Population Dynamics, Adrenocortical Function, and Pathology in Microtus pennsylvanicus

Levels of renal pathology (pyelonephritis) and hepatic parasitism (cysticercosis) were studied in three populations of the meadow vole, Microtus pennsylvanicus. The prevalence of pyelonephritis was lowest during the year of increasing numbers compared to the years of peak or declining numbers. Cysticercosis showed similar trends. Adrenocortical function in mature females was related to density in these populations. The changes in the prevalence of the two pathological conditions can be explained by reduced immunocompetence, which is related to adrenocortical function.