Time course Haemophilus parasuis infection reveals pathological differences between virulent and non-virulent strains in the respiratory tract.

Haemophilus parasuis is a common inhabitant of the upper respiratory tract of pigs and the etiological agent of Glässer's disease. However, the host-pathogen interaction remains to be well understood. In this work, 33 colostrum-deprived pigs were divided in 4 groups and each group was inoculated intranasally with a different H. parasuis strain (non-virulent strains SW114 and F9, and virulent strains Nagasaki and IT29755). Animals were necropsied at different times in order to determine the location of the bacteria in the respiratory tract of the host during infection. An immunohistochemistry method was developed to detect H. parasuis in nasal turbinates, trachea and lung. Also, the co-localization of H. parasuis with macrophages or neutrophils was examined by double immunohistochemistry and double immunofluorescence. Virulent strains showed a biofilm-like growth in nasal turbinates and trachea and were found easily in lung. Some virulent bacteria were detected in association with macrophages and neutrophils, but also inside pneumocyte-like cells. On the other hand, non-virulent strains were seldom detected in nasal turbinates and trachea, where they showed a microcolony pattern. Non-virulent strains were essentially not detected in lung. In conclusion, this work presents data showing differential localization of H. parasuis bacteria depending on their virulence. Interestingly, the intracellular location of virulent H. parasuis bacteria in non-phagocytic cells in lung could allow the persistence of the bacteria and constitute a virulence mechanism.