Host in reserve: the role of common shrews (Sorex araneus) as a stable
supplementary source of tick hosts in small mammal communities
influenced by rodent population cycles
Abstract
In recent decades, warming temperatures and changes in land use are
supposed to have enabled several tick species to expand their
distribution limit northwards. The progression of ticks to new areas may
introduce new and emerging tick-borne pathogens as well as increase
existing diseases. Aside from climatic conditions, ticks are dependent
on hosts for survival, and rodents often act as important hosts for
ticks and as pathogen reservoirs. At northern latitudes, rodents often
undergo multi-annual population cycles, and the periodic absence of
hosts may inhibit the further progression of ticks. We investigated the
potential role of common shrews (Sorex araneus) to serve as a stable
host source to immature life stages of a generalist tick Ixodes ricinus
and a specialist tick I. trianguliceps, during decreasing abundances of
bank voles (Myodes glareolus). We tested whether ticks would have a
propensity to parasitize a certain host type dependent on host
population size and composition in two high latitude locations in
southern Norway, by comparing tick burdens on trapped animals. We found
that I. ricinus larvae showed an equal propensity to parasitize both
host types as the host population composition changed, but voles had a
higher level of parasitism by nymphs (p< 0.004). An overall
larger host population size favored the parasitism of voles by larvae
(p= 0.027), but not by nymphs (p= 0.074). I. trianguliceps larvae showed
a higher propensity to parasitize shrews, regardless of host population
size or composition (p= 0.004), while nymphs parasitized shrews more as
vole abundance increased (p= 0.002). The results indicate that common
shrews may have the potential to act as a replacement host during
periods of low rodent availability, but long-term observations
encompassing complete rodent cycles may determine whether shrews are
able to maintain tick range expansion despite low rodent availability.