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MPI Surveillance Reports
SURVEILLANCE REPORTS
BIOSECURITY NEW ZEALAND
Biosecurity New Zealand’s Incursion Investigation team investigates suspected cases of emerging or exotic diseases in New Zealand. They are interested in hearing about any equine health issues that seem out of the ordinary, including but not limited to:
• Parasites or unexplained disease in one or more horses associated with recent international travel • An unusual or unexplained outbreak in a group of horses, particularly involving: – acute respiratory disease – neurological signs – abortion/reproductive signs – unexplained mortalities
If you suspect you’ve seen an exotic or emerging disease, please phone Biosecurity New Zealand on 0800 80 99 66 as soon as reasonably practical.
Digested from Surveillance, June 2021; 48 (3) PDF available on SciQuest HERE
SUSPECTED EQUINE EXOTIC DISEASES Paul Bingham
Paul.Bingham@mpi.govt.nz
Accessed and edited from Surveillance 2021; 48 (4): 10-24
EQUINE HERPESVIRUS MYELOENCEPHALOPATHY CONFIRMED
A veterinarian called the MPI exotic pest and disease hotline to report mild neurological deficits in the hindlimbs of a heavily pregnant brood mare. The affected mare was in the same paddock as a mare that had aborted 12 days earlier and Equine herpesvirus-1 (EHV-1) had been detected by PCR on fetal lung tissue. The property had experienced an outbreak of equine herpesvirus myeloencephalopathy (EHM) in February 2014, with the neuropathogenic genotype D752 confirmed (McFadden et al. 2016). A few months after the 2014 EHM outbreak and immediately prior to the following foaling season, two mares on the farm aborted, one at 9 months and one at 10 months gestation. The cause of abortion in both cases was confirmed to be EHV-1 with the same genotype as in the EHM outbreak (McFadden et al. 2016). Since that time the farm has carried out comprehensive EHV-1 vaccination of all mares. At the time of this notification the affected mare was otherwise bright, alert and eating well. On clinical examination she had hindlimb muscle fasciculations and mild weakness with ataxia on circling or stepping. This mare subsequently aborted and, with little improvement in the neurological deficits, was euthanased. The remaining nine paddock mates showed no neurological signs and went on to foal normally. Nasal, blood and fetal tissue samples from the neurological case were submitted to a commercial veterinary laboratory and EHV-1 was detected by PCR in both the nasal swab and fetal tissues. Further investigation and management of the case was co-ordinated by the farm veterinarian, in consultation with the New Zealand Equine Health Association (NZEHA; https://www.nzeha.org.nz/) and MPI incursion investigation team. Samples from both aborted fetuses and the neurologically affected in-contact mare were submitted to the AHL (Wallaceville) for virus isolation and molecular analyses. The farm instituted strict quarantine measures following the NZEHA biosecurity code (NZEHA 2022). Virus isolates obtained from aborted fetuses underwent molecular sequencing, which identified EHV-1 with the amino-acid coding change of the DNA polymerase (D/N752) associated with an increased risk of neurological disease (Nugent et al. 2006; Oladunni et al. 2019). It should be noted, however, that only about 80 percent of EHV-1 strains from horses with clinical EHM have this genetic indicator. It appears that many factors apart from the viral strain, including sex, age, immune status, reproductive status and stage of gestation, and perhaps the route of transmission, determine the clinical manifestations and variable incidence of EHM associated with EHV-1 infection (Oladunni et al. 2019). To inform disease management, the veterinarian instituted a high level of molecular and serological surveillance, including two rounds of nasal swabs and acute and convalescent blood sampling of all horse cohorts on the farm. In addition, mares and stallions were closely monitored with tail sensors that constantly recorded their body temperatures. No further horses were affected, and foaling proceeded normally, with no further premature births or abortions. Repeated PCR testing of nasal swabs found no further evidence of EHV-1, and serological screening by ELISA identified no seroconversion. The early and efficient implementation of quarantine and other biosecurity controls appeared to have helped contain spread: there was no evidence of spread of the
virus to in-contact horses, and in consultation with NZEHA and MPI, after 28 days biosecurity protocols were relaxed, and the investigation was closed.
