Experimental transmission of porcine circovirus type 2 (PCV2) in weaned pigs: a sequential study.

Weaned specific pathogen-free pigs were inoculated intranasally with porcine circovirus type 2 (PCV2) and killed in groups of two or three animals at 6, 13, 20, 27 and 34 days post-inoculation (dpi), together with appropriate uninfected controls, for examination by histopathological, immunohistochemical (immunogold silver staining; IGSS), polymerase chain reaction (PCR) and viral isolation techniques. Serum samples were also collected for detection of antibodies. No major clinical signs were observed in infected pigs, and gross lesions were essentially limited to the lungs and lymph nodes of some of the animals. Histologically, no lesions were seen at 6 dpi, but bronchointerstitial pneumonia was invariably noted from 13 dpi onwards. Granulomatous inflammation, with or without intracytoplasmic inclusions, was present in lymphoid tissues (e.g. lymph nodes, thymus, spleen and tonsil) from day 20 onwards, being most severe at days 20 and 27 dpi. Liver inflammation was present at days 13, 20 and 27 dpi. Virus was demonstrated in the tissues by isolation and PCR methods throughout the experiment. PCV2 antigens were detected by IGSS in bronchial and bronchiolar epithelial cells, in mononuclear cells and multinucleated giant cells within inflammatory lesions, and in mononuclear cells of apparently normal tissues (e.glamina propria of the small intestine and the bronchus-associated lymphoid tissue). The lesions were consistent with those of postweaning multisystemic wasting syndrome (PMWS), although not all previously reported PMWS lesions were seen. PCV2 antibodies were detected in infected pigs from day 13 onwards. The results demonstrated widespread distribution of PCV2 after infection and persistence of the virus in vivo for at least 34 days. It would appear that PCV2 can induce PMWS lesions in weaned pigs in the absence of porcine parvovirus and other common swine pathogens.