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Seasonal epidemics erupt commonly in nature and are driven by numerous
mechanisms. Here, we suggest a new mechanism that could determine the size
and timing of seasonal epidemics: rearing environment changes the
performance of parasites. This mechanism arises when the environmental
conditions in which a parasite is produced impact its
performance—independently from the current environment. To illustrate the
potential for ‘rearing effects’, we show how temperature influences
infection risk (transmission rate) in a Daphnia-fungus disease system
through both parasite rearing temperature and infection temperature.
During autumnal epidemics, zooplankton hosts contact (eat) fungal
parasites (spores) reared in a gradually cooling environment. To delineate
the effect of rearing temperature from temperature at exposure and
infection, we used lab experiments to parameterize a mechanistic model of
transmission rate. We also evaluated the rearing effect using spores
collected from epidemics in cooling lakes. We found that fungal spores
were more infectious when reared at warmer temperatures (in the lab and in
two of three lakes). Additionally, the exposure (foraging) rate of hosts
increased with warmer infection temperatures. Thus, both mechanisms cause
transmission rate to drop as temperature decreases over the autumnal
epidemic season (from summer to winter). Simulations show how these
temperature-driven changes in transmission rate can induce waning of
epidemics as lakes cool. Furthermore, via thermally-dependent
transmission, variation in environmental cooling patterns can alter the
size and shape of epidemics. Thus, the thermal environment drives seasonal
epidemics through effects on hosts (exposure rate) and the infectivity of
parasites (a rearing effect). Presently, the generality of parasite
rearing effects remains unknown. Our results suggest that they may provide
an important but underappreciated mechanism linking temperature to the
seasonality of epidemics.
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