A veterinary pathologist contacted MPI to report a suspect exotic disease in a Southland racehorse. The Thoroughbred mare had developed limb oedema after training. Serum amyloid A was mildly increased (60 mg/L; reference range 0–8 mg/), indicating a low level of systemic inflammation. Serum was sent to the AHL for exclusion of exotic causes of equine oedema including Theileria equi, Babesia caballi, equine viral arteritis virus and equine infectious anaemia virus. All these agents were excluded by serology. The veterinarian noted that this was the second horse in the stable to develop similar signs, and that the oedema seemed to be related to infected lower-limb abrasions sustained on the training track. This was considered a likely pathogenesis of the oedema and, together with the serology results, excluded exotic agents as the cause of disease.
A pathologist at a commercial veterinary laboratory called the exotic pest and disease hotline to report that a 10-yearold Thoroughbred mare had been losing weight over the previous 3–4 weeks and now had clinical signs including dependent oedema of the head and lower limbs. The mare had been referred from a private veterinary clinic to Massey University Veterinary Teaching Hospital at Palmerston North for a diagnostic workup. The presentation was potentially consistent with an arteritis that might be caused by exotic agents including equine arteritis virus (EAV) and equine infectious anaemia (EIA) virus. Serum submitted to the AHL (Wallaceville) tested negative by AGID test for EIA, and negative by VNT for EAV. In addition, serum was tested by cELISA, which enabled the exclusion of the exotic agents Theileria equi and Babesia caballi. The mare improved temporarily but then developed a septic carpal arthritis and deteriorated. She died and a full postmortem was carried out. Pathological changes were evident in multiple organs and systems, including hepatitis, nephritis, dermatitis (pruritic epidermal ulceration and crusting affecting predominantly the head, neck and distal limbs), and fibrinosuppurative (septic) arthritis and tenosynovitis of the right carpal joint. The skin, kidney and liver changes were characterised histologically by multifocal eosinophilic granulomas and/or eosinophilic, lymphocytic and plasmacytic infiltrates. The main lesions present in this case were considered likely to have been multifactorial, including equine multisystemic eosinophilic epitheliotropic disease (MEED). Some of the clinical signs and lesions present in this case such as anorexia, weight loss, severe pruritus, subcutaneous oedema, exfoliative eosinophilic dermatitis and eosinophilic inflammation on internal organs such as the liver, were consistent with those described in horses with MEED (Black & Mace 2004; Bosseler et al. 2013). The case described here did not demonstrate a blood eosinophilia, which was consistent with a case series described in the literature where only 6/44 (14%) had eosinophilia (Bosseler et al. 2013). Although the pathogenesis is not well established, according to some authors recurrent episodes of type 1 hypersensitivity associated with ingested, inhaled or parasitic antigens may be involved in the development of MEED (Pucheu-Haston & Del Piero 2013). Exotic disease was excluded and the investigation closed.
An equine veterinarian in Canterbury notified MPI of an unusual case of anaemia in a 6-year-old male castrated Warmblood horse. Causes of anaemia are many and include exotic infectious diseases such as equine infectious anaemia (EIA), equine viral arteritis (EVA) and equine piroplasmosis. Whole blood and serum from the horse were obtained, and testing excluded EIA, EVA, Anaplasma phagocytophilum and the equine piroplasms Theileria equi and Babesia caballi. Further workup by the veterinarian indicated that the horse was suffering from right dorsal colitis, and treatment to support this condition was instituted. The horse gradually improved, and the investigation was closed.
An equine veterinarian in Ca nterbury contacted the incursion investigation team to report severe thrombocytopaenia in a 13-year-old Warmblood mare. The mare had developed sudden inappetence and depression, and on bloodwork was found to be anaemic (haematocrit 0.23; normal range 0.33–0.55) with a panleucopaenia and increased fibrinogen (9 g/L; normal range 1-4 g/L). Exotic infectious causes of anaemia in horses include equine viral arteritis (EVA) virus, equine infectious anaemia (EIA) virus, the bacterium Anaplasma phagocytophilum and the equine piroplasms Theileria equi and Babesia caballi. Serum and whole blood from this mare were obtained for testing, and assays were negative for all these agents. The mare’s condition further deteriorated, and she developed a severe thrombocytopaenia and mild nasal epistaxis. She was euthanased and a full necropsy was performed. Histology showed the absence of cellular bone marrow, suggestive of an aplastic process, which would explain the mare’s disease process. Exotic agents were excluded and the investigation was closed.
