Juvenile hosts and natal dispersers are protected in the early stages of epidemics

Parasite prevalence varies in time and space. Thus, hosts may escape infection by dispersing out of habitats where parasites are present. However, it is not clear if the advantage of avoiding parasites outweighs the cost of dispersing. Juvenile hosts are expected to be relatively protected from environmentally-transmitted parasites, and we hypothesize that this age bias in transmission could magnify the benefits of juvenile (i.e., natal) dispersal. We tested these ideas in the model nematode Caenorhabditis elegans, a host with discrete life stages and natal dispersal, and its environmentally transmitted microsporidian parasite Nematocida parisii. We found that under standardized exposure conditions, larger C. elegans individuals (corresponding to older life stages) acquired many more parasites than smaller (younger) individuals. We found this same bias during multigeneration epidemics, especially during early stages of the epidemics. We also found that C. elegans dispersal larvae were less likely to be infected and harbored less severe infections than the population mean. We conclude that the early stages of an epidemic can provide young hosts with a window of opportunity to escape infection by dispersing.