Resistance to a Pathogenic Challenge Lacks Seasonal Specificity in Prairie Voles (Microtus ochrogaster) and Cotton Rats (Sigmodon hispidus)

Abstract

We tested the hypothesis that resistance to a pathogenic challenge would demonstrate seasonal variations in populations of cotton rats (Sigmodon hispidus) and prairie voles (Microtus ochrogaster) using the bacteria Listeria monocytogenes in a host-resistance assay. Cotton rats and prairie voles were livetrapped in spring, early autumn, and winter from spring 1996 to spring 1997. Animals were returned to the laboratory, inoculated intraperitoneally with a lethal dose (LD50) of L. monocytogenes, and monitored for mortality for 120 h post-challenge. We found that resistance to L. monocytogenes lacked significant seasonal variations in cotton rats and prairie voles, two species with clear seasonal rhythms in many individual measures of immunity. Prairie voles were more susceptible than cotton rats to L. monocytogenes in spring 1997. Our results indicate that although seasonal variations in selected immunological parameters may exist in these species, overall resistance to L. monocytogenes varied little across seasons. Because resistance to L. monocytogenes involves both innate and adaptive immunological responses, it appeared unlikely that seasonal environmental stressors were capable of reducing both innate and adaptive responses to a point of decreasing resistance to L. monocytogenes.