is spread either genitally or respiratorily. Genital strains have been found to be non-syncytial. Respiratory strains may be of relatively low or of high virulence. In Europe, syncytial strains have been found to be highly virulent. Epidemiology Populations of wild boar, or feral hogs (Sus scrofa), in the US commonly contract and spread the virus throughout their range. Mortality is highest in young piglets. Pregnant sows often abort when infected. Otherwise healthy male adults (boars) are typically latent carriers, that is, they harbor and transmit the virus without displaying signs or experiencing disability. Swine (both domestic and feral) are usual reservoirs for this virus, though it does affect other species. Aujeszky's disease has been reported in other mammals, including brown bears, and black bears, Florida panthers, raccoons, coyotes, and whitetail deer. In most cases, contact with pigs or pig products was either known or suspected. Outbreaks in farm fur species in Europe (mink and foxes) have been associated with feeding contaminated pig products. Many other species can be experimentally infected. Humans are not potential hosts. Cattle have been found to be infected either by the respiratory or the vaginal route (iatrogenic cases disregarded). Primary infection of mucous membranes of the upper respiratory tract is associated with head pruritus, while lung infection results in chest pruritus. Vaginal infection of bovines, which regularly show pruritus of the hindquarters, has been found to be associated with a concurrent genital infection in swine on the same premises, and investigations have evidenced that the vaginal infection of cattle had been sexually transmitted by man from infected sows. Genital infection in swine herds has been closely correlated with the use of boars from boar centres for natural service of sows. Transmission Aujeszky's disease is highly contagious. The infection is commonly considered to be transmitted among swine through nose-to-nose contact, because the virus is mostly present in nasal and oral areas. This notion, however, is contradicted by results from epidemiological studies, according to which the decisive spread within herds occurs by air currents over many meters. Correspondingly, the risk of airborne transmission of highly virulent virus strains from acutely infected herds to other swine herds has been found to be very high. The infection has been found transmitted over distances of many kilometers. Otherwise, the infection is most often transmitted into herds by introduction of acutely or latently infected pigs. Concerning transmission to cattle, see section above.