HORSES

Ontario Racing Commission Death Registry:  2014 postmortem summary

The Ontario Racing Commission (ORC) has a long-established record and takes pride in its proactive approach to advancing the welfare of the racehorse and the safety of the participant. 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 our knowledge of why these tragic events occur. The ORC Death Registry continues to provide excellent data regarding the causes of morbidity and mortality in racehorses in this province.  Summaries of postmortem submissions to the AHL under this program and diagnoses by body system are provided in the following tables.   

Table 1. Breed distribution of ORC Death Registry submissions to the AHL, 2003-2014

Table 2. Postmortem diagnoses of ORC Death Registry submissions by body system, 2003-2014.

Table 3. Musculoskeletal injuries in ORC Death Registry submissions by breed and anatomic site, 2014.

Table 4. Non-musculoskeletal diagnoses in ORC Death Registry submissions, 2014.

Potomac horse fever (Neorickettsia risticii) infection in 2 aborted Ontario foals  

Andrew Vince, Andrew Brooks, Hugh Cai

In December and January of 2014-2015, we received 2 aborted foals, the first at 5 months gestation, the second at 7 months gestation – one from eastern Ontario, one from southwestern Ontario. In the first foal, there was 100 mL blood-tinged free fluid in the peritoneal cavity and diffuse firmness of both lungs. The second foal had generalized icterus; multifocal hemorrhages in the skin, skeletal muscle, and small intestinal mucosa; subtle enlargement and pallor of the liver; and large quantities of fluid in the small intestine and colon. Tissues from both animals were negative by PCR for equine herpesvirus-1 (EHV-1). No significant pathogens were isolated on bacterial culture of lung, stomach content, or placenta (as available).

Despite divergent gross lesions, histologic lesions were similar in both cases, and  included hepatitis, myocarditis, thymic necrosis/thymitis, and most distinctively erosive enteritis (an unusual lesion in an aborted foal). The first foal also had a distinctive vasculitis in placenta, lung, and thymus. More uncommon differential diagnoses for equine abortion were discussed, including leptospirosis, equine viral arteritis (EVA), and Potomac horse fever (PHF). The first fetus was negative for EVA (PCR on lung) and leptospirosis (immunohistochemistry on various tissues). PCR tests of spleen and liver for PHF was positive in both foals. This result strongly implicated PHF as causal in both abortions.

Neorickettsia risticii is the causal organism underlying Potomac horse fever, a disease characterized by acute enterocolitis in horses first identified in 1979. It is principally associated with disease in spring, summer, and fall, and is most common in farms bordering rivers. This is a very infrequently identified cause of equine abortion in Ontario.

Abortion typically occurs months after infection and clinical disease in the mare, and is associated with fetal infection resulting in enterocolitis, hepatitis, myocarditis, placentitis, and variable hyperplasia or depletion of lymphoid organs. The gross and histologic appearance may initially resemble EHV-1 (though without classic microscopic herpesviral inclusions), and PHF should be considered a differential diagnosis for abortion in such cases if EHV-1 PCR/IHC testing is negative, regardless of season. PCR testing of tissues (usually lung, liver, spleen, commonly collected for EHV-1 PCR in such cases) is available at the AHL.