out by a negative fowl pox PCR for the tumour tissue. A virus culture on white blood cells from this flock did produce REV but not Marek serotype 1 or avian leukosis virus. Although REV has been found in several European countries, this is the first time REV has been detected in commercial poultry in the Netherlands. The moment of introduction into the flock and the route of infection are unknown.
REV in laying breeder poultry
From December 2024 onwards, GD has also found reticuloendotheliosis virus in several laying breeder flocks of various chicken breeds. After identifying REV in Dutch commercial laying hens for the first time in July, this detection in Dutch laying breeder flocks was also a first. There seems to be no direct link between the various flocks. When and how the flocks became infected is, as yet, unknown. It is known that REV can be transmitted to offspring via the egg. However, REV infections can also occur later in life. Together with the concerned parties, GD is conducting further investigations to determine whether the offspring of the infected laying breeder flocks were also infected.
REV screening examination
Following the infections in laying breeder flocks and after joint consultations, all of the involved parties have started serological screening of their flocks. This includes the offspring of infected laying breeder flocks. No infected animals have so far been found in this offspring-testing programme.
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Animal health monitoring
Since 2002, Royal GD has been responsible for animal health monitoring in the Netherlands, in close collaboration with the veterinary sectors, the business community, the Ministry of Agriculture, Fisheries, Food Security and Nature, vets and farmers. The information used for the surveillance programme is gathered in various ways, whereby the initiative comes in part from vets and farmers, and partly from GD Animal Health. This information is fully interpreted to achieve the objectives of the surveillance programme – rapid identification of health issues on the one hand and monitoring trends and developments on the other. Together, we team up for animal health, in the interests of animals, their owners and society at large.
Via Veekijkernieuws houden wij u elk kwartaal op de hoogte van nieuws uit de monitoring van diergezondheid bij pluimvee. Mocht er tussendoor iets belangrijks spelen dan sturen wij u daarover een e-mail.
Reticuloendotheliosis virus infection has subsequently been detected in adult laying hens submitted to GD for necropsy in early 2025. It seems likely that REV has spread further within the Dutch poultry sector. To get a clear picture of this, anonymous screening was started of blood samples submitted for regular blood testing in laying flocks (egg production sector) and broiler breeder flocks. The blood is tested anonymously and positive results will not be traceable to specific poultry farmers. This screening will provide a better idea of the prevalence of REV infections in Dutch commercial poultry.
Increased incidence of fowl pox
Typically, only a few outbreaks of fowl pox occur every year. From September to December 2024, GD received an unusually high number of cases for pathological examination (see figure). In 2024, GD confirmed it in 28 flocks.
Number of flocks confirmed by GD to have fowl pox diphtheria
number of flocks
Fowl pox occurs in a dry form, with scabs and lesions around the eyes and beak, and a wet form, with ulcers in the mouth and throat (see photos). In about half the cases, there was also a drop in production, although this was unlikely to be solely attributable to fowl pox. In early December, GD called for poultry with fowl pox to be sent in for genetic analysis because of the possibility of a mutated strain as reported in other European countries, with reticuloendotheliosis virus (REV) embedded in the fowl pox virus genome. The relevance of REV in the fowl pox virus is unknown. Results of the genetic analysis are expected in early 2025.
Figure: Number of flocks confirmed by GD to have fowl pox diphtheria (necropsy) (Source: GD)
Photos: Dry and wet forms of fowl pox (L and R respectively) (Source: GD)
Animal health barometer for poultry
Disease/disorder/ health characteristic
Execution decree (EU) 2018/1882 of the Animal Health Law (AHL) (EU) 2016/429 (Category A disease) Avian influenza (AI) in the Netherlands (H5/H7)
(Source: GD, WBVR, national government)
ND in the Netherlands (Source: GD, WOAH)
Highly pathogenic AI (H5/H7)*: (first detection in flock)
* In commercial poultry and in backyard situations with >50 birds.
