With multiple equine herpesvirus (EHV-1) outbreaks popping up across the United States, owners need to take precautions to protect their horses from the potentially fatal disease. EHV-1 manifests in three forms: respiratory disease, abortion, and/or neurologic disease.
Dr. Steve Reed, internal medicine specialist at Rood & Riddle Equine Hospital in Lexington, Ky., clarified the same pathogen that causes EHV-1 is responsible for the neurological form (equine herpesvirus myeloencephalopathy or EHM). But the virus sometimes mutates, causing it to become more virulent.
“So when it does have the mutated strain, it seems like there's a little more likelihood you're going to get neurologic disease,” he said. “We think part of the reason for that is because it replicates too rapidly, and you're going to have so much more virus present.”
Although commercial vaccines exist both in killed and modified-live virus forms, they do not guarantee protection against the deadly neurologic form. But Reed said it is reasonable to expect the vaccine to mitigate the symptoms if the horse does develop the disease. Once your horse has been properly vaccinated against EHV-1, he suggests administering an EHV-1 booster with enough lead time for it to take effect if your horse will be in contact with outside horses.
Reed warned horsemen to follow the manufacturer's recommendations for the frequency of vaccination. In 2003, he worked on a large EHV-1 outbreak at the University of Findlay in Ohio. Among the horses that became infected horses that had been vaccinated every 60 days were the ones that developed neurological symptoms.
“There is some evidence that too frequent vaccination might have some negative impact,” Reed said. “You'd be better off to just follow the manufacturer's recommendations and make sure the horse has a complete series, and then do everything else to maintain normal good health.”
Good husbandry is the only way to keep your horse from contracting EHV-1.
Exposure and Transmission
Direct contact with a horse shedding the disease or contact with the virus sneezed or coughed into the air by an infected horse (aerosol) are the primary means of transmission. Other means include contaminated buckets, tack, caretakers' clothing, soiled bedding, and other items. The disease also can be spread if a horse comes into contact with a contaminated aborted fetus, its placenta, or the fetal fluid.
From the time of exposure, the incubation period is three to 14 days before a horse begins to show symptoms, typically including a fever and nasal discharge. If EHV-1 develops into EHM, neurological symptoms usually become evident eight to 12 days after the initial fever.
EHV-1 can lie dormant, making a horse without symptoms a latent carrier until the animal is subjected to stress that depresses its immune system. One such form of stress is extended transport.
Protection and Precaution
Reed said it's best to keep your horse in top health so if it is exposed, its immune system will be able to mount a defense. Proper vaccination, nutrition, and hygiene play crucial roles. Herd health also is important. If your horse is housed with other horses, they need to be healthy enough to resist infection, too.
“Having the horse vaccinated for all the major important pathogens is going to be critical,” he said. “If you're going to be moving the horse from place to place, then keep it as healthy as you can prior to moving because one thing we know is that if you put a horse on a van ride for as short as four to five hours, that causes some suppression of the immune system.”
Reed said administering an immunostimulant to fortify a horse's immune system is a good idea, especially before any anticipated physical or mental stress. Some veterinarians are using the natural immune booster interferon to attempt to block the pathogen locally at the nostril before it gets into the horse's respiratory system.
Most importantly, Reed urged horsemen to take their horses' temperatures twice daily and to act quickly if a horse develops a fever.
“One thing about this particular virus is that it causes biphasic fever spikes,” he explained. “So they'll have a fever and it might be up for a day or two and then it will go back down. Then four to five days later, they'll have another fever spike, and that's usually when the neurologic or other signs start to show up after that. The first fever spike might get missed. But if you're temping your horse once or twice a day, you're less likely to miss that.”
Any horse that spikes a fever or shows clinical signs of infectious disease should be quarantined immediately at least 35 feet from other horses for 21 days beyond resolution of those clinical signs and negative test results. New horses should be quarantined for a minimum of two weeks, even if they appear healthy.
Do not allow horses to share water buckets, and don't place the water hose in the bucket when filling it. Buckets should be scrubbed individually with a brush soaked in disinfectant, then rinsed and returned to the respective horse's stall.
Bridles with bits, lip chains, twitches, tongue ties, nose rags, and any tack that is exposed to the horse's saliva or nasal discharge should be disinfected with chlorhexidine (Nolvasan) solution and then rinsed thoroughly with water before use on another horse.
Horse trailers should be disinfected between uses. When disinfecting trailers and premises, use a scrub brush and phenolic disinfectant. Do not use a power washer because it can cause disease particulates to aerosolize.
Veterinarians should heighten their disinfection protocol for endoscopes and other instruments that could be a source for spread of disease. Farriers should disinfect their hands, tools, and shoes if they have been on a premises where there is illness.
Sick horses should be handled last, and only designated personnel should handle them. Disinfectant foot baths should be used when entering and exiting a quarantined horse's stall. Soon after working with sick animals, those personnel should remove and wash their clothes, disinfect their shoes, and shower to prevent spread of the disease. The farm veterinarian may recommend use of protective clothing such as latex gloves, disposable coveralls, boot covers, and surgical masks and hats.
“I don't know why we're seeing more and more outbreaks now,” Reed said. “Part of it is that everybody is so much more vigilant than they used to be. I don't think the virus is undergoing any new mutations, and in fact, most of the new outbreaks — even the neurologic ones –have been the wild-type virus, the one that's been seemingly around for the longest time.”
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