ehv-1 shedding

Equine herpesvirus-1 (EHV-1) is a highly infectious disease that can cause respiratory and neurologic signs in adult horses, as well as abortion in broodmares. Even without clinical signs of disease or evidence of the virus in the blood (viremia), horses can shed the virus and, thus, spread it via nasal discharge and aborted placental tissues and fluids. Perhaps introducing yet another biosecurity challenge, veterinarians recently determined mares can shed the virus via the vagina, too.

One of the challenging parts of preventing EHV-1 spread is this latent spread of infection—apparently healthy horses shedding the virus without ever showing clinical signs or developing viremia. That’s why Carina Cooper, DVM, a pathobiology PhD candidate at the University of Guelph’s Ontario Veterinary College, in Canada, set out to confirm the suspicion that healthy mares could be shedding EHV-1 vaginally—putting their foals and barn mates at risk of infection. She presented her study findings at the 2018 American Association of Equine Practitioners Convention, held Dec. 1-5 in San Francisco, California.

In her study Cooper aimed to not only determine the prevalence of EHV-1 in healthy broodmares but also examine the effects of vaccinating them against the virus while pregnant. She evaluated 385 healthy broodmares of various breeds from 45 farms across Ontario, collecting blood samples and nasal and vaginal swabs every two months throughout the course of the 2016-2017 breeding season.

Cooper isolated EHV-1 DNA from 85% of the mares at least one time during the study and found that 69% of mares were shedding from the nose (33%) and/or vagina (55%).

“To be clear, the test I was using was very sensitive,” she said, “and so was detecting really low levels of viral DNA—below what is normally found with the commercially available tests.”

Interestingly, Cooper said, mares harbored less virus in their blood later in pregnancy. “This is counterintuitive, since we know EHV-1 causes late-term abortion,” she said. But Cooper emphasized that this study was only for one year, so “we can’t make any seasonal, cyclical, or viral-behavior conclusions just yet.”

While a mare’s odds of testing positive for viremia were lower within two months after being vaccinated against EHV-1, vaccination is probably not very useful in preventing mares from becoming latent carriers, said Cooper.

“Mares had likely long before become infected with the virus,” she said, theorizing that vaccination was now reducing viral activity in the blood without reducing shedding in these healthy broodmares. “It did not appear to ‘prevent’ infection, either,” she said.

Cooper’s main take-home? Healthy broodmares shed EHV-1 vaginally. “The virus can be found in the vagina and might be a bigger source of environmental contamination than previously thought,” she said. “The vagina should be investigated (in future studies) as a potential source of infection or viral spread during disease, as well as the nose.

“We need to change the belief that a horse can be ‘cured’ or can clear a herpesvirus infection,” she continued. “They can’t—it stays with them for life. So, our isolation and hygiene protocols are really important to prevent disease spread.”