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Special attention in this review is given to published work on genetic, antigenic and pathogenic variations of the virus of porcine reproductive and respiratory syndrome (PRRS) and the potential implications of these properties for vaccine efficacy. Genomic organization of type 1 and type 2 strains of the PRRS virus is presented: the former being the representative of the European strains and the latter of the North American isolates. Among the European isolates 4 subtypes are differentiated. More homogenous are the strains of the Northern American type. Despite very high differences in antigenic properties of both European and North American isolates, it is possible to differentiate them, using serological methods, particularly the immunofluorescence test. The ELISA test proved to be useful in diagnosis of PRRS. However, antigens of the PRRS virus are of no use in the selection of strains for vaccine production. The high degree of variation among PRRS virus isolates is also demonstrated in pathogenic properties. Generally, the heterogeneity of genetic, antigenic and pathogenic properties, being a continuous process, is the explanation for the persistent infections resulting from the selection of mutants that escape neutralizing antibodies or cytotoxic T lymphocytes. Because of the heterogeneity of the mentioned biological properties of the field strains, the immunoprophylactic value of the available vaccines is not high. Better results are obtained when live, attenuated vaccines are used. In relation to this topic the proposition of Mengeling, called SWINE, is presented. The first letter means sensitization of the pig by vaccine injection; the second letter means waiting for more than several weeks until the level of neutralizing antibodies reaches low values; the letter I means immunization; N means neutralization of the virus; the letter E means eradication, in other words the elimination of PRRS. The last step must be supported by biosecurity and additional veterinary measures. Another method of immunoprophylaxis is the vaccination of pigs introduced to a herd using viremic serum of pigs of the same herd, which provides the best homogeneity of immunity and the highest efficacy.
The epidemiology of the PRRS is much more complicated in comparison to other viral diseases, relating to its spread and persistence of the virus, which is a marked characteristic. For this reason the immunological status was evaluated in different groups of animals. Herds were located in different regions of Poland, with large, medium and small populations of pigs. The evaluation of the titres of antibodies against-PRRS virus were carried out in 48 herds located in selected regions of Poland (in 2 voivodships from regions of large pig populations, in 4 voivodships from regions of medium and in 4 voivodships from regions of small pig populations). A total of 1515 pigs were submitted for the study. In regions of large pig populations 26 herds were tested. In 19 herds (73%) antibodies of anti-PRRS virus were detected. In 3 herds (11%) a stable inactive immunological status was indicated and in 16 herds (62%) an unstable immunological status was indicated. In 7 herds (27%) antibodies of anti-PRRS virus were not observed. In regions of medium pig population 11 herds were tested. In 6 herds (73%) antibodies of anti-PRRS virus were detected. In 1 herd (10%) a stable inactive immunological status was indicated and in 5 herds (45%) an unstable immunological status was indicated. In 5 herds (45%) antibodies of anti-PRRS virus were not observed. In regions of small pig populations 11 herds were also tested (1 herd was not classified). In 5 herds (50%) antibodies of anti-PRRS virus were detected. In 1 herd (10%) a stable inactive immunological status was indicated and in 4 herds (40%) an unstable immunological status was indicated. In regions of small pig populations there was the highest percentage (50%) of herds that were free from the PRRS virus (5 herds).
The study evaluated the status of pigs immunological systems during streptococci and mixed infections of the respiratory tract caused by S. suis, Mhp and PPRSV. T lymphocytes, including the proportions of Th and Tc subpopulations, as well as B lymphocytes were examined. Additionally, CD2+ and CD8+ co-expression with the receptor for IL-2 (CD25) was analyzed. Immunodetection was performed using pairs of monoclonal antibodies against CD2 and CD21, CD4 and CD8 as well as CD5 and CD8, with isotype specific polyclonal antibodies conjugated to fluorochromes. Infections caused by S. suis as well as mixed infections of the respiratory tract resulted in an increase in the total blood leukocyte count, noted in the peak of the disease as well as 4 weeks later. However different mechanisms of specific cellular defense were apparent during streptococci and poliethiological respiratory infections in the pigs. An increase in the percentage of Tc lymphocytes and double positive CD4+CD8+ cells without a decrease in the number of B lymphocytes was noted in animals infected with S. suis. An increase in the number of Th and Tc double positive lymphocytes as well as NK cells was observed during infections caused by S. suis, Mhp and PRRSV. Additionally, a significant decrease of CD2+ and CD8+ activated T cells co-expressed with CD25 was observed in this group of animals accompanied by an increase in the population of the above cells without the expression of IL-2R receptor.
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