Features of porcine circovirus-2 disease: correlations between lesions, amount and distribution of virus, and clinical outcome.

Tissue sets from 36 snatch-farrowed colostrum-deprived (SF/CD) and 71 Caesarian-derived gnotobiotic swine infected with porcine circovirus type 2 (PCV-2) as neonates were examined and scored for the types and tissue distribution of histologic lesions associated with this viral infection. The occurrence and severity of these lesions were correlated with qualitative and quantitative determinations of viral burden in tissues by immunohistochemistry (IHC) and tissue titrations for infectious virus, respectively. These measures were, in turn, related to 1 of 3 categories of clinical disease expressed in PCV-2-infected swine as subclinical infection, preclinical postweaning multisystemic wasting syndrome (PMWS), and clinically evident PMWS, respectively. Statistically significant (P < 0.05 to 0.001) associations between both measures of viral burden, the severity of histologic lesions and the stage of disease were obtained. Discrimination between and among categories of disease was best accomplished by a combination of IHC and histopathology. The results of this study confirm that viral burden in PCV-2-infected tissues, specifically lymphoid tissues and liver, directly correlate with severity of clinical disease expression in PCV-2 infected swine.