Infectious dose affects the outcome of the within-host competition between parasites.

The ecological and epidemiological processes underlying the success of parasites competing within individual hosts are not yet clear. We investigated one idea: that increasing one parasite's infectious dose might decrease the success of its competitor. We reared uninfected larvae of the mosquito Aedes aegypti and exposed them to two concentrations of the microsporidium Vavraia culicis and of the protozoan Ascogregarina culicis. The rate at which Vavraia produced its infectious spores depended on its dose and that of its competitor: when the dose of Vavraia was high, only the higher dose of Ascogregarina slowed the production of spores, but when the dose was low, either dose of Ascogregarina did. Ascogregarina was least likely to produce oocysts when its competitor's dose was high and the mosquito was reared with little food. This was due to the mosquito's preadult mortality induced by Vavraia. Our results show that increasing the dose of a parasite can increase its deleterious effects on a coinfecting parasite. Since dose increases with a parasite's prevalence, such dose effects could lead to an epidemiological feedback that ultimately eliminates one of the parasites.