Stallions, on the other hand, can harbor the disease in their reproductive system as long as 15 years and shed the virus continuously or only periodically. When stallions are 'shedders,' they can infected mares during breeding.
Infected mares can also infect their foals in utero, or prior to delivery.
Foals are vaccinated between the ages of six and 12 months, while blood-test negative breeding stallions can be inoculated yearly. Mares that will be bred to infected stallions should be vaccinated at least three months prior to breeding, with a booster administered three or four months after the initial vaccination. Abortions, foal deaths and lost breeding seasons resulting from this disease can substantially impact the equine industry. Most worrisome is the challenge to guard against the spread of disease by infected stallions. States urged to consider testing positive imports to attempt virus isolation.