A Subset of Equine Gastric Squamous Cell Carcinomas Is Associated With Equus Caballus Papillomavirus-2 Infection.

Squamous cell carcinoma (SCC) is the most common neoplasm of the equine stomach. However, the mechanisms underlying malignant transformation are unknown. As Equus caballus papillomavirus-2 (EcPV-2) is a likely cause of some genital SCCs, we hypothesized that EcPV-2 is associated with a subset of equine gastric SCCs. To this aim, we performed polymerase chain reaction (PCR) and in situ hybridization (ISH) for EcPV-2 E6/ E7 oncogenes on 11 gastric SCCs and on gastric samples from 15 control horses with no SCC. PCR for EcPV-2 was positive in 7/11 (64%) gastric SCCs; non-SCC gastric samples were all negative. Intense hybridization signals for EcPV-2 E6/E7 nucleic acid were detected by ISH within tumor cells in 5/11 (45%) gastric SCCs, including distant metastases. No hybridization signals were detected within any of the non-SCC gastric cases. This study provides support for a potential association between EcPV-2 infection and a subset of equine gastric SCC.

Intestinal-peritoneal Coelioscopy for the Diagnosis of a Mesothelioma in a Pet Chicken.

An adult female chicken, from a small backyard flock, was presented to the Ontario Veterinary College Avian and Exotics Service for evaluation of dyspnea and recurrent ascites. An antemortem diagnostic evaluation included a coelomocentesis, coelomic ultrasound, and a coelioscopy procedure. A sample of the fluid collected during the coelomocentesis was submitted for analysis and was determined to be a nonspecific modified proteinaceous transudate. The coelomic ultrasound examination identified numerous coalescing fluid-filled and solid nodules throughout the coelom. However, no site of origin of the nodules could be identified. A coelioscopy of the intestinal-peritoneal cavity was performed by a ventral midline approach, and biopsies collected during the procedure were submitted for histologic examination. The pathologic diagnosis of the biopsy samples was a disseminated neoplasia, presumptively coelomic adenocarcinoma. The chicken received palliative treatment which included periodic coelomocentesis, meloxicam, antibiotics, and deslorelin following the diagnosis of a disseminated neoplasia. Three months following initial presentation the patient was euthanatized. A postmortem examination with histopathology confirmed the tissue biopsy results of coelomic neoplasia. Further immunohistochemistry supported mesothelioma as the definitive diagnosis. This case documents the usefulness of intestinal-peritoneal coelioscopy in identifying neoplasia as the cause of ascites in a pet chicken as well as describing the clinical features and progression of a mesothelioma in this species.

URINARY BLADDER TRANSITIONAL CELL CARCINOMA AND CARCINOMATOSIS IN A LINNAEUS'S TWO-TOED SLOTH, CHOLOEPUS DIDACTYLUS.

A 19-yr-old female Linnaeus's two-toed sloth (Choloepus didactylus) with a history of urinary incontinence, ascites, and behavioral changes was euthanized after diagnostic imaging revealed a large bladder mass. On gross necropsy, the sloth had a severely thickened bladder mucosa, partial urinary obstruction, and nonseptic exudate in the peritoneal cavity. Histopathology showed a malignant and highly invasive transitional cell (urothelial) carcinoma with transmural and intra-abdominal invasion and diffuse carcinomatosis. Immunohistochemistry for expression of pancytokeratin (AE1/AE3), cytokeratin 7 (CK7), cytokeratin 20 (CK20), and uroplakin III was performed to confirm the diagnosis of transitional cell carcinoma. Neoplastic cells had a strong cytoplasmic immunoreactivity with the antipancytokeratin antibody clone AE1/AE3, which was consistent with a neoplasm of epithelial origin. Neoplastic cells were negative for expression of CK20. This is the first detailed report describing the antemortem diagnosis of urinary bladder transitional cell carcinoma with carcinomatosis in a two-toed sloth.

Uterine angioleiomyoma in an African green monkey (Chlorocebus aethiops sabaeus).

BACKGROUND: A uterine neoplasm was observed, as an incidental finding, during post-mortem examination of a 26-year-old female multiparous African green monkey (Chlorocebus aethiops sabaeus). The intramural, expansile, 2 to 3 cm well-demarcated, dark-red, nodular neoplasm was located on the anterior uterine body (corpus) wall. METHODS: The mass was examined by light microscopy and by immunohistochemistry. RESULTS: The mass was confirmed as a cavernous uterine angioleiomyoma (syn. vascular leiomyoma) characterized by abundant intratumoural vasculature lined by Factor VIII-positive endothelial cells and surrounded by smooth muscle actin-positive cell proliferations. CONCLUSION: Angioleiomyoma sharing the characteristics of intramural human cavernous uterine angioleiomyoma should be considered in the differential diagnosis of uterine tumours in non-human primates.

