Equine Herpesvirus-1 Transmission Risk After Quarantine

Q.Veterinarians recently diagnosed a case of equine herpesvirus-1 (EHV-1) at a local barn. The farm was quarantined for a period of time after the diagnosis, but it’s now reopened and many people in our area go there to school their horses. One of my horse’s pasturemates is headed there this weekend where she might make contact with a horse that traveled to the quarantined farm during the incubation period. The pasturemate was vaccinated earlier this year, but I’m not sure if she got a six-month rhino booster. Furthermore, her owner won’t be riding her at the show, and I’m not sure how well her new rider will follow biosecurity protocols.

What I can do to keep my horse safe? My horse is up-to-date on her vaccinations but is due for a flu/rhino booster . Since she is a senior, should I give her the booster early?

—Caroline, via e-mail

A.Your question did not specify whether the horse with EHV-1 was diagnosed with the more common upper respiratory type infection or the comparatively rare neurologic strain. Typically, quarantine protocols are quite strictly enforced by state veterinarians for the neurological strain, which is referred to as equine herpes myeloencephalopathy (EHM), so it would be unlikely that a horse who was potentially exposed would not also be quarantined itself.

State veterinarians don’t always enforce a quarantine for non-neuropathic/respiratory strains of EHV-1 or -4, but responsible owners and boarding facilities certainly should do so of their own volition.

Horses should be quarantined for 10 days after their last fever.

Equine herpesvirus usually has a two to 10 day incubation period. Then, infected horses generally develop a fever, cough, and nasal discharge; it might also cause pregnant mares to abort. Young horses are more susceptible to rhinopneumonitis infections than older horses; seniors have almost certainly been previously exposed at some point during their lives and so have some degree of natural immunity.

It would be prudent to go ahead and have your horse’s EHV-1 and -4 vaccines boostered a little early, but keep in mind that it takes about two weeks for vaccines to result in a full immune response, so protection is not immediate. Also remember that no vaccine confers 100% protection. However, your horse should still be fairly well-protected by the vaccines from four months ago. Do keep in mind that there’s no vaccine licensed to prevent EHM, the neurologic strain of EHV-1.

If it were possible to keep your horse separate (no nose-to-nose or direct contact) from the pasturemate for 10 days after it returns from the show, that might be wise. However, overall, I’d say it sounds like the risk of infection is fairly low for your horse. Again, I would be much more concerned about the risk if we are talking about EHM than the respiratory form of EHV-1.