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1. Several studies have shown that genetic variation exists in response to various Salmonella strains in mammals and poultry. In the current study immunocompetence traits related to natural resistance to Salmonella were measured in 7 genetic groups of meat-type chickens (in total 296 chickens involved). 2. Variables were measured of both innate (phagocytic activity) and adaptive immune responses that are important after a natural or experimental Salmonella enteritidis infection. Two traditional Old Dutch Breeds (groups 1 and 2), four commercial broiler groups (groups 3 to 6), and one experimental broiler group (group 7) were used. In two periods, birds of each group were killed for examination at ages between 14 and 35 d post hatch. 3. Significant differences between groups were found for most immune variables measured, with significant correlations between several of them. All groups produced an adequate immune response, of either the innate or the adaptive type. 4. In the current study, group 2 showed the highest overall natural resistance, though none of the groups was uniformly superior with respect to all traits measured. 5. In conclusion, for reliable measurements of general immunocompetence or resistance to Salmonella, for example, it is important to measure several aspects of the immune system.

Various leukocytes are involved in the reaction to counter Salmonella infection in chicken. The various leukocyte types react differently after an infection, since some clear the infection while others may cause dissemination of Salmonella throughout the chicken. Therefore, we investigated in vitro the entry and survival of Salmonella enterica serotype Enteritidis in chicken cell lines of various cell types, including two macrophage cell lines, HD11 and MQ-NCSU (NCSU), two B-cell lines LSCC-1104-X5 (1104) and LSCC-RP9 (RP9), and a T-cell line MDCC-MSB-1 (MSB-1). The macrophages were able to internalize high numbers of S. Enteritidis. In contrast and as expected, cells of the T-cell line MSB-1 and the B-cell line RP9 internalized bacteria at a much lower level. After S. Enteritidis entered the macrophages, the number of intracellular S. Enteritidis decreased over time, so that after 48h no more than 20% of the bacteria, which had entered, survived intracellularly. In contrast to macrophages, the number of S. Enteritidis in cells of the T-cell line MSB-1 and the B-cell line RP9 increased rapidly within 12h post-inoculation. Thereafter the number of intracellular S. Enteritidis decreased only slowly. In conclusion, all three different cell types were able to control and to start clearing S. Enteritidis, although macrophages were far more effective compared to T- and B-cells. However, none of the cell lines were able to clear S. Enteritidis fully within 48h. These results suggest that the three cell types play an important but different role in the dissemination and elimination of S. Enteritidis throughout the animal.

Four broiler lines were inoculated orally with Salmonella enteritidis phage type 1 at the age of 7 days (experiment A: lines 1 and 2) and at the age of 1 day (experiment B: lines 3 and 4). At various days post-infection chickens were sacrificed and the number of Salmonella in the caeca, liver, and spleen were determined. Furthermore, phagocytic activity, cellular immune responses, and humoral responses were determined using, respectively, single-cell suspensions of spleen or intestine and serum. In both experiments, similar trends were seen. Increased numbers of S. enteritidis were found in the caeca of lines 1 and 3, whereas at the same time a decreased colonization was found in the spleen and in the liver, as compared to lines 2 and 4. In the latter two lines, the phagocytic activity of the phagocytes was higher and the humoral responses were lower. Observations from this study suggest that lower activity of phagocytes and higher humoral activity prevent systemic S. enteritidis infection.