Virulence of recurrent infestations with Borrelia-infected ticks in a Borrelia-amplifying bird.

Lyme disease cases caused by Borrelia burgdorferi s.l. bacteria is increasing steadily in Europe, in part due to the expansion of the vector, Ixodes ricinus. Wild reservoir hosts are typically recurrently infested. Understanding the impact of these cumulative parasite exposures on the host's health is, therefore, central to predict the distribution of tick populations and their pathogens. Here, we have experimentally investigated the symptoms of disease caused by recurrent infestations in a common songbird (Parus major). Birds were exposed three times in succession to ticks collected in a Borrelia endemic area. Health and immune measures were analyzed in order to investigate changes in response to tick infestation and Borrelia infection rate. Nitric oxide levels increased with the Borrelia infection rate, but this effect was increasingly counteracted by mounting tick infestation rates. Tick infestations equally reduced haematocrit during each cycle. But birds overcompensated in their response to tick feeding, having higher haematocrit values during tick-free periods depending on the number of ticks they had been previously exposed to. Body condition showed a similar overshooting response in function of the severity of the Borrelia infection. The observed overcompensation increases the bird's energetic needs, which may result in an increase in transmission events.