EQUINE INFLUENZA EXCLUDED
An MPI Verification Services veterinarian called MPI via the exotic pest and disease hotline to report that a recently imported Thoroughbred gelding had spiked a fever on the last day of 2 weeks quarantine at a Transitional Facility in Auckland. There were no other clinical abnormalities. The imported cohort was retained in quarantine pending diagnostic testing to rule out the possibility of exotic disease and to try and find the cause of the pyrexia. Nasal swabs and blood samples sent to the AHL returned negative PCR test results for influenza A, equine arteritis virus and equine piroplasmosis. ELISA test results were negative for equine piroplasmosis (Theileria equi and Babesia caballi), and the AGID test was negative for equine infectious anaemia virus. Two blood samples were taken 24 hours apart and tested at a commercial veterinary laboratory, where they
revealed an initial slight neutrophilia with left shift, indicative of inflammation, which then returned to normal. Biochemistry showed mild non-specific abnormalities. The horse’s faeces softened during testing, and the vet was suspicious of a mild self-limiting colitis. The cohort was released following the horse’s full return to normothermia and results from the exotic disease testing.
[NOTE: Could this have been a case of equine coronavirus disease?]
BABESIA CABALLI EXCLUDED
An AHL scientist contacted MPI to report a positive test for Babesia caballi in a healthy 4-year-old Thoroughbred gelding being prepared for export to Singapore. To meet access requirements, horses must return negative cELISA results for both B. caballi and Theileria equi. The horse had returned a low-titre positive result in the cELISA test for B. caballi. Repeat blood samples were collected from the gelding 3 weeks later and submitted for follow-up testing at the OIE Collaborating Centre for Surveillance and Control of Animal Protozoan Diseases at Obihiro University of Agriculture and Veterinary Medicine in Hokkaido, Japan. The horse was confirmed negative to B. caballi by IFAT, competitive ELISA and nested PCR testing methodologies carried out at the OIE reference laboratory, and subsequently was able to travel to Singapore. Equine piroplasmosis was excluded and the investigation was closed.
References
McFadden AMJ et al. (2016). The first reported outbreak of equine herpesvirus myeloencephalopathy in New Zealand. NZ Vet J 64(2), 125–134.
New Zealand Equine Health Association (2012) Biosecurity Code for the control of Equine Contagious Diseases. https:// www.nzeha.org.nz/. Accessed 15 Feb 2022
Nugent J et al. (2006). Analysis of equid herpesvirus 1 strain variation reveals a point mutation of the DNA polymerase strongly associated with neuropathogenic versus nonneuropathogenic disease outbreaks. J Virol 80(8), 40474060.
Oladunni FS, Horohov DW, Chambers TM (2019). EHV-1: A Constant Threat to the Horse Industry. Front Microbiol 10, 2668.
EQUINE CLINICAL DIAGNOSTIC LABORATORY CASES
IDEXX LABORATORIES
Two Quarterhorse mares on a Queenstown-Lakes property presented with diarrhoea. Blood tests revealed low selenium levels in both, at 1,180 and 1,060 nmol/L (reference range 1,600–3,260 nmol/L). A 4-year-old Thoroughbred filly in Kapiti presented with coughing. Cytological evaluation of a tracheal wash sample revealed a neutrophilia with moderate numbers of bacteria (coccobacilli), both extracellularly and intracellularly (within neutrophils). Cultures yielded a moderate growth of gramnegative rods that were identified by MALDI-TOF mass spectrometry as Raoultella terrigans. This soil bacterium is closely related to other genera of enterobacteria, particularly Klebsiella. It is a rarely reported medical and veterinary opportunistic pathogen. Reports in the medical literature speculate that it has a role in device-associated nosocomial opportunistic infections and is thus related to multi-drug resistance.
