| Summary: | Factors influencing Salmonella serotype-host specificity were assessed by characterising the interactions of five serotypes with the bovine host in vivo. S dublin and S. tryphimurium were highly virulent in calves infected intravenously whereas S. choleraesuis was only moderately virulent. S. dublin and S. choleraesuis were also virulent in calves infected orally. However, S. gallinarum and S. abortusovis were avirulent by either route of infection, demonstrating that genetic differences exist between serotypes, which contribute to host specificity in calves. Additionally, S. dublin was consistently recovered from systemic tissues 24 hours after oral inoculation of calves whereas S/ gallinarum and S. abortusovis were recovered only intermittently. The ability of different serotypes to invade the intestinal mucosa and induce enteropathogenic responses was assessed. S. typhimurium, S. dublin and S. gallinarum invaded bovine intestines in vivo in greater numbers and induced greater enteropathogenic responses than either S. choleraesuis or S. abortusovis. A bovine intestinal cannulation model was developed to assess systemic translocation of S. dublin and S. gallinarum, from the intestinal lumen, and host responses to infection. Both serotypes were present within the mucosa and efferent lymph in a predominantly cell-free niche. In addition, S. gallinarum was primarily cell-free within the mesenteric lymph nodes (MLN), although there was some association of both serotypes with CD14+ leucocytes in this tissue. Significantly, S. dublin was able to pass through the MLN in higher numbers than S. gallinarum, a factor that may influence the outcome of Salmonella infection in calves. However, host cell death did not appear to influence the pathogenesis of either serotype. Taken together, these results suggest that while initial interactions with the intestinal mucosa do not directly correlate with host specificity, the ability to survive within host tissues appears to be crucial for the induction of both systemic and enteric disease in orally infected calves.
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