Sometimes everything appears to add up in an outbreak of what resembles equine herpesvirus-associated myeloencephalopathy, the dreaded neurologic condition generally caused by equine herpesvirus-1 (EHV-1) infection that causes ataxia or paralysis. Everything, that is, except for the test results. In these cases, consider the fact it might be another herpes strain causing the illness.

At the 46th Congress of the British Equine Veterinary Association (BEVA) held Sept. 12-15, 2007, in Edinburgh, Scotland, Lutz Steffen Goehring, MSc, Dipl. ACVIM, PhD, assistant professor in equine medicine at Colorado State University’s veterinary school, presented a study in which EHV-4 was identified in lesions of a horse’s spinal cord. The Standardbred stallion with signs of equine herpesvirus-associated myeloencephalopathy (EHM) was examined during a farm visit in the province of North-Holland, The Netherlands.

The culprit virus ended up being EHV-4, a herpesvirus known for being rather common among horses, but one that rarely causes neurologic signs. EHV-4 was easily isolated from the spinal cord of this horse during post-mortem examination.

"While this is not the first report of an EHV-4 associated myelopathy, it is so far still considered an extremely rare presentation," said Goehring. "While EHV-4 seroprevalence is generally very high, EHV-4 viremia (presence of virus in the bloodstream) is less common than it is during EHV-1 infection; however, viremia is a prerequisite to develop EHM