Further studies on vertical transmission and persistence ofSalmonella entericaserovar Enteritidis phage type 4 in chickens

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Abstract
One-week-old commercial layers were infected orally with 108 colony forming units of Salmonella enterica serovar Enteritidis phage type 4. No mortality was observed. The inoculated organism was isolated in decreasing viable numbers from a number of tissues, particularly the spleen, liver and caeca. Organisms present in the spleen were primarily localized within macrophages. No Salmonella Enteritidis organisms were isolated between 10 and 24 weeks of age, when the experiment was terminated after several weeks of lay. When two groups of adult hens, housed with males, were infected, contaminated eggs were found within 2 weeks of infection in one of the experiments only. Progeny hatched from these eggs showed no mortality unless they were infected artificially with the S. Enteritidis strain. In this case, the percentage mortality fell as the hatches progressed, indicating increasing immunity to infection. The faecal excretion of the inoculated phage type 4 strain by infected but healthy progeny was followed. Although most birds ceased to excrete by 11 to 12 weeks of age, a small number of the birds continued to excrete until they themselves came into lay. The small numbers of birds in which this occurred indicates that tolerance to infection does not occur readily following infection of hens laying fertile eggs or in progeny birds infected before or within hours of hatching. Birds infected when they were less than 24 h old remained persistently infected until they were well into lay. However, control birds infected when 1 week old, on this occasion, showed a high level of excretion until the birds began to lay at 18 weeks. Inbred lines of chickens showing differences in their susceptibility to systemic salmonellosis did not show significant differences in the extent to which S. Enteritidis localized in the organs of the reproductive tract or in the number of infected eggs produced.