[NOTE: It is important to study neutrophil cytology in order to get a better handle on the significance of a positive TTW culture (Jocelyn et al. 2018)].
SVS LABORATORIES
In early July, a mare from Bay of Plenty was examined for suspected chronic mud fever lesions on white parts of both the hind pasterns and fetlocks. The lesions were not improving and biopsies were taken. Histopathology revealed erosive and ulcerative dermatitis with a superficial dermal vasculopathy, suggestive of pastern leukocytoclastic vasculitis. The exact pathogenesis of this syndrome is not known, but the lesions may represent a common pathological response to a number of insults including immune dysregulation, superficial bacterial infections with such organisms as Staphylococcus spp., drug reactions, and photosensitivity, but can be idiopathic. In many cases the lesions involve only non-pigmented skin of distal extremities.
A 19-year-old Clydesdale gelding from Hawkes Bay presented with a quittor wound. A swab from the affected site yielded a moderate growth of Staphylococcus aureus and a light growth of E. coli. The S. aureus isolate was resistant to cefoxitin in vitro and therefore consistent with a methicillin-resistant Staphylococcus aureus (MRSA). This isolate was also resistant to tetracycline and gentamicin.
In early September a 3-week-old foal from Waikato presented with respiratory signs. Consolidated lung tissue was seen on ultrasound, and purulent material was present in a tracheal wash. Cytology of a tracheal wash sample revealed suppurative inflammation with intracellular and extracellular cocci and short rods. On culture there was a heavy growth of Rhodococcus equi.
GRIBBLES VETERINARY PATHOLOGY
An 8-year-old Miniature horse mare in Waikato had a mass on the flank that was 4 x 3 cm, subcutaneous, and attached to underlying muscle but with the overlying skin freely mobile. Histopathological examination of a biopsy revealed fibrous connective tissue and muscle disrupted by numerous eosinophils and scattered pockets of neoplastic mast cells, consistent with a mast-cell tumour.
Mast-cell tumours are uncommon in horses and when they do occur, they are typically low-grade and tend to expand locally but only rarely metastasise.
A fixed sample of third eyelid from a 13-year-old Quarterhorse cross gelding from Auckland was submitted for histopathological examination. One area of the conjunctiva was expanded by broad anastomosing cords and sheets of dysplastic epithelial cells. The cells were pleomorphic, displaying nuclei of variable size and shape, and there was keratinisation and necrosis of individual cells. These findings were consistent with ocular squamous cell carcinoma in situ. The third eyelid and limbic bulbar conjunctiva are common locations for squamous cell carcinoma in horses and, as in other species, ultraviolet light is thought to play a role in their development. It is thought that there may also be genetic risk factors in certain breeds. Often pre-cancerous lesions are present in both eyes. These progress to squamous cell carcinoma in situ and then become locally invasive. Metastasis is uncommon and may be restricted to the local lymph nodes but can be seen a long time after treatment of the initial lesion.
A 10-year-old Gypsy x Welsh Cob gelding from Canterbury had longstanding skin inflammation of the pastern regions. Skin scrapings were submitted to the laboratory for cytological examination. All three of the smears examined contained hundreds of live and dead mites in various stages of maturity. The morphology of the mites was consistent with a Chorioptes species, most likely Chorioptes bovis (formerly C. equi), consistent with a diagnosis of chorioptic mange. This is the most common cause of mange in horses and occurs in all breeds, but draught horses are predisposed. Lesions are pruritic and most commonly affect the distal limbs. Initially, papules develop, followed by alopecia, crusting and, in chronic cases, a moist dermatitis.
A subcutaneous mass adjacent to the carpus of a 10-yearold horse from Westland was excised and submitted for histopathological examination. It consisted of an infiltrate of neoplastic mast cells with many eosinophils and scattered foci of necrotic eosinophils. Although only infrequently seen, mast-cell tumour is nevertheless one of the more common skin tumours of horses.
References
Jocelyn NA, Wylie CE, Lean M, Barrelet A, Foote AK. Association of neutrophil morphology with bacterial isolates in equine tracheal wash samples. Equine Vet J. 2018; 50(6): 752-758. doi: 10.1111/ evj.12837.
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