The pathogenesis of mucosal disease.

The pathogenesis of MD is complex and remains somewhat obscure. Clearly, the disease occurs in cattle persistently infected with noncytopathic BVDV. It also is clear that cytopathic BVDV is associated with MD, and is the likely trigger of the cellular destruction that leads to clinical disease. Whether the cellular destruction is attributable directly to the cytopathic virus, or occurs as the result of other mechanisms remains unclear. Although immunotolerance is involved in MD, it can be broken and its role in the disease process needs further research. It is logical, and well supported by research, that noncytopathic BVDV is the source of cytopathic BVDV. It also is likely that most outbreaks of MD are the result of a spontaneous mutation of noncytopathic to cytopathic virus within a PI animal. Antigenic homology between viruses would be expected in those outbreaks. MD also occurs when PI cattle are exposed with a cytopathic BVDV that is antigenically heterologous with the resident noncytopathic BVDV. In those situations, it may be a race between the cytopathic virus and the immune system.