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After presenting the definition of immunosuppression the paper concentrates on the immunosuppressive properties of porcine circovirus type 2 (PCV2). Its role in the etiology of the post-weaning multisystemic wasting syndrome (PMWS) and the other porcine circovirus diseases (PCVD) is characterized, indicating that following the immunosuppressive action of PCV2 facultatively pathogenic microorganisms join the etiology of the disease syndrome which becomes multifactorial. The lymphocytopenia resulting from PCV2 infection is particularly manifest among the B lymphocytes, followed by the T lymphocytes. In vivo analyses have shown that PCV2 is most frequently associated with monocytes, macrophages and dendritic cells (DC). PCV2 infection of macrophages and DCs is the key factor determining the consequences for immune defense competence. The differences between conventional DCs (cDCs) and plasmacytoid DCs (pDCs) are mentioned. The term cDCs defines cells with antigen presenting functions and the term pDCs represents the interferon, type 1 producing cells. Examples of immunosuppressive action of other pathogens have been presented. Classical swine fever virus inhibits the interferon production in the infected DCs. In addition it causes leucopenia. These changes enablr opportunistic Pasteurella multocida and Streptococcus sp. strains to cause septicemia in the infected pigs. Immunosuppressive action of pseudorabies virus is mentioned resulting in secondary infection by other species of viruses or bacteria. Moreover, as presented in the paper, porcine reproductive and respiratory syndrome virus demonstrates immunosuppressive properties.
The aim of the study was to assess the occurrence of the postweaning wasting syndrome (PMWS) in Polish pig herds. Thirty-two herds were selected from 195 visited pig production sites based on the appearance of clinical lesions. The population of breeding herds ranged from 25 to 2000 sows and the percentage of losses varied from 2 to 20%. In each farm 3 pigs affected by wasting were autopsied and lymph nodes were collected. Sections of lymph nodes were examined for the presence of histopathological lesions and tested with in situ hybridization to detect the DNA of porcine circovirus type 2 (PCV2), an etiological agent of PMWS. The results were considered as positive when characteristic histopathological changes were detected along with an abundant amount of viral DNA. PMWS was recognized in 26 pigs (27.9%) from 16 farms (50%). In two of those farms PMWS was diagnosed as sporadic, based on the analysis of production parameters and low rate of losses. In 12 herds (37.5%) no pig had DNA of PCV2 in lymph nodes, while in the remaining 4 herds (12.5%) low amounts of DNA were detected in lymph nodes from at least one pig. PMWS was confirmed in all tested farms with breeding herds of more than 1000 sows, while none of the small farms with breeding herds of less than 100 sows were affected. In two other groups of farms counting 100-499 and 500-1000 sows the percentage of affected herds was respectively 33.3% and 57.1%. The established rate of PMWS occurrence was similar to that claimed by most other European countries. Results obtained in this study suggest that the incidence of PMWS outbreaks increases with the size of the herd. A higher number of properly selected samples markedly enhances the probability of disease detection.
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