Classical signs are a fever for 1 - 5 days; sometimes upper respiratory signs (inflammation and mild discharge from nostrils); weakness; depression; off feed; swelling (oedema) of lower legs, scrotum and sheath; abortion in the pregnant mare from 2 months pregnancy onwards; and respiratory disease and enteritis in young foals. Severity is greater in old, young, and debilitated horses. The virus is transmitted from horse to horse in 2 ways; via the respiratory route (nasal discharges and aerosols) from the newly infected horse (this lasts up to 19 days, then stops. The horse develops a life-long immunity); or via the semen in an infected stallion (this ability to spread the virus can last a lifetime in the stallion).
A vaccine exists that gives life-long immunity to the vaccinated horse. In general, this is used in stallions after ensuring they are not currently infected or shedding the virus in their semen. Prevention of spread on studs is achieved by isolating new arrivals from other horses for 3 weeks, and using good hygiene (the virus is very easily killed with disinfectants). Horses can be blood tested to see if they have had contact with this virus, and if a stallion tests positive, its semen can be tested to see if he is a shedder of the virus.
Castration of a shedder stallion stops him being a shedder. Isolation of newly infected horses to protect other non-infected horses protects this latter group. There is no other treatment for infected horses, apart from symptomatic treatment