Comparison of intestinal invasion and macrophage response of Salmonella Gallinarum and other host-adapted Salmonella enterica serovars in the avian host

The purpose of this investigation was to study the host specific infection of Salmonella Gallinarum in chickens and to determine the contribution of intestinal invasion and macrophage survival in relation to systemic infection in the host. This was carried out by comparing the kinetics of infection of S. Gallinarum to that of other Salmonella host-adapted (S. Cholerae-suis, S. Dublin and S. Typhimurium) and host-specific (S. Pullorum and S. Abortus-ovis) serovars. Establishment of the rate of colonisation in intestinal tissue, bursa and systemic sites was carried out by oral infection in day-old and week-old birds. Salmonella Gallinarum was the only serovar capable of causing systemic infection in chickens, however, general colonising ability in the intestine and bursa demonstrated no apparent selective advantage for S. Gallinarum. Further quantification of gastrointestinal invasion was carried out using ligated loops in the small intestine. Invasion in the jejunum of the chicken intestine over 3 h demonstrated that Salmonella Typhimurium invasion was statistically higher (P < 0.01) when compared with S. Gallinarum. Specific sites of high lymphoid tissue concentration in the chicken, including the bursa of Fabricius and caecal tonsils, were also targeted in invasion assays to investigate possible areas of tissue tropism. S. Typhimurium demonstrated significantly higher (P < 0.01) invasion at these sites when compared with S. Gallinarum. Infection of chicken macrophages with S. Gallinarum did not demonstrate increased multiplication and survival intracellularly when compared with other Salmonella serotypes. The only difference seen was with S. Abortus-ovis, which demonstrated a significantly lower (P < 0.05 to 0.001) intracellular survival. Together these data suggest that although S. Gallinarum host specificity in the chicken correlates with systemic infection, intestinal and lymphoid tissue invasion in the bursa and caeca, and macrophage survival does not influence this outcome.