Update on porcine circovirus and postweaning multisystemic wasting syndrome (PMWS)

Postweaning multisystemic wasting syndrome (PMWS) is a recently emerged disease of nursery and grower pigs associated with type 2 porcine circovirus (PCV2) infection. A proposed case definition of PMWS requires that a pig/group of pigs have all of the following: 1) clinical signs characterized by wasting/failure to thrive, with or without dyspnea or icterus; 2) histologic lesions characterized by depletion of lymphoid tissues and/or lymphohistiocytic to granulomatous inflammation in any organ, typically lungs and/or lymphoid tissues; and 3) PCV2 within characteristic lesions. Immunohistochemistry and in situ hybridization applied to formalin-fixed tissues are the preferred methods for demonstrating PCV2 antigen or genome, respectively, within PMWS lesions. Since available evidence indicates that PCV2 infection is much more common than PMWS, demonstrating PCV2 exposure via serology or infection via isolation or PCR without localizing PCV2 within characteristic lesions cannot constitute a diagnosis of PMWS. To date, consistent experimental reproduction of severe PMWS has only been accomplished via PCV2-parvovirus or PCV2-PRRSV co-infection. Effective control of PMWS will require an understanding of the viral and nonviral cofactors that enable PCV2, an apparently ubiquitous and generally nonpathogenic agent, to induce PMWS.