Disease ecologists have historically focused on understanding how a single pathogen causes widespread mortality of multiple hosts. However, evidence suggests that hosts are regularly infected by multiple pathogens (i.e., coinfection), which have been implicated in drastic host declines across multiple ecosystems. The goal of our project was to understand the disease ecology of coinfections within hosts using an experimental set-up. Specifically, we asked if order of infection affects host feeding rates, growth rates, and immunological responses. We used 650 pacific tree frog, Pseudacris regilla, tadpole hosts, which were placed in either uninfected (control) or three infected (treatment) groups by exposing them to one of three infectious agents: ranavirus, Batrachochytrium dendrobatidis (Bd), or trematode cercariae. We then exposed tadpoles to one of the two remaining infectious agents. We expected the interactive effect (facilitative, antagonistic, or neutral) between the primary and secondary parasites to depend on if the primary parasite stimulates or inhibits host immune response. We euthanized, dissected, and sampled host tissues related to ranavirus, Bd, and trematode intensities from 25-40 individuals per day. We also examined blood samples for the abundance of lymphocytes, basophils, eosinophils, and neutrophils to measure immunological responses.
Results/Conclusions
Regarding host feeding rates, there was no significant difference in the quantity of food all infected individuals consumed when compared to uninfected individuals. However, there were significant differences between the growth rates of all infected individuals, whether individuals experienced single infections early in development, late in development, or were co-infected (p < 0.05). Tadpoles infected with trematode cercariae late in development experienced the largest negative growth rates (i.e. they grew smaller) compared to all other infection combinations. Individuals that were exposed to single infections early or late in development showed a decrease in lymphocyte production as well as a decrease in total immunological response (lymphocyte, basophil, eosinophil, and neutrophil counts combined) while co-infected individuals showed an increase in lymphocyte production and total immunological response. However, the differences between the immunological response variables were not significantly different between early single infections, late single infections, and coinfections. In summary, tadpoles grow less when infected during later developmental stages as well as when they are co-infected. While immunological responses did not differ significantly among single infections and coinfections, we observed a consistent pattern of individuals allocating energy away from growth to combat infections by ranavirus, Bd, or trematode cercariae.