Evaluation of the effect of umbilical hernias on play behaviors in growing pigs.

Umbilical hernias (UH) are common in pigs and are an animal welfare concern. This study used an assessment of play behavior to evaluate the welfare of pigs with UH. Twenty-one grower pigs with UH and 17 without hernias (WUH) were assigned to 16 playing groups (PG) of 2 or 3 pigs (with at least 1 UH pig per PG). The time each animal was engaged in any of the defined playing behaviors for locomotor/social or toy play behaviors was recorded. Mixed Poisson or negative binomial and linear models were used to determine the effect of UH and day of session, accounting for the cluster of pigs within groups, on the frequency of each play behavior, and playing times. Pigs with UH had the same frequency of most play behaviors and playing times as pigs without hernias. There was no indication that the presence of UH-affected play behavior or performance in pigs.

Évaluation de l'effet des hernies ombilicales sur les comportements de jeu chez les porcs en croissance. Les hernies ombilicales (HO) sont communes chez les porcs et elles représentent une préoccupation liée au bien-être animal. Cette étude a utilisé une évaluation du comportement de jeu afin d'évaluer le bien-être des porcs ayant une HO. Vingt-et-un porcs en croissance ayant une HO et 17 sans hernies (SHO) ont été assignés à 16 groupes de jeu (GJ) de 2 ou 3 porcs (avec au moins 1 porc HO par GJ). Le temps consacré par chaque animal pour participer à l'un des comportements de jeu définis pour les comportements de jeu de locomotion/social ou de jouet a été consigné. Des modèles mixtes de Poisson ou binomiaux et linéaires négatifs ont été utilisés pour déterminer l'effet de HO et le jour de la séance, en tenant compte des regroupements de porcs au sein des groupes, de la fréquence de chaque comportement de jeu et des moments de jeu. Les porcs avec une HO avait la même fréquence pour la plupart des comportements de jeu et des moments de jeu que les porcs SHO. Il n'y avait aucune indication que la présence d'une HO affectait le comportement de jeu ou la performance des porcs.(Traduit par Isabelle Vallières).

Retro-orbital and disseminated B-cell lymphoma in a yellow-collared macaw (Primolius auricollis).

A yellow-collared macaw was presented with unilateral left exophthalmia. The complete blood cell count and biochemistry revealed a heterophilic leukocytosis and elevation in liver parameters, respectively. A computed tomography scan showed a contrast-enhancing retrobulbar mass and hepatomegaly. Cytology of the liver was consistent with a round cell tumor, most likely lymphoma. The bird died after 2 months of palliative care. Postmortem examination confirmed a retro-orbital and disseminated B-cell lymphoma.

Lymphome B rétro-orbital et disséminé chez un ara à collier jaune(Primolius auricollis). Un ara à collier jaune a été présenté avec de l'exophtalmie unilatérale gauche. La formule sanguine complète et la biochimie ont révélé une leucocytose hétérophile et une élévation des paramètres hépatiques, respectivement. La tomodensitométrie à l'aide d'une injection de milieu de contraste a montré une masse rétrobulbaire et une hépatomégalie. La cytologie du foie était conforme à une tumeur à cellules rondes, le plus probablement un lymphome. L'oiseau est mort après 2 mois de soins palliatifs. L'examen postmortem a confirmé un lymphome B rétro-orbital et disséminé.(Traduit par Isabelle Vallières).

Persistence of clinical signs associated with rotavirus following an outbreak of porcine epidemic diarrhea (PED) on a farrow-to-grow swine operation in southwestern Ontario.

Clinical signs attributed to porcine epidemic diarrhea (PED) persisted for several months in a southwestern Ontario farm following an outbreak of PED. Extensive testing revealed rotavirus infection and absence of PED in nursing and nursery pigs, highlighting the importance of repeated diagnostic testing following a disease outbreak.

