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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.
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