Acute infection of postnatal pigs, including pregnant dams that subsequently develop reproductive failure, is usually subclinical. However, in young pigs and probably in older breeding stock as well, the virus replicates extensively and is found in many tissues and organs with a high mitotic index. Viral antigen is especially concentrated in lymphoid tissues (Fig. 3A, B). Many pigs, irrespective of age or sex, have a transient, usually mild, leukopenia sometime within 10 days after initial exposure to the virus. PPV and other structurally similar viruses have been identified in the feces of pigs with diarrhea. However, there is no experimental evidence to suggest that PPV either replicates extensively in the intestinal crypt epithelium or causes enteric disease as do parvoviruses of several other species. PPV also has been isolated from pigs with lesions described as vesiclelike. The causal role of PPV in such lesions has not been clearly defined.