Persistance des signes cliniques associés au rotavirus après une éclosion de diarrhée épidémique porcine (DEP) dans une exploitation porcine de type naisseur-finisseur dans le sud-ouest de l'Ontario. Les signes cliniques attribués à la diarrhée épidémique porcine (DEP) ont persisté pendant plusieurs mois dans une ferme du sud-ouest de l'Ontario après une éclosion de DEP. De nombreux tests ont révélé l'infection par le rotavirus et l'absence de DEP chez les porcs à l'allaitement et de la pouponnière, ce qui souligne l'importance de tests diagnostiques répétés après l'éclosion d'une maladie.(Traduit par Isabelle Vallières).

Detection of Leptospira spp. in wildlife reservoir hosts in Ontario through comparison of immunohistochemical and polymerase chain reaction genotyping methods.

A total of 460 kidney samples from wildlife (beavers, coyotes, deer, foxes, opossums, otters, raccoons, skunks) were obtained from road-kill and hunter/trapper donations in Ontario between January 2010 and November 2012. The objectives of the study were to detect Leptospira spp. by immunohistochemistry and polymerase chain reaction (PCR), to map presence of leptospires in wildlife relative to livestock and human populations, and to characterize positive samples by sequencing and comparison to leptospires known to affect domestic animals and humans. The proportion of samples that tested positive ranged from 0% to 42%, with the highest rates in skunks and raccoons. Leptospira spp. were present in kidneys of wildlife across Ontario, particularly in areas of high human density, and areas in which livestock populations are abundant. The PCR was too weak in most samples to permit genotyping and examination of the relationship between the leptospires found in this study and those affecting domestic animals and humans.

Détection deLeptospiraspp. chez des hôtes du réservoir faunique en Ontario par la comparaison des méthodes de génotypage de réaction immunohistochimique et d'amplification en chaîne par la polymérase. Un total de 460 échantillons de reins provenant de la faune (castors, coyotes, cerfs de Virginie, renards, opossums, loutres, ratons-laveurs, moufettes) ont été obtenus d'animaux tués sur la route et de dons de chasseurs et de trappeurs en Ontario entre janvier 2010 et novembre 2012. Les objectifs de l'étude étaient de détecter Leptospira ssp. par immunohistochimie et amplification en chaîne par la polymérase afin de cartographier la présence des leptospires dans la faune en rapport avec les populations de bétail et d'humains et de caractériser les échantillons positifs par le séquençage et la comparaison avec des leptospires reconnus comme affectant les animaux domestiques et les humains. La proportion des échantillons qui ont montré un résultat positif s'échelonnait de 0 à 42 %, et les taux les plus élevés se retrouvaient chez les moufettes et les ratons-laveurs. Leptospira spp. était présent dans les reins de la faune partout en Ontario, particulièrement dans les régions à forte densité humaine et dans les régions où les populations de bétail sont abondantes. L'amplification en chaîne par la polymérase était trop faible dans la plupart des échantillons pour permettre le génotypage et l'examen de la relation entre les leptospires trouvés dans cette étude et ceux touchant les animaux domestiques et les humains.(Traduit par Isabelle Vallières).

Focal thoracolumbar spinal cord lymphosarcoma in a ferret (Mustela putorius furo).

A 6-year-old, castrated male domestic ferret (Mustela putorius furo) was euthanized following progressive hind limb paresis and atonia of the bladder of 1-year duration. Neurological evaluation localized the lesion to the thoracolumbar spinal region, and magnetic resonance imaging showed a focal intramedullary spinal cord lesion. Histopathology revealed an extensive, unencapsulated, poorly demarcated mass within the thoracolumbar spinal cord, diagnosed as lymphosarcoma.

Lymphosarcome thoraco-lombaire localisé dans la moelle épinière chez un furet(Mustela putorius furo) . Un furet domestique (Mustela putorius furo) mâle castré âgé de 6 ans a été euthanasié après une parésie progressive des membres postérieurs et une atonie de la vessie d'une durée de 1 an. L'évaluation neurologique a repéré la lésion dans la région de la moelle épinière thoraco-lombaire et une imagerie par résonance magnétique a indiqué une lésion intramédullaire localisée dans la moelle épinière. L'histopathologie a révélé une masse importante, acapsulée et faiblement démarquée dans la moelle épinière thoraco-lombaire qui a été diagnostiquée comme un lymphosarcome.(Traduit par Isabelle Vallières).

Pathology and diagnosis of avian bornavirus infection in wild Canada geese (Branta canadensis), trumpeter swans (Cygnus buccinator) and mute swans (Cygnus olor) in Canada: a retrospective study.

