Interactions between parasites within the host: helminth infection reduces lice abundance in feral pigeons ( Columba livia )

Within a host, parasites form a community and interact with each another. They can interact directly through competition for space or indirectly through competition for resources, or even via the host’s immune response. These interactions can influence infection patterns within the host. In particular, helminths are capable of facilitating the infection of other parasites through their immunosuppressive effects or hindering it by activating immune responses that reduce the ability of other species to infest the host.

In this study, we aimed to test the existence of parasite interactions in the wild pigeon host ( Columba livia ). More specifically, we experimentally tested whether helminth infestation influences the infestation dynamics of ecto- and endoparasites in pigeons.

Our predictions were tested on a sample of pigeons captured in Paris, half of them were exposed to an anthelmintic treatment. We then assessed the effects of this deworming treatment on the abundance of blood parasites (hemosporidian parasites, Haemoproteus spp., Plasmodium spp., and Leucocytozoon spp.), lice ( Columbicola columbae and Campanulote compar ), as well as on the intensity of antiparasitic behavioral responses (preening).

We detected a negative relationship between helminth presence and lice abundance, with higher lice abundance when helminths were reduced (with anthelmintic treatment) and lower lice abundance when helminths were more abundant (without anthelmintic treatment), indicating inhibitory effects of helminths on ectoparasite abundance. However, these results were not supported by differences in preening activity between the two experimental groups, suggesting that other mechanisms are involved. Our results showed no effect of anthelmintic treatment on the abundance of blood parasites. The mechanism by which helminths induced ectoparasite repulsion remains to be demonstrated.

Our study highlights the importance of considering interactions between different types of parasites in an eco-epidemiological approach when taking into account the factors affecting parasite prevalence and abundance.