Quality matters: Resources and disease dynamics in a planktonic host-parasite system

Background/Question/Methods: Why do epidemics end? We offer a potential answer to this perennially challenging question: increases in resource quality of hosts could actually inhibit disease -- at least in our study system. During fungal outbreaks in several, intensively sampled Daphnia populations, poor food conditions at the start of epidemics elevated as epidemics waned. Why would enhancement of food conditions for hosts catalyze termination of disease?

Results/Conclusions: For answers, we turned to short-term, lab-based experiments using field-collected and lab-reared algae (varying in species identity, phosphorus content, and/or fatty acid content) to examine links between food quality and core components of the host-parasite interaction. These experiments showed that increases in food quality enhance production of spores per infected host. This step is vital for spread of this free-living parasite because Daphnia become infected after eating spores released from dead hosts. However, improved food quality (usually) decreased transmission rate. Thus, variation in food quality imposed a negative relationship between spore production and transmission. An epidemiological model that tracks dynamics of free-living parasites and hosts (susceptible and infected classes) illustrated how this quality-induced tension between transmission and spore production could explain the field patterns. Improvements in food quality for hosts inhibit disease when the lowered transmission effect outweighs improvements in spore production. Conversely, poor resource quality could inhibit disease spread if high transmission rates are not accompanied by sufficient production of parasite to sustain the epidemic. Both conclusions highlight the potential for resource dynamics of hosts to influence outcomes of disease.