Nine hundred and fifty-five pathology cases collected in Ontario between 1992 and 2011 from wild free-ranging Canada geese, trumpeter swans and mute swans were retrospectively evaluated for the pathology associated with avian bornavirus (ABV) infection. Cases were selected based on the presence of upper gastrointestinal impaction, central nervous system histopathology or clinical history suggestive of ABV infection. The proportion of birds meeting at least one of these criteria was significantly higher at the Toronto Zoo (30/132) than elsewhere in Ontario (21/823). Central, peripheral and autonomic nervous tissues were examined for the presence of lymphocytes and plasma cells on histopathology. The presence of virus was assessed by immunohistochemistry and reverse transcriptase-polymerase chain reaction (RT-PCR) on frozen brains and on formalin-fixed paraffin-embedded tissues. Among selected cases, 86.3% (44/51) were considered positive on histopathology, 56.8% (29/51) were positive by immunohistochemistry, and RT-PCR was positive on 88.2% (15/17) of the frozen brains and 78.4% (40/51) of the formalin-fixed paraffin-embedded samples. Histopathological lesions included gliosis and lymphoplasmacytic perivascular cuffing in brain (97.7%), spinal cord (50%), peripheral nerves (55.5%) and myenteric ganglia or nerves (62.8%), resembling lesions described in parrots affected with proventricular dilatation disease. Partial amino acid sequences of the nucleocapsid gene from seven geese were 100% identical amongst themselves and 98.1 to 100% identical to the waterfowl sequences recently described in the USA. Although ABV has been identified in apparently healthy geese, our study confirmed that ABV can also be associated with significant disease in wild waterfowl species.

An investigation of ear necrosis in pigs.

Porcine ear necrosis was investigated in 23 conveniently chosen farms, consisting of 14 case farms and 9 control farms. Biopsies of lesions and oral swabs from pigs on 11 case farms were examined by histology and bacterial culture. All farms were visited for observations and a survey on management, housing, and the presence of other clinical signs or behavioral vices. Histological examination revealed that the lesions began on the surface and progressed to deeper layers, and that vascular damage did not appear to be the initiating cause. Spirochetes were only rarely observed in histological examination and were not cultured from biopsies and oral swabs. Staphylococcus aureus and Staphylococcus hyicus were cultured from 91% and 66% of samples, respectively. Ear biting and a humid environment were associated with ear necrosis. On some farms large numbers of pigs were affected and lesions were sometimes extensive. The condition appears to be an infectious disease beginning on the surface of the skin; contributing environmental and management factors are likely.

Enquête sur la nécrose des oreilles chez les porcs. La nécrose des oreilles porcines a fait l'objet d'une enquête dans un échantillonnage de convenance de 23 fermes, composé de 14 fermes de cas et de 9 fermes témoins. Les biopsies de lésions et des prélèvements oraux faits chez des porcs dans 11 fermes de cas ont été examinés par histologie et culture bactérienne. Toutes les fermes ont été visitées pour recueillir des observations et effectuer un examen de la gestion, du logement et de la présence d'autres signes cliniques ou vices de comportement. L'examen histologique a révélé que les lésions commençaient sur la surface et progressaient vers des couches plus profondes et que les dommages vasculaires ne semblaient pas être la cause initiale. Des spirochètes ont été rarement observés à l'examen histologique et n'ont pas été cultivés dans les biopsies et les prélèvements oraux. Staphylococcus aureus et Staphylococcus hyicus ont été cultivés dans 91 % et 66 % des échantillons, respectivement. Les morsures d'oreilles et un environnement humide étaient associés à la nécrose des oreilles. Dans certaines fermes, un grand nombre de porcs étaient affectés et les lésions étaient parfois importantes. L'affection semble être une maladie infectieuse qui commence à la surface de la peau; des facteurs contributifs liés à l'environnement et à la gestion sont probables.(Traduit par Isabelle Vallières).

Lawsonia intracellularis-associated ulcerative and necro-hemorrhagic enteritis in 5 weanling foals.

This report describes 5 cases of fatal Lawsonia intracellularis-associated ulcerative and necro-hemorrhagic enteritis in weanling Thoroughbred and Standardbred foals. The lesions are similar to those of the L. intracellularis-associated ulcerative and necro-hemorrhagic enteritis syndrome in pigs. Two foals had concurrent severe typhlo-colitis as a result of a large burden of encysted cyathostomes. The clinical, diagnostic, and therapeutic challenges, and the potential complications encountered during the management of such cases are discussed.

