Different paths - the same virulence: experimental study on avian single and co-infections with Plasmodium relictum and Plasmodium elongatum.

Co-infections are prevalent worldwide, however, we are still struggling to understand interactions between different parasites and their impacts on host fitness. In the present experimental study we analysed the infection dynamics of two avian malarial parasites Plasmodium elongatum (genetic lineage pERIRUB01) and Plasmodium relictum (genetic lineage pSGS1) and their impacts on host health during single and co-infections. We reveal that P. elongatum intensity of parasitemia is enhanced by the presence of P. relictum during co-infection, while the parasitemia of P. relictum stays the same. This illustrates how development of a parasite (P. elongatum) which infects both mature and young (polychromatic) red blood cells (RBCs) is facilitated during co-infection with a parasite which specialises in adult RBCs only (P. relictum). The virulence of co-infections was similar to that of the more virulent parasite (P. elongatum). However, the profile of infection and the mechanisms that caused mortality were different. Birds infected with P. elongatum only start to die due to non-regenerative anaemia, when intensity of parasitemia is light and the number of polychromatic RBCs decrease dramatically. Meanwhile, co-infected birds start to die when the mean intensity of parasitemia reaches 10% and the number of polychromatic RBCs increases abnormally, reflecting regenerative anaemia. Our findings reveal that typically measured parameters of virulence (e.g., mortality rate, level of hematocrit) can be the same during single and co-infections, but the mechanisms responsible for the observed virulence can be different. This information serves a better understanding of the processes underpinning the interactions of co-infected parasite species.