WHEN EHV-1 TURNS DEADLY
Equus|Autumn 2020
WHEN EHV-1 TURNS DEADLY
There’s no sure-fire way of stopping an equine herpesvirus infection from developing into a potentially devastating neurological disease. But you can take steps to protect your horse.
Heather Smith Thomas with Laurie Prinz

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 to severe and can include cough, fever, nasal discharge, loss of appetite and general malaise. Most sick horses recover uneventfully within a week or two with rest and supportive care.

“About 99 percent of cases occur in young stock,” says Nicola Pusterla, DVM, PhD, DACVIM, of the University of California, Davis. “Rhinopneumonitis is self-limiting and the horse gets over it. Foals, yearlings and 2-year-olds get it and overcome it, and that’s it.”

But like other herpesviruses, EHV-1 and -4 share a unique characteristic. Even after the horse recovers from his initial illness, the virus remains in his body in a latent form, “hiding” from the immune system. While latent, the virus may exist harmlessly in tissues, such as the lymph nodes. “Horses just continue to carry the virus for the rest of their lives,” says Amy Johnson, DVM, DACVIM, of the University of Pennsylvania. “The virus itself is everywhere in the equine population. The vast majority of horses are infected without any serious side effects.”

Yet when the horse is stressed---by travel, training or other major events ---EHV-1 or -4 may revert to its active form and cause a new bout of illness. These later reinfections tend to be mild, even unnoticeable, but they pose a risk to herdmates. Sometimes a horse who is carrying the reactivated EHV will show no signs of illness at all, but he will shed the virus in his nasal secretions, potentially spreading it to others. If the horse is shedding a strain of the virus that is new to his companions, he may trigger an outbreak of illness. This explains how outbreaks of rhinopneumonitis can appear seemingly out of nowhere, even in a herd that is never exposed to newcomers.

“Unfortunately, a small percentage of horses are dealing with a different subset of this virus,” says Pusterla. There are multiple strains of EHV-1 and -4, and some are more dangerous. These strains of EHV-1 and -4 can pass beyond the respiratory membranes, infect white blood cells called lymphocytes and circulate throughout the horse’s body. If the circulating viruses gain a foothold in the epithelial cells lining the uterus of a pregnant mare, the resulting inflammation can reduce the blood supply to the placenta, which in turn can starve the fetus and may cause an abortion. If the infection occurs later in the pregnancy, the fetus may survive but be born weakened. The greatest worry with broodmare bands is that one mare shedding the virus may trigger an “abortion storm” as the infection sweeps quickly through the entire herd.

EHM occurs when the EHV-1 attacks the cells lining the central nervous system, damaging blood vessels serving the brain and spinal cord. “The neurologic impairment often doesn’t occur until a little bit later in the course of the infection,” says Johnson. Signs may include decreasing coordination, urine dribbling, weakness in the hindquarters and loss of muscle tone in the tail. The weakness in the hind-quarters may progress into a horse leaning against wall for balance, sitting like a dog or full recumbency with an inability to rise.

If the horse can be kept alive with supportive care, the inflammation will subside and the damage will heal---at least to some degree. Many horses recover fully and return to normal work, while others may survive with some permanent effects.

More severe cases---especially if the horse is recumbent---may be fatal.

The statistics vary somewhat among different outbreaks, but on the whole, up to 30 percent of horses who contract EHV-1 during an EHM outbreak will develop EHM, and anywhere from 5 percent to 50 percent of those EHM cases may require euthanasia. In some outbreaks, the number of horses with EHM who were put down approached 80 percent.

Chances are, your horse is already carrying at least one strain of EHV-1, and should an outbreak occur in your area, there is no way to guarantee that he will not develop EHM. But you can take steps to protect his health---and the health of other horses. Here are five things you can do to reduce his risk.

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Autumn 2020