Equine Herpesvirus Myeloencephalopathy in Virginia

Four cases of mysterious illness in Northern Virginia horses have been attributed to equine herpesvirus (EHV) myeloencephalopathy, a rare neurologic version of EHV type 1, which is typically recognized in its respiratory form as rhinopneumonitis. Three horses at Fox Chase Farm in Middleburg have been euthanized, and one pony is recovering at the farm under supportive care. The incidents have area horse owners concerned about travel and exposure of their horses, and veterinarians are alert for any other unusual symptoms.

Joseph P. Garvin, DVM, is laboratory director of the Virginia Department of Agriculture and Consumer Services' Warrenton Regional Animal Health Laboratory, where the autopsies of the second and third horses were performed. While test results at Garvin’s laboratory and the National Veterinary Services Laboratory in Ames, Iowa, have been “inconclusive,” scientists have ruled out other possible neurologic diseases and have strong evidence from clinical signs and histopathology reports that EHV is what caused the disease. “In this instance, we do have negative tests for West Nile virus, equine encephalitis, and for EPM (equine protozoal myeloencephalitis),” said Garvin. “There are still tests pending, and I have my fingers crossed that we’ll have definitive answers. The results have been very suggestive of equine herpesvirus myeloencephalopathy.”

Garvin explained that tests of the tissue are conclusive only if EHV-1 is grown out from the tissue or its DNA is found through one of many very sensitive laboratory tests. To complicate the matter, the virus might not be present in the horses by the time of their deaths, so tests might come back negative.

“The organism causing equine herpesvirus-1 can cause three different forms of the disease,” he explained, “rhinopneumonitis, a respiratory disease of mostly young horses, abortions in pregnant mares, and this neurologic disease.” There are at least seven other strains of equine herpesviruses, named in order of their discovery. (See Article #32 at www.TheHorse.com for more on herpesviruses.)

Typically, horses suffering from EHV-1 myeloencephalopathy have a fever prior to developing neurologic disease symptoms. According to Garvin, the Virginia horses did not have a fever. However, each displayed “hindlimb ataxia, loss of control of their bladder, a flaccid tail, and they eventually went down. One of the horses had forelimb ataxia, which occurs a certain percentage of the time,” he explained. “The horse that had the forelimb ataxia also had a head tilt and histologic lesions on the brain.

“With the history, clinical signs, and the tests that we’ve run, it all fits together very nicely,” Garvin said. “It seems like (the horses) read the textbook as far as the signs they showed.”

The first affected horse was not submitted for testing. Symptoms were recognized in the second horse on April 19 and the horse was euthanized on April 22. The third horse became affected on April 24 and was euthanized on April 26.

The neurologic form of EHV-1 often is curable with supportive care, but unfortunately once a horse is down, it is difficult to nurse the horse back to health, and owners and veterinarians elected to euthanize these three animals.

According to reports, the horses were vaccinated regularly for rhinopneumonitis, so the natural inclination of horse owners has been to ask why the horses were infected. Veterinarians have said that no vaccine manufacturer has made a vaccine that says that it will prevent the neurological form of EHV-1 from occurring. The duration of immunity to EHV is not as long as it is with some other viruses, so generally veterinarians vaccinate twice a year.

The source of infection remains unknown, but reports have suggested that one of the horses might have contracted the virus at a horse show. The Virginia Department of Agriculture and Consumer Services has not made a recommendation as to whether or not shows should be canceled or farms quarantined in the light of the incomplete test results, but many farms have taken precautionary measures and have not moved horses.

Fox Chase Farm made the decision to quarantine its horses and keep horse owners in Middleburg and surrounding areas abreast of any developments in the situation. “They have been very open, and very good citizens about this,” said Elaine Lidholm, the director of communications for the Virginia Department of Agriculture and Consumer Services. “They quarantined themselves immediately and are to be commended for that, for taking that proactive stance."


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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