The present study describes the role of Pasteurella (P.) multocida, in the multifactorial etiology of the porcine respiratory disease complex (PRDC). The losses due to PRDC are significant, particularly among weaners and finishers in middle-sized and large farms. They are connected with retarded growth and the necessity of eliminating some percentage of animals before the completion of the finishing process. The primary etiological factors of PRDC are: the porcine respiratory and reproductive syndrome virus (PRRSV), Mycoplasma hyopneumoniae, the swine influenza virus (SIV), Actinobacillus pleuropneumoniae (APP), Streptococcus suis, type 2, and the pseudorabies virus. One of the most important secondary etiological factors of PRDC is P. multocida. That is why the mechanism of the pathogenicity of this facultatively pathogenic microorganism is described here. The capsular and somatic antigens, which characterize the most important pathogenic serotypes, i.e. A:3, A;5, and D:3, are discussed. However, even representatives of these serotypes do not demonstrate pathogenicity without other factors weakening the innate immunity of swine. These immunosuppressive factors include environmental deficiencies which decrease the welfare of the swine population, and the above-mentioned pathogens, which are the primary etiological agents of PRDC. Despite controversial results concerning the importance of toxins of the P. multocida strains isolated from swine lungs, their contribution to the pathogenesis of PRDC can not be excluded. The role of the bacterial capsule and the ability of P. multocida (serotypes A and D) to attach themselves even to ciliated epithelial cells of the respiratory tract is of minor importance in explaining the participation of these microorganisms in the pathogenesis of PRDC, and further research is needed. The study comments on the tests used for the identification of serotypes of P. multocida, important in the etiology of PRDC, the indirect, passive haemagglutination test being “the golden standard”. However, at present this method is being increasingly replaced by PCR. Molecular typing of P. multocida strains is also performed by restriction enzyme digestion with or without subsequent hybridization with a standard probe or the sequencing of multiple loci of predominantly house-keeping genes. These modern methods contributed to a new perspective on the role of P. multocida in swine pneumonia: it came to be perceived as its secondary pathogenic agent rather than a major etiological factor. During the last decade these tests have also enabled the discovery of other, more important microorganisms, which would not have been possible earlier, when only classical diagnostic tests were used for identifying fenotypic properties of microorganisms. Segregated early weaning is the recommended method of preventing PRDC.
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