Host resource levels influence the quantity but not quality of parasite spores

Background/Question/Methods

Host-parasite interactions are embedded within food webs that are dynamic, with factors such as resource levels fluctuating over time. These factors have the potential to modify host-parasite interactions, including a parasite's ability to infect a host, its performance within the host, and its reproductive output. Because resource levels can change rapidly over the course of epidemics (sometimes as a result of the disease outbreak), we were interested in understanding how key epidemiological traits of parasites responded to variation in resource availability experienced by their hosts (and indirectly by parasites themselves). We hypothesized that lower food concentration would cause reduction of i) quantity and/or ii) quality of produced spores, with impacts on the ability to infect hosts and the virulence of those infections. To test this, we reared two freshwater parasite species, a fungus Metschnikowia bicuspidata and a bacterium Pasteuria ramosa, in hosts (Daphnia dentifera) receiving low, medium or high food abundance. We quantified the number of propagules produced in each resource regime and used these spores to experimentally infect a new cohort of D. dentifera. We measured the prevalence of infection, as well as reproduction and lifespan of the infected hosts.

Results/Conclusions

For both parasite species, lowering the amount of food resources provided to the host substantially reduced parasite spore yield, but did not change infectivity or virulence. Thus, the main impact of variation in the amount of resources was on the quantity of spores rather than their quality. Overall, poor host nutrition decreases the fitness of its parasites; therefore, seasonal increases in food abundance for the host might fuel disease outbreaks. This study provides evidence that changes in the food web can indirectly modulate the magnitude and timing of epidemics.