The bovine syncytial virus, a member of the retroviral subfamily Spumavirinae, causes a persistent, asymptomatic infection in cattle. Named for its characteristic cytopathic effect, BSV causes the formation in infected tissue of syncytial cells which are giant multinuclear cells formed by the fusion of several cells. BSV has a predilection for the lower respiratory tract, where it can cause varying degrees of pneumonia by itself. Secondary bacterial infections, frequently involving Mannheimia (Pasteurella) haemolytica, causes disease of greater severity or death. Human transmission is via respiratory aerosols or from direct contact with infected cattle.