Geographical variation in blood parasites in feral pigeons: the role of vectors

Prevalence and intensity of blood parasites are known to vary in space with in a same species, yet the causes underlying such variation are poorly know n. Theoretically, blood parasites variation can be attributed to difference s to exposure to parasite vectors and/or to differences in host susceptibil ity. Here, we show that prevalence of Haemoproteus columbae in feral pigeon s Columba livia varied among five near-by populations (range 15%-100%), par alleled by variation in the abundance of its main vector, the louse flies P seudolynchia canariensis. Geographic variation in intensity of blood parasi tes did not covary with abundance of vectors. Within populations, older ind ividuals had a higher probability of being parasitized than younger ones, w hereas younger birds, when infected, suffered higher intensities. Furthermo re, we found no evidence of sex-related differences neither in prevalence n or in intensity of blood parasite infections. To demonstrate that geographi cal variation in prevalence was actually due to differences in vector expos ure, we conducted two experiments based on translocation of unparasitized p igeons From a vector-free area to an area where both the parasite and vecto r were abundant. With the first experiment, we demonstrated that unparasiti zed pigeons were not resistant to the parasite because when transmission wa s possible pigeons became parasitized in a few months. With the second expe riment, in which half of the pigeons were prevented from contacts with the vector, we ruled out the possibility that pigeons we considered as unparasi tized would have suffered from patent infections. Therefore, both observati onal and experimental evidence supports the view that vector abundance is t he major factor influencing the spatial variation in prevalence of H. colum bae in pigeons.