A preliminary survey of male Microtus montanus trapped at Jackson Hole, Wyoming, indicated that 100% of these animals harbored one or more endoand/or ectoparasites. Spleen histology and differential white blood cell counts were both consistent with the suggestion that these animals have persistent parasitic infections, which serve as a stimulus to the immune response. It has also been demonstrated that there is a high percentage of splenomegaly in these animals. These results support the hypothesis that splenomegaly in Microtus montanus is caused by a parasitic infection, and therefore, that splenomegaly can be used as a survey marker to determine the extent of parasitism in field populations. It has also been shown in adult males captured during a 3-year period that there was a significant inverse relationship between spleen size and gonadal weights. It is assumed that smaller gonads denote reduced reproductive potential. Therefore, parasitic infections, by reducing this potential, may have a greater effect on the control of population density than previously suggested. Other hypotheses on the mechanisms of population density control are discussed.
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Seed et al. (1978) studied this question.