Interactions between ectoparasites and immune function in free-living rodents

Background/Question/Methods

Parasites and pathogens do not infect all hosts equally. Heterogeneity within a host community can be caused by host species, sex, and previous exposure to infectious agents. Differences in host immune response have been proposed as a mechanism for heterogeneity in parasite distribution on hosts. We studied small mammal hosts and tick vectors to investigate interactions between host immune function and ectoparasite load. Two hypotheses that could explain observed relationships are: 1) hosts with lower immune function will have higher ectoparasite burden due to reduced ability to mount a response, and 2) hosts with higher immune function will have higher ectoparasite burden due to high investment in immune function. We live-trapped white-footed mice (Peromyscus leucopus) and prairie voles (Microtus ochrogaster) at eight sites in southern Indiana. We collected ticks and blood samples from each individual to relate host immune function to ectoparasite load. We measured general, innate immune function with bacterial killing assays using host blood serum.

Results/Conclusions

There were significantly more ticks found on Peromyscus than Microtus (p <0.0001), and overall, males had higher tick burdens than females (p = 0.01). Likewise, Peromyscus serum had a higher bacterial killing ability than Microtus (p <0.0001). Within Peromyscus, males had higher killing ability than females (p = 0.07), but there were not sex differences within Microtus. Our results support hypothesis 2 because Peromyscus had the highest ectoparasite burden and the highest bacterial killing ability, suggesting parasite exposure may be a strong selective pressure on host immune function. The sex differences between male and female Peromyscus may be caused by reproductive trade-offs in females due to reallocation of resources during the breeding season. However, these rodents could be infected with vector-borne pathogens, which would also stimulate an immune response. Current work is integrating host infection profiles into this analysis which may better explain patterns of immune function seen in the field and reveal linkages between host immune function and vector-borne disease risk.