In recent weeks, several horses in Illinois have developed clinical signs of fever and respiratory disease. Of the six such cases submitted to the University of Illinois Veterinary Diagnostic Laboratory, molecular analysis determined that equine herpesvirus type 1 (EHV1) was the etiological agent in only two of the horses, both adults. Additional testing showed that the virus infection in these two adults was due to the equine herpesvirus myeloencephalopathy (EHM) variant.
Usually EHV1 will result in systemic infection manifesting as respiratory and possibly neurologic disease in the foals. In the older animal, it primarily causes problems in pregnant mares, with the potential for in utero infection and ensuing abortion or the neonate born with a systemic infection. All horses surviving the infection remain latently infected for life and experience periodic episodes of virus reactivation with possible virus transmission to other horses through respiratory exudates. Complicating this is an EHV1 variant, equine herpesvirus myeloencephalopathy (EHM), which may cause neurologic disease even in the adult, EHV1-vaccinated horse. The EHM variant is considered neuropathogenic due to its ability to replicate to higher viremic titers and cause higher neurologic morbidity and mortality as compared to the wild type EHV1. Protection against clinical disease due to EHM virus requires a robust cell-mediated immune response. As most EHV-1 vaccines are only considered effective for 90 days, boosters are warranted during such outbreaks. However, in general, immunization does not guarantee prevention from infection. Rather, vaccination only hopes to prime the immune response to keep the infection subclinical, reduce the duration of viremia as well as decrease the titer and duration of virus shedding.
The appropriate samples to submit for EHV testing are both nasal swabs (both nares) and whole blood (EDTA-treated). In addition, the Molecular Section of the VDL is able to pathotype EHV1 positive samples upon request. — Dr. Gail Scherba