WHEN EHV-1 TURNS DEADLY

In this age of pandemic, everyone has become familiar with, if not expert in, measures that control the spread of disease: Social distancing. Disinfection. Quarantine. These fundamentals of biosecurity might once have been abstractions but now have taken on practical importance in our lives. Of course, the horse world has long made the control of certain diseases a priority. And thanks to another pillar of disease prevention---vaccination---we’ve largely been successful in protecting horses from rabies, equine encephalomyelitis, West Nile virus and other infectious scourges.

Yet some pathogens continue to pose a threat despite even the most stringent hygiene measures and vaccination programs. Equine herpesvirus-1 (EHV-1) is one such organism. Even without physical contact, this highly contagious respiratory virus can spread rapidly from horse to horse through nasal discharge or aerosol droplets.

Although most cases cause mild-to-moderate respiratory illness (rhinopneumonitis), EHV-1 infection occasionally leads to a life-threatening neurologic disease known as equine herpesvirus myeloencephalopathy (EHM). The mechanisms through which EHV-1, and even more rarely EHV-4, produce neurologic disease are not yet understood. So your best bet is to reduce your horse’s exposure to pathogens in general.

You might think this year’s disrupted horse-show season and event schedules, along with depopulated equestrian venues, would have eliminated the threat of EHM, but that’s not the case. A quick scan of the Equine Disease Communication Center database shows that EHM cases occurred this spring in California, Iowa, Indiana and Maryland, even amid lockdowns and shelter-in-place orders. Regardless of where and how it occurs, a single EHV-1 case that is not promptly contained can easily turn into a widespread outbreak that puts all nearby horses at risk---and if EHM develops, the fatality rate can be high.

In other words, even as public health measures to stop the spread of COVID-19 continue, now is not the time to let down your guard when it comes protecting your horse from the unique threat posed by equine herpesvirus.

WIDESPREAD YET INSIDIOUS

EHV-1 and EHV-4 are respiratory viruses that, when inhaled, penetrate the epithelial cells lining the horse’s airways, setting off an inflammatory reaction called rhinopneumonitis, which is not unlike the common cold in people. Signs range from mild