Virulence of mixed fungal infections in honey bee brood

We show for the first time that single infection with a pollen fungus A. atra may induce some mortality and that co-infection with A. atra and A. apis resulted in higher mortality of honey bees compared to single infections with A. apis. However, similar single and mixed infections with A. proliperda did not increase brood mortality.Our results show that co-infection with a closely related fungal species can either increase or have no effect on host mortality, depending on the identity of the second species. Together with other studies suggesting that multiple interacting pathogens may be contributing to worldwide honey bee health declines, our results highlight the importance of studying effects of multiple infections, even when all interacting species are not known to be specialist pathogens.Variation in virulence (i.e. disease-induced host mortality) among pathogens is shaped by evolutionary pressure emanating from the combined life histories of pathogens and hosts [1]. However, virulence is difficult to predict when interactions are not restricted to a single host and pathogen, but involve multiple infections [2,3]. Conflict between co-infecting strains can lead to different within-host dynamics, with consequences for virulence, transmission and host resistance [4,5]. Examples illustrating aspects of this interaction-complexity are Daphia magna infections with bacteria Pasteuria ramosa and microsporidium Octosporea bayeri [6], helminths occupying mammalian guts [7], Plasmodium chabaudi clones infecting mice [8], and Metarhizium anisopliae var. anisopliae and Aspergillus flavus fungi infecting leaf-cutting ants [9]. Increased virulence of mixed infections can be due to increased pathogen densities [10-12] or to the host immune system being less efficient in clearing up multiple infections e.g. [13]. However, in some cases, the virulence of a mixed infection merely reflects the virulence of the most virulent strain/species e.g. [5,14,15]. The outcome of parasite i