Entérite ulcérative et nécro-hémorragique associée àLawsonia intracellularischez 5 poulains sevrés. Ce rapport décrit 5 cas mortels d'entérite ulcérative et nécro-hémorragique associée à Lawsonia intracellularis chez des poulains Thoroughbred et Standardbred. Les lésions sont semblables à celles du syndrome de l'entérite ulcérative et nécro-hémorragique associée à L. intracellularis chez les porcs. Deux poulains étaient atteints d'une typhlo-colite grave concomitante en raison d'une charge importante de cyathostomes enkystés. Les difficultés cliniques, diagnostiques et thérapeutiques ainsi que les complications potentielles rencontrées durant la gestion de ces cas sont analysées.(Traduit par Isabelle Vallières).

Vasculitis Associated with Parrot Bornavirus 4 Infection in a Rose-Crowned Parakeet (Pyrrhurarhodocephala).

A 3-month-old, female rose-crowned parakeet (Pyrrhura rhodocephala) was found dead after a 24-h course of lethargy and passing blood-tinged faeces. Fine white streaks were seen in the pectoral muscles on necropsy. Microscopic examination revealed typical lesions of avian ganglioneuritis and vascular necrosis in the pectoral muscles, myocardium, kidneys, air sacs, adrenal glands, pancreas and thyroid gland. These lesions were characterized by mural fibrinoid necrosis of small and medium-calibre arteries and arterioles, associated with lymphoplasmacytic inflammation, necrosis, atrophy and fibrosis of the surrounding tissues. Parrot bornavirus (PaBV) nucleoprotein was demonstrated by immunohistochemistry in smooth muscle and endothelial cells of many vessels. An avian bornavirus was isolated from kidney tissue and its identity confirmed as PaBV-4 by sequencing and phylogenetic analysis. We postulate that the vascular lesions could have been immune-mediated and that PaBV-4 may have played a role in its pathogenesis.

Equine primary liver tumors: a case series and review of the literature.

Hepatoblastoma (HB) is an uncommon pediatric liver tumor in humans and horses. In humans, HB is most frequently diagnosed in fetuses, neonates, and young children, whereas hepatocellular carcinoma (HCC) affects juvenile and adult humans. Hepatoblastoma in the horse is rare, with only 9 reported cases. Affected horses ranged in age from late-term aborted fetuses to 3 years. The current study describes 3 new cases of primary liver tumors in horses and reviews findings in relation to other reports on this condition. Tumors classified as HB were identified in a male Standardbred aborted fetus and in a 4-year-old Thoroughbred filly. Hepatocellular carcinoma was diagnosed in a 15-month-old Paint filly. In the Standardbred fetus, the tumor was only present in the liver. In the Thoroughbred and Paint fillies, primary tumors were in the right liver lobe and at the hilus, respectively, and there were metastases to other lobes (HB) and mesenteric lymph nodes (HCC). Tumors were sharply demarcated from adjacent tissue, nonencapsulated, compressive, and invasive. Consisting of cords and nests, or disorganized sheets of epithelial cells, tumors had variable stromal and vascular components. The fetal tumor contained areas of smaller, less differentiated cells with a pronounced mesenchymal component interpreted to be embryonal hepatic tissue. Diagnoses were based on tumor histomorphologic features, resemblance to hepatocyte developmental stages, age of the animal, and patterns of metastasis. Tumors classified as HB were alpha-fetoprotein immunoreactive. Primary hepatic tumors in the horse are diverse in morphology and include subtypes compatible with classification criteria applied to human tumors.

Avian bornavirus is present in many tissues of psittacine birds with histopathologic evidence of proventricular dilatation disease.

Proventricular dilatation disease (PDD) is a neurologic disease of psittacine birds suspected to be caused by a recently identified Avian bornavirus (ABV). In the current report, data supporting the causal association of ABV with PDD are presented. Immunohistochemistry (IHC) with rabbit polyclonal antiserum raised against ABV nucleocapsid protein was used to identify cell and organ distribution of viral antigen. The ABV antigen was most consistently detected in brain, spinal cord, adrenal gland, pancreas, and kidney. Histopathologic evaluation was correlated with ABV-specific polymerase chain reaction (PCR) and immunohistochemical tests in multiple tissues from 16 psittacine birds with and without PDD. Using histopathologic diagnosis as the gold standard, the sensitivity and specificity of IHC for ABV antigens were found to be 100% and 100%, respectively. Many more tissues were positive for ABV RNA by reverse transcription PCR than were positive for pathologic changes or viral antigens by IHC, indicating the presence of subclinical or asymptomatic infection at many sites.

