Virology and immunology of a spontaneous and experimental mucosal disease-like syndrome in sheep recovered from clinical border disease.

A disease similar to mucosal disease (MD) of cattle has occurred spontaneously in sheep clinically recovered from Border disease (BD). Evidence is presented to suggest that, in such animals, a specific and dynamic equilibrium exists between an attenuated form of the virus and the immunotolerant host. Upset of this equilibrium either by injection of BD virus of the same strain or by some unknown spontaneous event appears to lead to the reassertion of viral pathogenicity with fatal consequences.