Differential virulence in two congeneric ticks infesting songbird nestlings

Abstract

SUMMARYVirulence is part of the proximate base to the understanding of host–parasite interactions, and therefore it is crucial to interpret parasite-induced damage as an outcome of co-evolution between parasite and host. We experimentally investigated the impact of 2 congeneric ticks with contrasting transmission modes and host specificity, on the health of songbird nestlings (Parus major). The nidicolousIxodes arboricolalives in tree holes where it obtains blood from adult birds and their nestlings, hence is prone to vertical transmission. All developmental stages of the field-dwellingIxodes ricinusfeed on different host taxa, hence are prone to horizontal transmission. Within each nest we infested 3 nestlings with varying loads ofI. ricinusand 3 withIxodes arboricola. Ixodes arboricolahad no negative effects on nestling health (haematocrit, inflammation, body size, body condition). Body size was positively associated with the number of feedingI. arboricolaadults. In contrast,I. ricinusinfestations by nymphs and adult ticks led to haematocrit reductions. Furthermore,I. arboricolawas a more prudent feeder, with longer feeding durations for smaller bloodmeals. Although the negative effects on nestling health were limited, our results are in line with current theories, stating that virulence increases with horizontal transmission, reduced dependence on individual hosts and reduced relatedness among co-exploiting ticks.