Relative importance of Ixodes ricinus and Ixodes trianguliceps as vectors for Anaplasma phagocytophilum and Babesia microti in field vole (Microtus agrestis) populations

Abstract

The importance of Ixodes ricinus in the transmission of tick-borne pathogens is well recognized in the United Kingdom and across Europe. However, the role of coexisting Ixodes species, such as the widely distributed species Ixodes trianguliceps, as alternative vectors for these pathogens has received little attention. This study aimed to assess the relative importance of I. ricinus and I. trianguliceps in the transmission of Anaplasma phagocytophilum and Babesia microti among United Kingdom field voles (Microtus agrestis), which serve as reservoir hosts for both pathogens. While all instars of I. trianguliceps feed exclusively on small mammals, I. ricinus adults feed primarily on larger hosts such as deer. The abundance of both tick species and pathogen infection prevalence in field voles were monitored at sites surrounded with fencing that excluded deer and at sites where deer were free to roam. As expected, fencing significantly reduced the larval burden of I. ricinus on field voles and the abundance of questing nymphs, but the larval burden of I. trianguliceps was not significantly affected. The prevalence of A. phagocytophilum and B. microti infections was not significantly affected by the presence of fencing, suggesting that I. trianguliceps is their principal vector. The prevalence of nymphal and adult ticks on field voles was also unaffected, indicating that relatively few non-larval I. ricinus ticks feed upon field voles. This study provides compelling evidence for the importance of I. trianguliceps in maintaining these enzootic tick-borne infections, while highlighting the potential for such infections to escape into alternative hosts via I. ricinus.

FIG. 1.

Mean number of Ixodes ricinus larvae per vole recorded over the duration of the study on both control and fenced sampling grids. Error bars represent standard errors of the mean.

FIG. 2.

Mean number of I. trianguliceps larvae per vole recorded over the duration of the study on both control and fenced sampling grids. Error bars represent standard errors of the mean.

FIG. 3.

Number of questing nymphs collected by blanket dragging over a 300-m² area on control and fenced sampling grids. Error bars represent standard errors of the mean.

FIG. 4.

Infection prevalence of Anaplasma phagocytophilum in field voles sampled on both control and fenced grids. Error bars represent exact binomial 95% confidence intervals.

FIG. 5.

Infection prevalence of Babesia microti in field voles sampled on control and fenced grids. Error bars represent exact binomial 95% confidence intervals.