[Salmonella infection in animals and its importance for man].
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An unusual abundance of Salmonella infections was studied in an island population of a wild marsupial, the quokka (Setonix brachyurus), which experiences starvation in summer associated with significant mortality. The frequency of infections was found to vary seasonally over most parts of the island, with high infection rates (70 to 100%) in summer and low infection rates (0 to 30%) in winter. In some samples, there was an average of as many as two isolations per animal, and up to five isolations were made from a single animal. By the end of summer, virtually all animals excreted Salmonella spp., with a median rate of excretion of approximately 3,000 Salmonella organisms per g of feces. The seasonal changes occurred over intervals of only weeks. The infections are believed to be associated with disruption of the digestive physiology of the animals caused by the poor quality of feed available in summer. This conclusion was supported by a quantitative study of the infections and by a field manipulation experiment which delayed the initiation of the infections as long as a food supplement was available. The proliferation of Salmonella spp. is discussed in terms of the ecology of the quokka and of the use of Salmonella spp. as indicators of environmental stress acting on the animals.
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Serum immunoglobulin G (IgG) and IgM were measured for individual Salmonella species by the enzyme-linked immunosorbent assay (ELISA). F344 rats were experimentally infected with Salmonella typhimurium (serogroup B), S. enteritidis (serogroup D), and S. rubislaw (serogroup G.) Endotoxin extracted from each serogroup served as the antigen in a classical indirect ELISA. Antibody specific for each Salmonella serogroup was detected by ELISA. Normal gut flora from control animals appeared not to cause cross-reactions in the ELISA. Specificity and sensitivity of the IgG ELISA were determined by statistically evaluating false-positives and false-negatives. Ideal values of 90% or better were achieved in nearly all instances. Each antigen was also tested with heterologous antisera in an effort to develop a polyvalent assay for Salmonella species. No single antigen detected all positive heterologous antisera. Therefore, a polyvalent antigen composed of the three serogroup antigens was tested. The results suggested that Salmonella infections can be detected by measuring serum IgG levels with a polyvalent ELISA 6 to 9 days postinfection.
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This paper discusses the hypothesis that a major factor in the epizootics of Salmonella infection in poultry is a declining host genetic diversity. A computer model is described which is based on models that have been previously used to investigate host-pathogen coevolution in cereal crops. It is shown that, as host genetic diversity declines, parasite diversity also declines to a lower equilibrium level. With a highly diverse host, parasite numbers decline to zero. With a homogeneous host population, after an initial decline, there is a rapid increase in parasite numbers, due to the selection of a particularly well adapted parasite strain. This simple computer simulation is used as the basis for a discussion of the literature supporting the suggestion that a major factor in the epizootic of Salmonella in poultry is related to the low genetic diversity of commercial poultry flocks.