Equine Herpesvirus: Anatomy of an Outbreak

Researchers connected to an outbreak of equine herpesvirus-1 (EHV-1) in Ohio at the University of Findlay's English riding facility several years ago have recently published a report correlating age and specific clinical signs to horses' incidence of neurologic disease and chance of survival. These findings could serve as guidelines for predicting which horses are at risk during future outbreaks and enable veterinarians to take prophylactic action.

The outbreak at Findlay occurred in 2003 and resulted in the death of 12 neurologic horses and neurologic signs in 30 others.

The report includes long-term prognosis for survivors of the neurologic form of the virus. The outcomes of two different treatment protocols used during the outbreak were also examined.

Scientists found that age and specific clinical signs were predictors of neurological disease and death. Horses older than five years, those with temperatures higher than 103ºF, and those experiencing a peak in temperature more than two days after the initial fever were more likely to suffer from neurologic signs or death.

Of the 135 horses at the facility during the outbreak, 117 displayed signs of EHV-1 infection, including respiratory signs and fever, and accompanied by neurologic signs in some horses. Some neurologic horses had lingering signs at a follow-up examination six months later, but all returned to normal within a year.

The Findlay outbreak began Jan. 11, 2003. At the time, 100 of the horses at the school were resident horses and 35 horses had been added to the facility in the seven days before the first clinical signs appeared. Resident horses were vaccinated against EHV every three months, and Findlay required arriving horses to be vaccinated against EHV and influenza 10 to 45 days prior to arrival.

The only pregnant mare on-site was the first to show neurologic signs on Jan. 16. An additional 41 developed signs within a week.

Veterinarians evaluated all horses' neurologic function each day during the outbreak. Horses were scored 0 to 5, with 0 indicating the absence of neurologic deficits and 5 designating recumbency (a down horse, unable to rise). Of the 42 neurologic cases, there were 12 Grade 5 horses, two Grade 4, 12 Grade 3, seven Grade 2, and nine Grade 1. None of the Grade 5 horses survived.

During the first week of the outbreak, veterinarians treated affected horses with fluxinin meglumine or phenylbutazone (Bute) as indicated for fevers. Neurologic horses also received DMSO (dimethyl sulfoxide, an anti-inflammatory). The treatment protocol was changed to include acyclovir (a human antiviral drug) when it became available on Jan. 20, 2003. All horses involved in a second wave of fevers survived. The prophylactic use of acyclovir was also correlated with a reduced incidence of neurologic signs.

"It appeared that the use of acyclovir, during our outbreak, resulted in a reduction in the severity of neurological disease and, thus, had an impact on the survival of the horses," said Findlay veterinarian Rick Henninger, DVM, MS, Dipl. ACVS.

The quarantine was lifted March 10, 2003, three weeks following the final febrile horse's return to a normal temperature.

Six months after the outbreak, veterinarians reevaluated 26 of the 32 surviving neurologic horses. Of the 12 horses that had Grade 3 deficits during the outbreak, half had returned to full work and the others had lingering signs scoring 2 or below. Five of the six Grade 2 horses had returned to full function, with the other horse scoring a 1. All of the horses that were originally Grade 1 were classified normal at the follow-up examination.

In a second exam, performed 12 months after the outbreak, all available surviving horses had returned to full function.

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care.

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

Free Newsletters

Sign up for the latest in:

From our partners