Lyme disease bacterium does not affect attraction to rodent odour in the tick vector

Abstract

Background: Vector-borne pathogens experience a conflict of interest when the arthropod vector chooses a vertebrate host that is incompetent for pathogen transmission. The qualitative manipulation hypothesis suggests that vector-borne pathogens can resolve this conflict in their favour by manipulating the host choice behaviour of the arthropod vector.

Methods: European Lyme disease is a model system for studying this conflict because Ixodes ricinus is a generalist tick species that vectors Borrelia pathogens that are specialized on different classes of vertebrate hosts. Avian specialists like B. garinii cannot survive in rodent reservoir hosts and vice versa for rodent specialists like B. afzelii. The present study tested whether Borrelia genospecies influenced the attraction of field-collected I. ricinus nymphs to rodent odours.

Results: Nymphs were significantly attracted to questing perches that had been scented with mouse odours. However, there was no difference in questing behaviour between nymphs infected with rodent- versus bird-specialized Borrelia genospecies.

Conclusion: Our study suggests that the tick, and not the pathogen, controls the early stages of host choice behaviour.

Figure 1

Proportion of active nymphs that chose the scented focal stick for the four trial types. The proportion of active I. ricinus nymphs that chose the focal stick scented with mouse odours is shown for each of the four trial types. The four trial types were: (A) March nymphs and odour from uninfected control mice (n = 10 trials), (B) April nymphs and odour from uninfected control mice (n = 10 trials), (C) April nymphs and odour from B. afzelii-infected mice (n = 10 trials), and (D) April nymphs and no mouse odour (n = 10 trials). Shown are the means and the 95% confidence limits.

Figure 2

Proportion of active nymphs that chose the scented focal stick for each Borrelia ecotype. The proportion of active I. ricinus nymphs that chose the focal stick scented with mouse odours is shown for each of the three groups of nymphs. The three groups were: uninfected nymphs (16.20% = 29/179), nymphs infected with the bird-specialist ecotype (21.05% = 16/76), and nymphs infected with the rodent-specialist ecotype (23.08% = 3/13). The differences in attraction to rodent odour between the three groups of nymphs were not statistically significant. Shown are the means and the 95% confidence limits.