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EHV cases reportedly on the rise
Outbreaks temporarily close tracks, trigger quarantines in four states


"If animals are to be moved, then I strongly recommend that before they are introduced into anther population, that they are kept isolated for 21 days, and this would apply to fillies that are coming off the track," Powell says. "They should be kept isolated for 21 days, which could mean in theory that they miss a breeding cycle, but I think that is something that people would be prepared to accept as distinct from the possibility of introducing what could be a very serious problem, especially if a stallion became infected and developed the neurological signs, then the consequences of that are obviously very serious."

Michigan and Pennsylvania also confirmed the presence of EHV-1 with the onset of neurological symptoms.

Stress and strain

A new strain of EHV-1 or a mutated neurological form of the virus could be the culprit of the recent outbreaks. Many suspect heightened stress and strain in preparation for winter racing might lower an animal's immunity enough to become ill.

"My estimation is that horses are under a lot of stress, especially at racetracks, where there is a lot of movement, and it is the perfect incubator," says Dr. Klaus Osterrieder, associate professor of virology at the Cornell University College of Veterinary Medicine.

An estimated 80 percent of horses more than 2 years old are carriers of EHV-1, which is contracted for life, Osterrieder says. The disease has always been with horse populations, and always will, making future outbreaks inevitable, and the onset of the neurological form of the disease carries a 30 percent to 50 percent mortality rate, he says.

"Unfortunately, the bottom line in well vaccinated horses is that we still cannot protect them from neurologic disease, but I think it is still reasonable to vaccinate with licensed, approved products for rhinopneumonitis," says Elizabeth Davis, DVM, PhD, Dipl. ACVIM, assistant professor of equine medicine and immunologist with Kansas State University College of Veterinary Medicine. "Right now we suspect that it is a single-point mutation that has occurred, and that's why we are seeing the difference in the disease because the disease is clinically worse than what we were seeing 10-20 years ago."


Aside from isolation, treatment options for infected animals are pretty dire, which propels the need for prevention. Unfortunately, even a meticulous vaccination protocol doesn't always ensure complete protection, Davis says.

"This is a virus that we do have latency with," she says. "So even in an individual that might have had disease and recovered from it, in conditions of severe stress, they might have recrudescence of that virus, so we could have shedding at a later time."

The outbreak also spurs another long-standing discussion about modified-live and inactive or killed vaccines. Some veterinarians support use of modified-live vaccines; others say killed products offer solid protection, while others support the combination approach.

Osterrieder released a study this year that he says shows a strong benefit for modified-live vaccines versus killed-virus vaccines. His 15-horse study exposed the group to EHV-1 after vaccinating one-third with a killed vaccine, one-third with modified-live vaccine and one third without a vaccine.

"It is widely accepted and known that you need a cellular toxic-immune response to fight off the infection, and this type of immune response is best induced by modified-live vaccines. All you can do is prevent the disease and vaccinate, and from all we know, vaccinations with modified live virus should prevent against both biotypes if they truly exist."

Others aren't so sure. The Gluck Equine Center is recommending vigilant vaccination protocols, but the killed vaccine might be just as effective if used as recommended.

"There is an emphasis on the synergy between vaccination and good common-sense management procedures of not mixing animals, restricting movement as much as possible, although we recognize that horses, more so than other species, there is a tremendous amount of movement, and that is why we have to rely on a good vaccination process," Powell says. "But I don't really feel like we have enough experience to differentiate between the two (killed versus live)."


Source: DVM360 MAGAZINE,
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