HORSES
Alcohol and Gaming Commission of Ontario (AGCO) Death Registry / Equine Incidents in Ontario Racing program: 2003 - 2018 postmortem summary
The Alcohol and Gaming Commission of Ontario (AGCO; formerly the Ontario Racing Commission, ORC) continues in its proactive approach to advance racehorse welfare and safety of human and animal participants. In 2003, Ontario became one of the first North American racing jurisdictions to require mandatory reporting of racehorse deaths, in order to monitor, research, and improve knowledge of why these events occur. Postmortem (PM) exams conducted at the Animal Health Laboratory (AHL) through the AGCO Death Registry (DR, 2003-2016) and Equine Incidences in Ontario Racing (EIOR, 2016-current) programs continue to provide comprehensive data regarding the causes of morbidity and mortality in racehorses in this province. To date, PM has been carried out on 1,125 horses through these programs (Table 1). Annual variation in the number of PM cases reflects the discretionary requirement for PM on the part of the Registrar of AGCO.
A summary of significant PM findings is provided in Table 2. A comprehensive review of AGCO PM cases was conducted in 2015 as part of a separate retrospective study and as a result, some cases have been reclassified from results presented in previous editions of the AHL Newsletter. Results of the study were published in the July 2017 edition of the Journal of Veterinary Diagnostic Investigation.1
Since 2015, computed tomography (CT) of fractured and contralateral limbs has been carried out on select DR and EIOR postmortem cases through collaboration with the Diagnostic Imaging section of the Ontario Veterinary College Health Sciences Center. The goal of this in-depth examination is to identify pre-existent lesions, primarily in bone, that contribute to catastrophic fractures. The procedure was continued in 2018, with CT imaging of all 29 limb fracture cases submitted for PM exam. Pre-existent lesions in bone were identified by CT and considered potentially predisposing to fracture in 18 of 29 (62%) cases.
Exercise-associated sudden death continues to be of special concern in the racing industry. At the AHL, a modified in-depth PM protocol is used in the evaluation of these cases, with special emphasis on cardiovascular and respiratory systems. In 2018, the cause of death was investigated in 5 horses that died while exercising. Significant pulmonary hemorrhage was evident in 4 horses, and no cause of death was identified in 1 horse. Among all sudden death cases from 2003-2018, significant pulmonary hemorrhage was identified in 88 of 177 (50%) horses. The cause of death in such cases is often attributed to exercise-induced pulmonary hemorrhage (EIPH), although the pathogenesis of pulmonary hemorrhage in these horses is not well understood. In 41 of 177 (23%) exercise-associated sudden death cases from 2003-2018, no potentially fatal lesions were identified and the cause of death remained undetermined. It has been speculated that exercise-associated cardiac dysrhythmia, leading to acute heart failure and pulmonary hypertension, may be the underlying cause of death among many of these horses, and may also contribute to pulmonary hemorrhage in these animals.2 Typically, no morphologic lesions are detected in the heart as a cause or result of fatal ventricular dysrhythmia, and the diagnosis cannot be confirmed based on PM findings.
Summaries of postmortem submissions to the Animal Health Laboratory under this program and diagnoses by body system for these cases are provided in the following tables. AHL
References
1. DeLay J. Postmortem findings in Ontario racehorses, 2003-2015. J Vet Diagn Invest. 2017;29:457-464.
2. Physick-Sheard PW, McGurrin MKJ. Ventricular arrhythmias during race recovery in Standardbred racehorses and associations with autonomic activity. J Vet Intern Med 2010;24:1158-1166.
Table 1. Breed distribution of AGCO Equine Incidents submissions to the AHL, 2003-2018.
Table 2. Significant postmortem lesions identified in AGCO Death Registry submissions by body system, 2003-2018.
Dicrocoelium dendriticum in the liver of a miniature horse
Murray Hazlett, Margaret Stalker, Mary Lake, Andrew Peregrine
Following a humane society investigation, an emaciated miniature horse was submitted to the AHL for postmortem. It had no fat stores and was estimated to be relatively young. It had been frozen, however routine histologic examination revealed areas in the liver with moderate periportal fibrosis. A cross-section of a small fluke could be seen, along with some operculate eggs. We were able to recover a fluke following fine dissection of the liver and bile ducts, and identified it as Dicrocoelium dendriticum based on morphology as well as characteristic eggs that were released from the specimen (Fig. 1).
Dicrocoelium dendriticum is a small fluke that usually infects sheep, cattle, and goats as well as camelids. It has a unique life-cycle involving land snails and ants. Briefly, infected snails release infected “slime balls” that are eaten by ants, in which the parasite develops in the nervous system. At temperatures < 15oC, the parasite causes the ant to stay on the tips of grass blades where it can be eaten (see the 2 references below for more of this fascinating story).1,2
Dicrocoelium dendriticum is very common in Europe. This trematode is thought to have been introduced to Canada in the 1930s, likely from Europe1 and is now often recognized in Ontario; lesions are commonly seen in sheep and goat livers in slaughter plants (Alexandra Reid, OMAFRA, pers. comm.). There are documented cases of D. dendriticum in horses in Europe, Asia, and Africa, however it has not been reported in a horse in North America before our recent report1 in the Canadian Veterinary Journal. AHL
References
1. Hazlett M, et al. Hepatic Dicrocoelium dendriticum infection in a miniature horse. Can Vet J 2018;59:863-865.
2. Stalker MJ, et al. Hepatic infection with the lancet fluke, Dicrocoelium dendriticum, in an Ontario sheep flock. AHL Newsletter, September 2007:22–24.
Figure 1. A. Dicrocoelium dendriticum cross section in remnants of a bile duct. Eggs can be seen in the uterus of the trematode (arrows). H&E. B. More longitudinal orientation of a partial D. dendriticum in a bile duct. The oral sucker is visible (arrow). H&E. C. Periportal fibrosis (arrows) including necrotic cellular debris in a larger duct (large arrow). H&E. D. Eggs of D. dendriticum released from the dissected frozen, unfixed, liver tissue. Wet mount. E. Autolyzed body of recovered intact D. dendriticum. Wet mount. Reprinted with permission from the Canadian Veterinary Journal (Can Vet J 2018;59:863-865).