Experimental reproduction of severe disease in CD/CD pigs concurrently infected with type 2 porcine circovirus and porcine reproductive and respiratory syndrome virus

Three-week-old cesarean-derived colostrum-deprived (CD/CD) pigs were inocul ated with porcine circovirus type 2 (PCV2, n = 19), porcine reproductive an d respiratory syndrome virus (PRRSV, n = 13), concurrent PCV2 and PRRSV (PC V2/PRRSV n = 17), or a sham inoculum (n = 12) to compare the independent an d combined effects of these agents. Necropsies were performed at 7, 10, 14, 21, 35, and 49 days postinoculation (dpi) or when pigs became moribund. By 10 dpi, PCV2/PRRSV-inoculated pigs had severe dyspnea, lethargy, and occas ional icterus; after 10 dpi, mortality in this group was 10/11 (91%), and a ll PCV2/ PRRSV-inoculated pigs were dead by 20 dpi. PCV2-inoculated pigs de veloped lethargy and sporadic icterus, and 8/19 (42%) developed exudative e pidermitis; mortality was 5/19 (26%). PRRSV-inoculated pigs developed dyspn ea and mild lethargy that resolved by 28 dpi. Microscopic lesions consisten t with postweaning multisystemic wasting syndrome (PMWS) were present in bo th PCV2- and PCV2/PRRSV-inoculated pigs and included lymphoid depletion, ne crotizing hepatitis, mild necrotizing bronchiolitis, and infiltrates of mac rophages that occasionally contained basophilic intracytoplasmic inclusion bodies in lymphoid and other tissues. PCV2/ PRRSV-inoculated pigs also had severe proliferative interstitial pneumonia and more consistent hepatic les ions. The most severe lesions contained the greatest number of PCV2 antigen -containing cells. PRRSV-inoculated pigs had moderate proliferative interst itial pneumonia but did not develop bronchiolar or hepatic lesions or lymph oid depletion. All groups remained seronegative to porcine parvovirus. The results indicate that 1) PCV2 coinfection increases the severity of PRRSV-i nduced interstitial pneumonia in CD/CD pigs and 2) PCV2 but not PRRSV induc es the lymphoid depletion, granulomatous inflammation, and necrotizing hepa titis characteristic of PMWS.