LB 12-157 - CANCELLED - Spore density alters within-host interactions between parasites, with consequences for parasite prevalence and host abundance

Background/Question/Methods:

Parasite dose and the presence of multiple parasite species both have important consequences for infection dynamics, but it is not clear how these aspects of infection biology interact — though they likely do. We disentangled these effects and their interactions by conducting a factorial dose experiment and using it to parameterize dose-dependent multi-parasite epidemic models.



The parasites in our study system (one bacterium and one fungus) form environmental spores that degrade at variable rates; this creates ranges of parasite doses over time and space. We used factorial exposures to quantify how different combinations of spore doses from the two parasites impacted likelihood of infection, host lifespan, and transmission potential. We used this data to parameterize epidemic models with and without the measured dose-dependent effects, and in one- versus two-parasite scenarios. Comparisons between these simulations allow us to detect emergent dose-dependent properties and/or interspecific interaction effects.

Results/Conclusions:

We found bacterial infection was strongly influenced by both bacterial dose (which positively related to infection) and fungal dose (with higher fungal doses reducing bacterial infections). In contrast, fungal likelihood of infection was determined by fungal dose alone. Impacts on host lifespan and parasite fitness were strong, though dose independent: infection with the fungus significantly shortened host lifespans and, when co-infecting with the bacterial parasite, drastically reduced bacterial fitness.

Epidemic models revealed that, while the bacterium suffered a fungus-dose-dependent reduction in its probability of infection, this negative effect was mitigated at the level of host populations. In fact, when this negative effect on the bacterium was removed from the model, total bacterial prevalence further declined and host populations increased. Overall, we found that multi-parasite scenarios were detrimental to the bacterium (and had little effect on the fungus), but dose-dependent interspecific interactions mitigated that harm and resulted in higher parasite prevalence and lower host density than would have occurred otherwise. Ultimately, our work underscores how negative dose-dependent interactions between parasites can scale up to alter parasite burdens and host abundance in unexpected ways.