Equine viral arteritis: further characterization of the carrier state in stallions.

Further characterization of the carrier state in stallions infected with equine arteritis virus revealed that there is considerable variation in the frequency of its occurrence among breeds. The frequency ranged from 12.5% (Holsteiner stallions) to 72.7% (Dutch Warmblood stallions), with a mean occurrence of 40.8% in the seropositive stallions (n=561) examined. More than 70% of the virus shedders were Standardbred stallions. The carrier state was not confirmed in any of the stallions that had been vaccinated against equine viral arteritis nor was there any evidence of intermittent virus shedding by carrier stallions. Most (98.2%) of the semen isolates of equine arteritis virus were obtained on first passage in RK-13 cell culture and most of the samples had very high virus infectivity titres. Intermediate term (3.5-7.0 months) and long-term (> or =1 year) carrier states were confirmed in various horse breeds. Long-term persistence of equine arteritis virus in individual stallions was common, and some animals continued to shed the virus in semen for 4-12 years. Spontaneous clearance of the carrier state was observed in 27 stallions after periods ranging from several months to many years. There was a considerable difference in the rate of clearance of the carrier state between Standardbred (4.3%) and Thoroughbred (42.3%) stallions. Reduction and eventual elimination of the carrier stallion reservoir of equine arteritis virus is the key to the success of any control programme for this disease.