Investigating within-population temporal variation in tick parasitism and immune function among rodent hosts: Why are some hosts consistently more parasitized?

Background/Question/Methods

Parasites are often unevenly distributed among hosts. This leads to a few hosts carrying the majority of the parasites. While it is known that some host species become infected with vector-borne pathogens more readily than others, factors contributing to within-population variation in disease transmission is less studied, yet important for understanding variation in pathogen prevalence and disease risk. We conducted field surveys of ticks and rodent hosts in southern Indiana and measured innate immune function to investigate if certain individuals in a host population are consistently highly parasitized and therefore have a disproportionately large influence on the spread and maintenance of tick-borne pathogens. Ticks are important vectors of human and wildlife pathogens such as the agent of Lyme Disease. We predicted  that there would be predictability in the number of ticks carried by individual hosts in a population over time, i.e., some hosts will always have high tick burdens while others would always have low burdens. Because the host immune system interacts directly with parasite and pathogens, variation in immune function may explain some of the variability in tick burden within host populations.

Results/Conclusions

There was a significant seasonal component to tick abundance and host immune function, with both increasing until early-June and subsequently decreasing. During times of peak tick abundance, linear regression analysis showed significant positive correlations between a host’s tick burdens across sampling periods. These results demonstrate that some individuals have a consistently higher probability of being highly parasitized than others. During periods of peak tick abundance there was also a significant negative relationship between host tick burden and immune function from the same sampling periods. These results suggest that highly parasitized hosts may be those that have a reduced innate immune response, leading to a more favorable environment for the tick. These results highlight the importance of considering small-scale variability when investigating large-scale patterns of disease dynamics. In addition, these results show the potential for superspreaders in these host populations, hosts that are disproportionately important for the maintenance and spread of vector-borne and pathogens. Analysis of ticks and hosts for infection by tick-borne bacteria will clarify whether the most heavily parasitized rodent hosts are indeed more likely to be infected with tick-borne pathogens.