Zoonotic diseases that afflict both humans and animals are often maintained in the environment by a complex network of ecological interactions. By investigating these interactions, we can better inform management and public health efforts to reduce the threat of disease to both wildlife and humans. Tick-borne relapsing fever (TBRF) is caused by a spirochetal bacterium from the genus Borrelia that is transmitted through the bite of a soft tick from the genus Ornithodoros. These long-lived ticks are generalist feeders that inhabit the burrows and nests of their hosts, but the true diversity of host species and the infection dynamics within and among them remains unclear. Our objective was to describe the seasonal and long-term dynamics of TBRF within communities of wild rodents and identify the species and conditions that contribute the most to disease transmission and maintenance. Using mark-recapture methods, we trapped rodents at five sites in the Sierra Nevada Mountains between 2009-2016. Blood samples from captured animals were tested for the presence of Borrelia spirochete DNA with qPCR, and when possible, positive samples were sequenced using Sanger sequencing.
Results/Conclusions
We tested 2538 blood samples and found that 172 were positive for Borrelia spirochetes. Chipmunks dominated the raw number of positive captures with 121 (70%), followed by mice with 27 (16%), and ground squirrels with 23 (13%), but mice had the highest overall infection prevalence with 8.5%, followed by chipmunks with 7.6%, and ground squirrels with 3.8%. Overall community prevalence was 6.8% and this remained relatively stable across all sites, months, and years. We successfully sequenced 98 positive samples and observed virtually no clustering based on geographic location, host species, or year. We also found that infection has a negative effect on survival for ground squirrels but not chipmunks, and that chipmunks typically clear their infections in 30 days or less. Our results show that TBRF is endemic to the Sierra Nevada Mountains. Infection prevalence within the rodent community does not fluctuate over time and is also consistent across sites, therefore rodent population dynamics are generally not a good proxy for human disease risk. Infection seems to affect wildlife species differently, but the diversity of infected species displays the general feeding habits of the tick, and we postulate that this facilitates endemicity. More research is needed on the ecology and infection prevalence of the ticks.