Serological monitoring by GD: (first detection in flock) (antibodies for H5/H7)
Commercial poultry: 0
Low pathogenic AI (H5/H7): (first detection in flock)
(>50 birds)
Execution decree (EU) 2018/1882 of the Animal Health Law (AHL) (EU) 2016/429 (Categories B through E) Avian influenza (AI) in the Netherlands (H5/H7)
(Source: GD, WBVR, national government)
Campylobacteriosis
Avian mycoplasmosis (Source: GD)
Mycoplasma gallisepticumA Serological monitoring by GD: Reproduction sector: Layer pullets: Layers:
not vaccinated and infected:
Cases in EWSC based on positive serology and/or voluntary PCR testing:
Reproduction sector: Layers: Turkeys: Backyard
M. meleagridis (Source: GD)
Salmonellosis (non-zoonotic salmonella) (Source: GD)
Salmonella arizonae
Salmonella Gallinarum (SG) Commercial poultry: Backyard poultry:
Salmonella Pullorum (SP)
West Nile fever Not monitored
Article 2.1 Designation of animal diseases in the ‘Rules for Animal Health’ of the Dutch Animal Act
Avian chlamydiosis (Source: GD)
Detected by GD: Commercial poultry:
Disease/disorder/ health characteristic Brief description (numbers at the farm level)
Article 2.2. Designation of zoonoses in the ‘Rules for Animal Health’ of the Dutch Animals Act Salmonellosis (zoonotic salmonella) (at the flock level) (Source: GD)
Salmonella Enteritidis Reproduction:
Salmonella Typhimurium Reproduction:
pullets:
Other types of Salmonella (S. Hadar, S. Infantis, S. Java, S. Virchow)
Reproduction:
Other WOAH-list poultry diseases in the Netherlands subject to compulsory notification Duck viral hepatitis (Source: GD)
Gumboro (IBD)
(Source: GD, EWS)
Infectious bronchitis (IB)
(Source: GD)
Infectious laryngotracheïtis (ILT)
(Source: GD, EWS)
Mycoplasma synoviae B
(Source: GD)
Turkey rhinotracheïtis (TRT)
(Source: GD)
Detected by GD:
Reported in EWSC : Broiler breeder pullets: Broilers:
Layer breeder pullets:
pullets:
poultry:
Types most commonly detected by GD: Broilers:
Reported in EWSC : Layer pullets: Layers:
Broiler breeders: Broilers: Backyard poultry:
Serological monitoring and/or dPCR GD:
Broiler pullets:
Broiler breeding:
Broiler breeder pullets:
Broiler breeders:
Layer pullets:
Layer rearing:
Layer breeder pullets
Layer breeders:
Layer pullets: Layers: Turkeys:
Detected by GD:
Broiler reproduction sector (incl. pullets):
Layer reproduction sector (incl. pullets):
Broilers:
Layer pullets:
P.O. Box 9, 7400 AA Deventer
Disease/disorder/ health characteristic
Other poultry diseases
Avibacterium paragallinarum
(Source: GD, EWS)
Histomonosis
(Source: GD)
Pasteurella multocida
(Source: GD)
Erysipelas (Erysipelothrix rhusiopathiae) (Source: GD)
T. +31 (0)88 20 25 575
info@gdanimalhealth.com www.gdanimalhealth.com
Reported in EWSC : Layers:
Small commercial farms (<250)
Backyard poultry:
Detected by GD:
Reproduction (meat sector):
Reproduction (layer sector):
Layer pullets:
Layers:
Meat turkeys:
Aangetoond bij sectie: Broiler breeder pullets:
Layers:
Turkeys:
Continuation of
of EWS reportsd
of farmsd
Detected by GD: Layers: Number of farmsd
Ç Moderate to marked increase
Ç Slight increase
- No change
È Slight decrease
È Moderate to marked decrease
A Based on serological monitoring
B Based on serological monitoring and/or differentiating Ms-PCR
C Early Warning System
