Worse off together: consequences of microsporidian prior exposure for virus infection outcomes and bumble bee host health

Host-parasite interactions do not occur in a vacuum, but in connected multi-parasite networks. Resulting co-exposures and coinfections during an individual host’s lifetime can affect host health and infectious disease ecology, including disease outbreaks. However, many host-parasite studies examine pairwise interactions, meaning we still lack a general understanding of the influence of co-exposures and coinfections. Using the bumble bee Bombus impatiens, we study the effects of larval exposure to a microsporidian Nosema bombi, implicated in bumble bee declines, and adult exposure to Israeli Acute Paralysis Virus (IAPV), an emerging infectious disease from honey bee parasite spillover. We hypothesize that infection outcomes will be modified by co-exposure or coinfection depending on relevant temporal interactions, due to changes in host immune allocation or condition. Nosema bombi is a potentially severe, larval-infecting parasite, and we predict that prior exposure will result in decreased host resistance to adult IAPV infection. We predict a double exposure will also reduce host tolerance, as measured by host survival. Although our larval Nosema exposure mostly did not result in viable infections, it reduced resistance to adult IAPV infection. Exposure to Nosema also negatively affected survival, potentially due to a cost of immunity in resisting the exposure. There was also a significant negative effect of IAPV exposure on survivorship, but in contrast to resistance, prior Nosema exposure did not alter this survival outcome. These results again demonstrate that infection outcomes within multi-parasite host networks can be non-independent, even when exposure to one parasite does not result in a substantial infection.