Equine Herpesvirus Type-1 Outbreak Resolving; Strain Might be Atypical

The worst is over in the unusual equine herpesvirus type-1 (EHV-1) outbreak that led to the death of 10 horses and has affected the remaining equine population at the University of Findlay’s English riding facility in Findlay, Ohio, since Jan. 12. Veterinarians have not detected any new cases of the respiratory and neurological illness in the last five days at the facility. Ten horses with neurologic signs are stable and under treatment at Findlay, while two horses remained in critical condition at The Ohio State University’s veterinary teaching hospital Jan. 28. Epidemiological studies are being completed to figure out where the virus came from, and scientists are comparing the strain to others found in past outbreaks.

The EHV-1 organism can spread quickly from horse to horse through aerosol droplets in the air or contact with equipment used by affected horses (only in the very early stages and not for a long time as the virus does not live in the environment for a long time) and can cause three different forms of disease, including rhinopneumonitis (a respiratory disease of mostly young horses), abortions in pregnant mares, and neurologic disease. The stricken horses in Findlay showed clinical signs of the respiratory and neurological forms. There are at least seven other strains of equine herpesviruses, named in order of their discovery. (See article #32 at www.TheHorse.com for more on herpesviruses.) The neurologic form of EHV-1 often is survivable with supportive care, but unfortunately once a horse is recumbent, it is difficult to nurse the horse back to health.

According to Steve Reed, DVM, Dipl. ACVIM, of The Ohio State University, the high degree of mortality made this strain of EHV-1 unusual. The morbidity (causing sickness) of those horses with the neurologic form of EHV-1 was a little higher than in other outbreaks, he added.

C. Michael Kerns, DVM, Director of University Equine Veterinary Services at the University of Findlay and Associate Professor of Animal Science, said that five to six days after the initial case was detected, 80-90% of the horses had fevers. “The younger horses seemed to develop snotty noses, and the older horses became neurologic,” he explained. He said that 35-40 of the horses developed obvious neurologic signs, whereas a few others had more subtle neurologic signs. “Those 35-40 with more pronounced neurologic signs were treated with IV fluids,” he said. “The others that were a little ataxic came out of it.” Officials announced a presumptive positive for EHV-1 on Jan. 22 after gross histologic lesions characteristic to EHV-1were found in a necropsy, and a polymerase chain reaction test was completed at the Ohio Department of Agriculture. Results were confirmed Friday, Jan. 24.

“We probably have 10 that we’re still working with (at the University of Findlay) that have the neurologic signs, but these seem really stable and are not getting any worse,” said Kerns.

Two of the horses remain in critical condition at OSU. “We have two of them down that can’t get up without assistance,” said Reed. “We keep their heads wrapped so they don’t destroy their eyes.”

Kerns felt that the virus was spread via the barn ventilation. “It went through the barn so fast it just had to be in the air, because you’re talking about 138 horses, and the barn is huge and kind of sectioned off. It’s not a big straight line of stalls, but with north, south, and east wings,” said Kerns. “The way it spread all at once, it almost had to be aerosol (transmission.)”

Reed said, “One of the key things that you see (at the University of Findlay and in other outbreaks) is the horses are extensively well-vaccinated—every 60 days for flu and rhino, and some of these have lived here for 10-12 years. It tells you that the vaccine, while it might lessen the respiratory form of the disease, it does not prevent the neurologic form, and does not prevent fevers.” He added that he’s spoken with George P. Allen, PhD, at the University of Kentucky’s Maxwell E. Gluck Equine Research Center, about the possibility of the virus being an atypical strain.

“We will be investigating further the character of this particular strain to see if it’s different from normal, non-neurological strains,” said Allen, whose research interest is equine herpesviruses.

Treatment of the Horses

Veterinarians chose to use a human anti-herpes drug called acyclovir in the Findlay outbreak, because it had been used with some success in an outbreak in Virginia a few years ago. Every university-owned horse was put on the experimental acyclovir, and all other owners were contacted by university officials and given the option of putting their horses on the drug. According to Kerns, administration of acyclovir began on Jan. 20.

Horses with neurologic signs received 5mg/kg IV, and the other horses received it orally.

“Theoretically, if you gave it early, you may be able to block the virus,” said Reed. “We know it works on other herpes at least in man, but we didn’t know about this,” Reed added.

Epidemiology of the Outbreak

While some are still nursing the horses back to health, others are figuring out how the virus ended up at the school in the first place.

Ohio State University officials Reed, Cheryl Sofaly, DVM, William P. Shulaw, DVM, MS, Dipl. ACVPM (a specialist in disinfection), and John A. E. Hubbell DVM, Dipl. ACVA, (Associate Dean of Academic Affairs at OSU’s veterinary school and horseman), traveled to the University of Findlay on Wednesday, Jan. 22 and divided into three diagnostic teams. The teams examined and graded horses with neurologic signs, collected blood, and took throat swabs.

Kern said that veterinarians were paying special attention to horses which had been transported in recently or had difficult and stressful travel in the weeks prior to the outbreak. Officials are considering all possibilities of virus arrival, including horses shipped from within the country and any which might have been recently imported.

Additionally, a 22-item questionnaire will be completed on every horse kept at the facility. Reed said that the virus could have been introduced by a single sentinel horse that arrived recently at the facility, or with a group of young horses which arrived for a breaking program. No one will bargain to say where it came from, but after collection of the questionnaires, “we’ll go back and look at those factors,” he said.

The good news is that the virus hasn’t been seen elsewhere in the vicinity of Findlay. Kerns said, “We haven’t seen any of it in the community and none at the Western (riding facility),” said Kerns. “Both farms are still quarantined and we’re still discussing when to release that (self-imposed) quarantine. We’re going to try and get more students back on the farm, probably next week. Things are looking better and improving all the way around, we’re really thankful for that,” he added.

The only horse which has been transported from the English facility since the outbreak is a Western horse which had been brought over to the English facility prior to the outbreak because it was showing colic symptoms. The Veterinary Services’ medicine facility and surgery suite are based at the English facility. Once the colic resolved without surgery, the horse was taken to a farm with no other horses on the premises.

All movements to and from the farm were closely watched and monitored “We don’t feel like any horses went back into the community or to shows,” he said. He and officials at the Ohio Department of Agriculture are not aware of any other farms in the area or in the state that are dealing with this virus.

A cooperative team has been working on the outbreak, including University of Findlay students, faculty, and staff, the Ohio Department of Agriculture and its diagnostic lab, Ohio State’s diagnostic lab, and group of virologists, neurologists, clinicians, neuropathologists, and infectious disease epidemiologists. According to Kerns, staff, faculty, and junior and senior English equestrian students spent many days treating the horses at the facility around the clock,said Kerns, and eventually he had to make the students go home because they were so exhausted. Officials then recruited students to volunteer who weren’t going to be in contact with any other horses.

As for the outlook of recovering horses at the University of Findlay, veterinarians aren’t certain. Reed remembers a horse which treated once with the neurologic form of EHV-1 that was down for more than a week, and in a sling for six more weeks. The horse was back to normal a year later. “They can recover fully,” said Reed, “but there is no guarantee.”

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More