With the possible exception of the uterine changes alluded to in the preceding paragraph, PPV-induced reproductive failure is caused by the direct effect of the virus on the conceptus. In the absence of an immune response, the virus replicates extensively throughout these tissues. By the time the conceptus dies, most of its cells contain large quantities of intracytoplasmic viral antigen that can be demonstrated by IF microscopy. The relative lack of nuclear fluorescence at the time of death, compared to earlier stages of the disease, indicates that when the conceptus is severely affected, mitotic activity and the associated conditions necessary for viral replication are suppressed more than phagocytic activity.