Poor resource quality lowers transmission potential by changing foraging behaviour

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. 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 high‐quality green alga and a low‐quality cyanobacterium. 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 potential – despite their large size – because the poor quality resource strongly depressed foraging. 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. 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.