Infection with Borrelia afzelii reduces moulting time of Ixodes ricinus ticks

Arthropod vectors carry vector-borne pathogens that cause infectious disease in vertebrate hosts, and arthropod-associated non-pathogenic microorganisms. Both types of micro-organisms can influence the fitness of their arthropod vectors, and hence the epidemiology of vector-borne diseases. The bacterium Borrelia afzelii , which causes Lyme disease in Europe, is transmitted among vertebrate reservoir hosts by Ixodes ricinus ticks, which also harbour a large diversity of non-pathogenic bacteria. The purpose of this study was to test experimentally whether B. afzelii and tick-associated non-pathogenic bacteria influence the fitness of I. ricinus . Eggs obtained from field-collected adult female ticks were washed with bleach, which reduced the abundance of non-pathogenic bacteria in the hatched I. ricinus larvae by 28-fold compared to larvae that hatched from eggs washed with water. The dysbiosed and control larvae were subsequently fed on B. afzelii -infected or uninfected control mice, and the engorged larvae were allowed to moult into nymphs under laboratory conditions. I. ricinus larvae that fed on B. afzelii -infected mice had a significantly faster larva-to-nymph moulting time compared to larvae that fed on uninfected control mice. In contrast, we found no evidence that B. afzelii infection or larval dysbiosis influenced the four other life history traits of the immature I. ricinus ticks, which included engorged larval weight, unfed nymphal weight, larva-to-nymph moulting success, and immature tick survival. A power analysis found that our sampling effort had sufficient power (>80%) to detect small effects (difference of 5% – 10%) of either B. afzelii infection or larval dysbiosis. Under the laboratory conditions of our study, B. afzelii appears to be a mostly passive passenger in I. Ricinus ticks.