Patterns of helminth parasite infections in cyclic common vole ( Microtus arvalis ) populations

Research on parasite-induced regulation has identified the conditions under which parasites can destabilise host population dynamics: high levels of aggregation, delayed density-dependence, and moderate negative effects on fitness (reproduction, survival). Gastrointestinal helminths with direct life cycles and a single definitive host provide ideal systems to test these predictions. In this study, we first determined which helminths infect common voles (Microtus arvalis) in NW Spain, where populations are cyclic. We showed that the helminth community is dominated by Syphacia sp., a gut-restricted, directly transmitted nematode.We then examined how the prevalence and abundance of Syphacia sp. varied with host sex, season, and population cycle phase (increase, peak, or crash), and tested if vole condition (relative body mass and organ hypertrophy) and female fecundity (litter size) correlated with the prevalence of Syphacia sp. Infections were highly aggregated in Syphacia sp. and parasite abundance peaked during the crash phase of the vole cycle. We found that vole condition did not vary with the prevalence of Syphacia sp., but vole litter size showed a season-dependent association, with infected females producing smaller litters in spring and summer.These findings suggest that even low-pathogenic, directly transmitted parasites could exert reproductive effects, potentially shaping host population dynamics in combination with ecological and demographic factors. Experimental approaches are required to clarify causality and potential regulatory feedback.