Age-related decline in the resistance of mice to infection with intracellular pathogens.

Resistance to infection with Toxoplasma gondii and Listeria monocytogenes in BALB/c female mice decreased with increasing age. The decrease was apparent as early as 9 months of age and was more marked as the animals aged further. This age-related decline in resistance was not restricted to BALB/c female mice, as male and female mice of the C57BL/6 strain exhibited similar responses. With both pathogens, aged mice showed a more marked susceptibility to the strain of lesser virulence. Transfer of normal serum from old mice to young or old mice before infection with T. gondii resulted in an increased susceptibility to this organism, suggesting the presence of inhibitory factors or the absence of potentiating factors that are present in the serum of young mice.