What within-hosts mechanisms lead to coexistence of parasites versus priority effects (alternate stable states)? We draw lessons from classic consumer-resource models to build a theoretical framework to understand the role of immunity and resources in mediating competition between parasites.
The ubiquity of multiple infections and its consequences for disease outbreak has led to examination of within-host competitive interactions. Specifically, studies have focused on outcomes of within-host competition when two parasites infect hosts simultaneously versus sequentially, mimicking natural variation in timing of infection. The results from these studies have had mixed outcomes (coexistence, priority effect, or no effect) with no clear demonstration of the mechanism leading to these inconsistent results. Predictive insight depends on mechanisms of competition involving host-immunity, and/or host resources between parasites. However, these has been very little empirical testing, or even theoretical knowledge on how these may serve to explain these outcomes. We propose to build a theoretical model where the interplay of parasites and host modules (immunity and resources uptake) can better explain these anomalies of co-infection studies. We draw parallels from classic consumer-resources models, particularly the intraguild predation and keystone predation models to build a framework for within-host competition. Specifically, we ask: What interactions between the parasites and the host modules lead to parasite coexistence, priority effects, or the host clearing out these pathogens?
Results/Conclusions
We used feedback loops to analyze within-host competition in a 2-parasite system engaged in exploitative competition that interact with shared resources and immunity. Hosts could either allocate baseline energy, or not towards immunity and were compared to the keystone predation model. Analysis indicates that loop schemes governing instabilities in the within-host models and keystone predation models are identical. Specifically, if either or both parasites have a positive effect on immune cells (leading to enhanced immunity) that can clear out the other parasite, relative to their impact on resources then strong positive feedbacks will be generated. These positive feedback that lead to alternate stable states are typically found at high resource gradients. Using 2D bifurcation diagrams, we show how interplay of resources gradients and parasite feeding rates can lead to regions of parasite coexistence, alternate-stable states, or host clearance. In conclusion, we propose a framework to better understand outcomes of co-infection studies by analyzing its within-host modules.