Pathogenesis and epidemiology of bovine virus diarrhoea virus infection of cattle.

An outline of the clinical diseases that arise following BVDV infection is given. Isolates of BVDV can be separated into two forms non-cytopathic and cytopathic depending on their effect on cell cultures. In utero infection of the foetus with non-cytopathic virus may result in a number of syndromes, such as abortions, stillbirths or weak calves. It may also result in the birth of calves with a persistent viraemia and these animals may later develop mucosal disease as a result of superinfection with a "homologous" cytopathic strain of BVDV. Acute infection of seronegative animals in usually mild and subclinical. A chronic disease also occurs and this can be protracted with progressive wastage and diarrhoea. This condition has not yet been well defined but it is suggested that it may be the result of superinfection of a viraemic animal with a "heterologous" cytopathic strain of virus. The maintenance of non-cytopathic virus within the cattle population can be either by the slow spread following acute infections of seronegative animals or, more importantly, by spread from persistently viraemic cattle. The cytopathic virus is usually found in association with cases of mucosal disease and may be maintained in the population only by continually arising, possibly by mutation, from the non-cytopathic virus. It is recommended that persistently viraemic animals are eliminated from the herd to avoid in utero infections and the possibility of subsequent mucosal disease.