It’s spring, and horses will be moving around more frequently for shows, trail rides and other events. Equine herpesvirus (EHV) has already reared its ugly head this year, and horse owners panic, rightly so, when they find their horse has been exposed to the disease.
Dr. Stephen Reed, internal medicine specialist at Rood and Riddle Equine Hospital in Lexington, KY, discusses EHV and some of the associated issues. There are many clinical signs associated with equine herpesvirus,” he said. “There are many herpesviruses, but the ones that we worry about most are EHV-1 and EHV-4. EHV-4 causes respiratory disease, but EHV-1 causes fever followed by respiratory disease and sometimes, unfortunately, abortion and equine herpes myeloencephalopathy (EHM). We need to remember that this virus can cause abortion storms.”
Reed says the respiratory form of EHV can appear similar to equine influenza. “The one thing that’s different about influenza is that it has a very short incubation period, and it causes a very dramatic hacking cough,” he said. “EHV can spread rapidly, but when it comes to the neurological form, one of the things that the mutation can do is cause the virus to replicate very fast. When it replicates fast, there are very high levels of viremia at the nose as well as in the bloodstream.”
Clinical signs of EHV include fever and a snotty nose, and that might be all. “With the neurological form, there’s an ascending paralysis,” said Reed. “It starts with the pelvic area. They get weak there, then progressively weaker, then there’s loss of tail and anal tone and urine dribbling.”
Reed says the EHV virus, wild or mutated, has been around for centuries, and explains that herpesviruses maintain themselves in the environment through latent infection – a virus that remains in the body and presents as disease under stress. Once infected, the horse may remain infected for life in latent form.
The wild strain of EHV is the oldest form of the virus. “But the mutated strain might have been around for a lot longer than we know,” said Reed. “In this virus, there’s a modification in the genetic code where there’s a substitution of one amino acid for another. That allows this virus to start replicating very rapidly. Once the virus is in the bloodstream, it will increase and finally cause damage through the respiratory system, the nose and then circulate inside cells. Once it’s in there, it can cause damage to blood cells. A lot of the neurologic signs are due to stroke-like involvement.”
Like many viruses, EHV lives inside cells and doesn’t survive long in the outdoor environment. If a horse is sick, then another horse goes into the same stall and uses the same feed and water bucket or if there’s nose-to-nose contact, there’s a chance for disease transmission. But Reed says generally, if the sick horse is segregated from others for 14 to 21 days beyond the last fever, the area is probably safe for other horses. “In actuality, if you haven’t had another onset within a week, you probably aren’t going to have a problem,” he said. “When it starts, it spreads fairly rapidly.”
Since the virus is harbored by a good percentage of the healthy horse population, it’s almost impossible to determine which horses are carriers. “Many, if not most horses, are infected early in life,” said Reed. “As long as they are not manifesting clinical signs, you would have very little ability to know whether the horse harbors the virus. Once the horse is exposed, it might be laying dormant in the trigeminal ganglion, part of the nervous system, or inside a lymph node. As long as it’s quiet, it isn’t stimulating the nervous system. Only when it’s in its active form is it likely to ‘turn on’ the immune system in the horse. That’s why it’s important to vaccinate horses that are latently infected on a regular basis.”
Reed mentions research that involved harvesting lymph nodes from horses that had died from other causes to determine whether the virus was present in those animals. The intent was to determine whether or not the mutated strain of EHV was present in a latent form, and the necropsies proved that the virus was present despite the horse having no respiratory signs at the time of death.
If a horse has mild clinical signs, would you notice? Most likely, and this is why owners and daily caretakers are the most important people in detecting EHV in individual horses and preventing potential problems. Those who interact with the horse daily know its habits, how much it eats and drinks, and other normal behavior for individual animals. “They know quickly when something is different,” said Reed. “They may or may not catch a temperature.”
What if a horse is diagnosed with EHV at a show while you’re there? Reed says it’s important to remember that because the virus is present as a latent infection, two to three percent of horses might be shedding the virus at any time. “If you’re at a show and you’ve done all you can to protect your horse – you’ve kept it well vaccinated, you’ve done the first part,” said Reed. “The rest is good hygiene and if you know there are horses showing neurologic signs, hopefully they will be segregated.”
Treatment options for respiratory EHV include NSAID medications such as bute, banamine or aspirin to deal with the fever and help the horse feel better and eat. Anti-viral drugs might help reduce the level of viremia and shorten the virus-shedding period.
Mature horses have a degree of immunity to EHV because it’s endemic in the environment, although breeding farm vaccinate regularly. “Some people use modified live in breeding live on advice of veterinarian,” said Reed. “The animal that has been vaccinated and is not showing or traveling and lives in a closed herd should still be protected, but there’s probably no need for excessive vaccination in any horse.”
Reed noted older horses appear to lose part of their immune system, and research showed the neurological form of herpes was easier to reproduce in older animals. “But the animals at shows are usually not old,” he said. “Disease outbreak is more likely initiated by transport, stress, and the level of virus they’re exposed to and whether the virus they’re exposed to is the mutated form.”