Equine Herpesvirus Type 1 in Canada

On the heels of the deadly equine herpesvirus type 1 (EHV-1) outbreak in Ohio came a report of a private farm in Canada with several horses dead of the same cause. Scott Weese, DVM, DVSc, Dipl. ACVIM, assistant professor of clinical studies at the University of Guelph’s Ontario Veterinary College, said Canada has had at least one big EHV-1 outbreak in the past six months, but sporadic “clusters” of EHV-1 cases are fairly common in Ontario. There have been a few standout, or “hot” strains that seem to have caused more morbidity (sickness rate) and mortality than other strains, so EHV-1 has received attention in Ontario recently.

“We looked at a cluster of cases the other day (at a private farm) with a lot of horses on a small piece of property, many of them geriatric,” said Weese. “It looks like a similar strain to that back in the October outbreak (based on clinical signs and outcome), and looks similar to the (current) Ohio strain,” something he suggests to be “hotter” than typical endemic strains.

The October outbreak occurred at a Standardbred auction in London, Ontario, at the end of October 2002 when 264 yearlings were sold. According to Weese, four horses died after showing acute severe neurological signs of EHV-1 following the sale. 

Weese said that veterinarians traced 177 (67%) of the yearlings which were sold and left the premises, 77 (43.5%) of which developed clinical signs of disease. According to an official report on the outbreak, 57 of the 77 (74%) horses had nasal discharge, 42 (55%) had fever, 36 (47%) had cough, six (8%) displayed anorexia, five (6%) were ataxic, two (2.5%) exhibited vasculitis, and four (5%) were the yearlings that died. Scientists were unable to isolate the virus in that outbreak, but according to Weese, the histology of tissues of horses which died was abnormal and not consistent with typical herpes strains.

“We get reports of sporadic cases (rather than outbreaks) all the time,” said Weese. “The London (outbreak) got us interested because there were a lot of deaths, a lot of transmissions, and they scattered and spread the virus to other horses. You’ll see clusters on farms commonly, and it’s not too remarkable,” and that has been the case in Ontario recently.

“Most cases are going to have mild fevers,” he said, with a small number of EHV-affected horses having abortion or neurologic signs. “The strain this year may be more prone to causing neurologic disease,it may have been introduced more heavily—it’s hard to say until we (isolate) the virus, or we just might be paying more attention because of a few larger clusters of disease.” He said that researchers in Guelph’s diagnostic laboratory are working to isolate and identify the strain in the current private farm’s outbreak.
Weese hasn’t recommended using acyclovir in the treatment of EHV-1-infected Canadian horses as they did in the Ohio outbreak. “We haven’t tried it, but there is not much evidence at this point that it is effective,” he said.

Weese explained that the neurological form is due to the immune-mediated vasculitis--the immune system is attacking the blood vessels in brain and spinal cord where the virus is located. He said it’s probably a better goal to get rid of the inflammation as a result of this infection rather than kill the virus. “The only reason I would use (acyclovir) is if I had a severe respiratory case in a foal,” he said. He doesn’t recommend using it for prophylactic treatment. One of the most important ways to handle the disease is proper management of the horses—know which horses have been exposed and control their movements.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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