Influence of prior infection on the dynamics of bacterial counts in calves experimentally infected with Salmonella dublin.

The aim of this investigation was to obtain information on the influence of immunity gained from a previous infection in calves on the dynamics of a subsequent salmonella infection. Immune and non-immune animals were orally infected with virulent S. dublin. After infection clinical symptoms and the development of bacterial counts in intestinal contents and intestinal mucosa, as well as in intestinal lymph nodes, spleen and liver were compared. The protection of the immune animals became evident as milder courses of infection and in lower bacterial counts in all examined organs. The greatest absolute and relative difference between the counts of immune animals and controls was in the intestinal mucosa. While absolute counts were lower, the proportion of bacteria was higher in the lymph nodes of immune animals. This apparently more intensive penetration in immune animals was feigned by a general and immunity independent negative correlation between concentration of salmonella in the mucosa and of salmonella found in the intestinal lymph nodes. This time course of counts provided evidence that enhanced defence of immune calves was particularly pronounced 3 and 4 days after the onset of infection.