First confirmed case of the bovine brachyspina syndrome in Canada.

Brachyspina syndrome is a congenital inherited lethal defect in Holstein cattle. Although cases have previously been reported from Europe, the first confirmed North American case is reported herein. This case was genetically related to the European cases, but pedigree analysis indicates that the founder may be more remote than previously expected.

Paraparesis in a polar bear (Ursus maritimus) associated with West Nile virus infection.

A polar bear (Ursus maritimus) housed at the Toronto Zoo presented with acute-onset, nonambulatory paraparesis. Physical examination 24 hr after onset was otherwise unremarkable, spinal radiographs looked normal, and blood tests indicated mild dehydration. With continued deterioration in its general condition, euthanasia was elected a day later. Necropsy did not reveal a cause for the major presenting clinical signs. Serum collected at the time of initial examination was positive for West Nile virus (WNV) antibodies in a serum neutralization assay and at the time of euthanasia was positive in both a competitive enzyme-linked immunosorbent assay and in a plaque reduction neutralization assay. The major microscopic finding was a mild-to-moderate nonsuppurative meningoencephalomyelitis. WNV was not detected by immunohistochemistry in brain or spinal cord or by real-time reverse transcription-polymerase chain reaction (RT-PCR) and cell culture of brain and kidney, but it was isolated and identified by RT-PCR in second passage cell culture of spleen. Retrospective immunohistochemistry on spleen revealed rare antigen-positive cells, probably macrophages. Prevention of exposure to potentially WNV-infected mosquitoes or vaccination of captive bears against WNV should be considered.

The emergence of a new strain of porcine circovirus-2 in Ontario and Quebec swine and its association with severe porcine circovirus associated disease--2004-2006.

In the late fall of 2004 more severe lesions of porcine circovirus-2 associated disease (PCVAD) than usual occurred during an outbreak of porcine circovirus-2 (PCV-2) infection in Ontario nursery and grower/finisher pigs. The lesions were of unprecedented severity and included diffuse bronchointerstitial pneumonia, granulomatous enteritis, vasculitis, interstitial nephritis, and new lesions of splenic infarction. Some affected herds had up to 50% mortality. The outbreak correlated with the sudden emergence of a variant PCV-2, with PCR restriction fragment length polymorphism (RFLP) type 321. Phylogenetic comparison of ORF2 sequences and full genome sequences showed the new variant to be different from the previously dominant RFLP type 422 viruses, and similar to viruses that had occurred in France and other European and Asian countries. A subsequent retrospective study showed a statistically significant increase in the frequency of histological lesions in lymph node, spleen, lung, small intestine, colon and kidney, for pigs spontaneously infected with RFLP type 321, compared with the older RFLP type 422 strain. Viral burden, based on IHC staining in lymph node, also showed a statistically significant increase in pigs infected with the newer variant RFLP type 321, compared with the older RFLP type 422 strain. This enhanced virulence in pigs infected with PCV-2 RFLP type 321 strain may be related to the genetic differences in this new strain of PCV-2. This virus is now the dominant strain of PCV-2 virus found in Ontario and Quebec swine.

Postmortem findings in Ontario racehorses, 2003-2015.

Postmortem lesions and cause of death were evaluated retrospectively for 963 horses examined as part of the Ontario Racing Commission Death Registry over a 13-y period. The Death Registry was established in 2003 to identify factors leading to death or euthanasia of racehorses in this Canadian province. Postmortem examination was carried out on 56% of horses reported to the Death Registry and included Standardbred, Thoroughbred, and American Quarter Horses. Musculoskeletal injury was the most common reason for death associated with racing or training among all racehorses and involved 68% of horses. A history of sudden death during or immediately following exercise was described for 31% of racing- or training-associated deaths, and in 16% of all horses in the study. Sudden death occurred in horses of all breeds, and our report describes lesions associated with sudden death in a series of Standardbreds. The cause of death in these cases was most frequently attributed to cardiopulmonary lesions. The cause of death was undetermined in 20% of sudden death cases, and it is speculated that cardiac arrhythmia may have contributed to these deaths. Injection-associated death was documented in 4% of the study population. Lesions among horses whose death was not associated with exercise were similar to those in other equine populations, and lesions involving the gastrointestinal system were most common. Standardization of pathology reporting of pulmonary and other lesions in racehorse postmortem cases would allow direct comparison of results among racehorse populations.