EHV cases reportedly on the rise

NATIONAL REPORT — An increase in the number and severity of equine herpesvirus Type-1-induced neurological disease outbreaks has forced the closure of racetracks, the quarantine of several barns in least four states this year and the humane destruction of several horses.

The most recent scare at presstime involved a horse at Bowie Training Center in Maryland; 29 horses were quarantined after one, previously treated for protozoal myeloencephalitis, continued to display neurologic signs consistent with the herpesvirus outbreaks. A collective sigh of relief from around the state was echoed when the tests came back negative Feb. 5, and no new cases had been reported in the state for two weeks at presstime.

The question remains: Has increased awareness and improved diagnostics facilitated the recognition of the infectious disease, or do real increases exist?

"Certainly we do feel that within the last few years, there has been an increase in the number of outbreaks of the neurological form of the disease in North America (United States and Canada)," says David Powell, BVSc, FRCVS, equine epidemiologist with the University of Kentucky Gluck Equine Research Center. "Previously, the number of neurological outbreaks that were reported were quite rare — one a year or even less than that. Within the last three years, we have seen an increase in the number of outbreaks in the neurological form of the disease in various parts of the country and in various breeds of horses. It doesn't seem to be restricted to any particular group."

Maryland has been plagued with numerous EHV events beginning with Pimlico Race Course, where three horses were euthanized in January; a fourth was destroyed at Laurel Park, and another horse was put down at a Kent County farm.

A total of 12 horses were either euthanized or isolated in three barns at the home of the Preakness. The Maryland Jockey Club proactively placed the Baltimore oval on quarantine on Jan. 21, restricting the movement of horses. Barn 9 at Laurel Park was under a hold order from the Maryland Department of Agriculture, but no new cases had been reported there for two weeks.

"Conditions at Pimlico and Laurel continue to progress in a positive manner," Maryland State Veterinarian Dr. Guy Hohenhaus said Feb 1. The hold order at Pimlico had just been lifted at presstime.

DVM Newsbreak

That didn't dissuade nearby Charles Town Races & Slots in West Virginia's panhandle from quarantining its facility to stave off the spread of equine herpesvirus Type-1 (EHV-1) in early February. Horses are not permitted to enter the facility, which houses more than 1,500 animals, and no animals will be permitted to return should they leave for another race. The duration of the preventive protocol was undetermined at presstime, but typical quarantines last about 21 days, including a thus-far successful movement ban at Turfway Park in Florence, Ky., where two horses were euthanized in mid January; the Kentucky outbreak, which also affected Western Kentucky Training Center 200 miles away, began Jan. 2. No new cases had been reported more than three weeks later, according to Kentucky State Veterinarian Dr. Robert Stout, and plans to lift the movement ban from the general population was anticipated in early to mid February.

"The catch word is biosecurity," Stout says. "I don't think you can ever do enough in terms of biosecurity."

Experts recommend the quarantine of at least 21 days with no new cases before reintroducing horses that might have been exposed to infected horses. The isolated horses should have their temperature taken at least once a day during that period to keep tabs on the possible spread within the isolation barn.

"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."

Prevention

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)."

With a heightened awareness, interest and inquiries about the disease from clients, veterinarians might be forced to admit that there is not much they can do aside from vaccinate to decrease the likelihood of spread and hope for the best. But there should be some solace in the fact that the recent outbreaks — besides the 2003 outbreak at a Findlay, Ohio riding school — have been predominantly at racetracks, says Kenneth Marcella, DVM, owner of Chattahoochee Equine Clinic in Georgia.

"Since the experts can't be sure that any one vaccine is better than the other, I'm going to recommend that we strive for the best protection we can get by doing a little bit of both. Maybe spring we'll do the modified live, and in the fall, we'll administer the killed vaccine," Marcella says. "There is not enough evidence that shows this is going to be a huge population problem without the stress factor, so I'd rather manage the stress.

"The other thing is that if you vaccinate too much with the killed vaccine, the more susceptible they are to contracting the disease."

Osterrieder agrees, saying some of the killed vaccines, especially the combination products that contain six antigens, drive the immune response in a direction where an animal will be unable to deal with the herpesvirus infection.

"They are driving more toward antibody generation, which is not useful against herpes," he says.

A practitioner who suspects a case of EHV-1 should contact the respective state board of animal health. Some tests require about a week to glean results, but some states and facilities, including Gluck, use a rapid-response polymerase chain reaction (PCR) test, which generates results in about two to three days.