Summary1. Resource quality can have conflicting effects on the spread of disease. High-quality resources could hinder disease spread by promoting host immune function. Alternatively, high-quality food might enhance the spread of disease through other traits of hosts or parasites. Thus, to assess how resource quality shapes epidemics, we need to delineate mechanisms by which food quality affects key epidemiological traits. 2. Here, we disentangle effects of food quality on 'transmission potential' -a key component of parasite fitness that combines transmission rate and parasite production -using a zooplankton host and fungal parasite. We estimated the components of transmission potential (i.e. parasite encounter rate, susceptibility and yield of parasite propagules) for hosts fed a highquality green alga and a low-quality cyanobacterium. 3. A focal experiment was designed to disentangle food quality effects on various components of transmission potential. The low-quality resource decreased transmission potential by stunting host growth and altering foraging behaviour. Hosts reared on low-quality food were smaller and had lower size-corrected feeding rates. Due to their slower grazing, they encountered fewer parasite spores in the water. Smaller hosts also had lower risk of an ingested spore causing infection (i.e. lower susceptibility) and yielded fewer parasite propagules. Hosts switched from high-to low-quality food during spore exposure also had low transmission potentialdespite their large size -because the poor quality resource strongly depressed foraging. 4. A follow-up experiment investigated traits of the low-quality resource that might have driven those results. Cyanobacterial compounds that can inhibit digestive proteases of a related grazer likely did not cause the observed reductions in transmission potential. 5. Our study highlights the value of using mechanistic models to pinpoint how resource quality can change transmission potential. Overall, our results show that low-quality resources could inhibit the spread of disease through effects on multiple components of transmission potential. They also provide insight into how disease outbreaks in wildlife may respond to shifts in resource quality caused by eutrophication or climate change.

Foraging-mediated hydras in epidemics

AbstractAll else equal, parasites that harm host fitness should depress densities of their hosts. However, parasites that alter host traits may increase host density via indirect ecological interactions. Here, we show how depression of infected host foraging rate can produce such a hydra effect. Using a foraging assay, we quantified reduced foraging rates of a zooplankton host infected with a virulent fungal parasite. We then parameterized a dynamical model of hosts, parasites, and resources with this foraging function, showing how foraging depression can create a hydra effect. Mathematically, the hydra arose when increased resource productivity exceeded any increase in resource consumption per host. Therefore, the foraging-mediated hydra effect more likely emerged (1) for hosts which strongly control logistic-like resources and (2) during larger epidemics of moderately virulent parasites. We then analyzed epidemics from 13 fungal epidemics in nature. We found evidence for a foraging-mediated hydra effect: large outbreaks depressed foraging rate and correlated with increased densities of both algae and hosts. Therefore, depression of foraging rate of infected hosts can produce higher host densities even during epidemics of parasites that increase host mortality. Such hydras might prevent collapse of host populations but also could produce higher densities of infected hosts.