Porcine circovirus type 2 (PCV2) is considered by most to be the etiologic agent of postweaning multisystemic wasting syndrome (PMWS), but it is also found in association with numerous other conditions. These conditions include porcine dermatitis and nephropathy syndrome (PDNS), porcine respiratory disease complex (PRDC), congenital tremors (CT) type AII, reproductive failure and enteritis. Although PCV2 has been determined to be associated with many of these conditions, there is still no scientifically sound proof that it is the causal agent. Much of the role played by PCV2 in today's swine industry is still a mystery. We do know that the incidence of PCV2 associated conditions continue to rise in the U.S. PCV2 associated diseases have increased from 37 cases in 1997 to 1,116 cases in 2002 at the Iowa State University Veterinary Diagnostic Laboratory (ISU-VDL).
PCV1 was detected in swine populations but, to date, has not been associated with any clinical disease. Although PCV1 and PCV2 are antigenically similar, they can be differentiated by molecular tests. Like PCV1, PCV2 is prevalent in virtually every herd. Much research is focused on trying to determine why the prevalence of PCV2-associated disease is relatively low while the prevalence of PCV2 is so high. There may be numerous contributing factors in the development of the various disease conditions.
PMWS was first recognized in high health Canadian herds in 1991 and is seen in almost every country now. Although morbidity is usually 100%, mortality can approach 5-50%. This syndrome most often affects pigs 4-12 weeks of age and is diagnosed by meeting all of the following three criteria:
Clinical signs and gross lesions - wasting, slowed growth, enlargement of inguinal lymph nodes and frequently dyspnea, noncollapsed and mottled lungs and occasionally jaundice
Characteristic histological lesions - lymphocyte depletion in secondary lymphoid tissues, granulomatous inflammation in numerous organs, interstitial pneumonia and often basophilic cytoplasmic inclusion bodies in macrophages
Demonstration of PCV2 within lymphoid tissue lesions by IHC, PCR or in situ hybridization; virus isolation is also offered by some laboratories.
There has been much uncertainty in the relationship between PCV2 and PMWS but Bolin et al demonstrated that Koch's postulates could be fulfilled by inoculating cesarean-derived, colostrum-deprived (CDCD) pigs with PCV2 and producing clinical signs of PMWS. Similar work has been performed using an infectious genomic clone of PCV2 in specific pathogen free (SPF) pigs to produce the gross and microscopic lymphoid lesions of PMWS. Although PCV2 has been shown to induce PMWS lesions in CDCD and SPF pigs, the presence of additional factors are required.