Ecological factors influencing small mammal infection by Anaplasma phagocytophilum and Borrelia burgdorferi s.l. in agricultural and forest landscapes.

Small mammals are key components of numerous tick-borne disease systems, as hosts for immature ticks and pathogen reservoirs. To study the factors influencing tick-borne infection in small mammals, we trapped small mammals and collected questing ticks in spring and autumn in 2012 and 2013 at 24 sites in a 10 × 15 km rural landscapes (Brittany, France). Tissue samples were screened by real-time PCR for Anaplasma phagocytophilum and Borrelia burgdorferi sensu lato. Of the two dominant small mammal species captured, bank voles (Myodes glareolus) had higher prevalence than wood mice (Apodemus sylvaticus) for both infections, presumably because of specific differences in immunological defenses. Prevalence of infections was higher in 2013 than in 2012, likely because small mammals were fivefold less abundant in 2013, favouring tick aggregation. Bacterial prevalence, which was higher in autumn, was not associated to questing Ixodes ricinus nymph abundance which was six times higher in spring, but rather to the structure of the small mammal community. These findings suggest the involvement of endophilic tick species, I. trianguliceps and/or I. acuminatus, in bacterial transmission. Our study highlights that the entire community of hosts and vectors, and their interactions, should be considered to fully understand the epidemiology of vector-borne diseases.