Susceptibility of aged guinea pigs to repeated daily challenge with Listeria monocytogenes

Epidemiological data suggests that certain groups such as the elderly are more susceptible to listeriosis than the general population. Repeated exposure to low levels of Listeria monocytogenes may increase the probability of infection. Increased susceptibility to infection in the elderly has been attributed in part to reduced activity of T cells. We investigated the impact of consecutive daily intragastric challenge with L. monocytogenes on the development of listeriosis within an aged guinea pig population. Approximately 63% of animals became infected following oral L. monocytogenes challenge with 10⁴ CFU daily for 3 days, based on recovery of the pathogen from the liver or spleen. At day 4 postchallenge, 100% of animals were infected based on recovery of the bacterium in spleen and liver, decreasing to 14% by day 6 and then steadily increasing to 83% by day 13. During the 13-day postchallenge period, in the blood, numbers of total CD3⁺ T cells decreased significantly; CD8⁺ T-cell population underwent two shifts; CD4 ⁺ T-cell population decreased and then increased. The results suggest that listerial infection can occur following repeated daily exposure to low levels of L. monocytogenes and that during infection, CD3⁺ T-cell immune response may be depressed, potentially increasing susceptibility to other diseases.