Epidemiology of bovine viral diarrhea virus.

Prevalence studies around the world show that BVDV is widespread in most cattle raising countries. There are significant differences, however, in prevalence between areas, probably the result of differences in cattle population structure and management practice. Direct contact with PI animals is probably the most important method of transmission of infection; however, field studies have shown that some limited spread of infection also occurs in the absence of PI animals. This may be due to contact with acutely infected animals or contact with other species infected with BVDV. Different ways of indirect transmission such as contaminated needles and equipment have been proven experimentally, and indirect transmission is considered to have some importance. If a PI animal is introduced directly into a dairy herd, most animals will be infected within a few months. On many occasions, however, a herd gets infected by other means than direct introduction of PI animals. In these cases, the infection is often spread to only a few animals after which the infections stops. The infection is then reinforced when PI animals are born. Slow and hence prolonged spread of infection in herds without PI animals also has been described, but the mechanism is not fully understood. Family lines of PI animals delivering PI calves are fairly common and can cause the infection to continue for several years. The clinical manifestations, acute BVDV, reproductive disorders, birth of malformed, weak and undersized calves, unthrifty PI animals, and mucosal disease often appear within certain periods. Large variation, however, can occur between herd outbreaks due to variation in virulence of the BVDV strain, housing of the cattle, and variation in transmission patterns. The extensive transmission of infection from PI animals makes different surveillance methods possible. Thus testing of a screening sample of a few young stock of antibodies and determination of antibody titer in bulk milk will often give good indication of presence of PI animals in herds not using BVDV vaccines. In herds using killed vaccine, determination of antibody titers among few young stock can show the presence of PI animals. The high incidence of infection combined with all the different damages that are seen after BVDV infection cause huge economical losses, which on a national level in the UK and Denmark (i.e., areas with widespread occurrence of infection) has been calculated as between 7 and 27 million pounds (between $11 and $42 million) per million calvings. Epidemiologic studies are important as a basis for selection of control strategy. Because of the variation in epidemiology between geographic areas, evaluation of a control strategy in an area preferentially should be based on epidemiologic studies in the same area.