Once a horse is infected with equine herpesvirus-1 (EHV-1), the dormant virus resides in the horse's nervous system, where it can reactivate when the horse's immune system is compromised by stress or illness.

Nicola Pusterla, DVM, PhD, Dipl. ACVIM, and his colleagues at the University of California, Davis, wondered if stress caused by transporting horses would reactivate a latent infection. They tested 302 healthy horses imported from Europe and quarantined at the UC Davis facility for EHV-1.

Typically a horse's trip from a European stable to their new home in the United States requires about 12-14 days and includes a plane ride and quarantine.

"That's a lot of stress for a horse," Pusterla said, noting the research team expected to see positive test results.

They found EHV-1 in just four horses; three with a reactivated infection, and one with a new infection.

"We did not detect EHV-1 in many horses, so the study might give the false impression that EHV-1 is not highly prevalent," Pusterla relayed. "We only tested horses at one time, which is one limitation of our study." He noted that horses only shed virus for a few days, so they might have missed the reactivated virus in some horses.

Horses with a reactivated EHV-1 infection don't always show clinical signs that owners would recognize. Pusterla advised horse owners to prevent nose-to-nose contact between new animals, and don't let horses share water troughs or equipment with newcomers.

"Owners should try to decrease the stress factor on the horse, support the horse with regular vaccination and good nutrition so that he has a solid immune system, and closely monitor the horse," Pusterla said. "Make sure he is eating, acting normally, has no fever, nasal discharge, or coughing. If any of these things do occur, then seek veterinary advice."

The study, "Prevalence of EHV-1 in adult horses transported over long distances," was published Oct. 17, 2009. The full text is available online for subscribers to The Veterinary Record.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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