Individuals vary greatly in their resistance to disease, even within a single population. This variation may have a large effect on the severity and duration of epidemics. According to the “dilution effect” hypothesis, unsuitable hosts that consume the infective stage of a parasite or interact with a vector without becoming infected may decrease the prevalence of infection among suitable hosts. While much support for the dilution effect hypothesis has come from studies of vector-borne diseases, recent studies have also shown a between-species dilution effect for non-vector-borne diseases, including a Daphnia-host, yeast-parasite system. We used a Daphnia dentifera-host, yeast-parasite system to investigate whether a dilution effect can occur between two clones of the same species. Daphnia hosts ingest the infective spores of the parasite while feeding, and clones differ substantially in their resistance. To test for a within-species dilution effect, we performed two separate experiments using one clone known to be highly susceptible to infection by this parasite and two different clones known to be relatively resistant to the parasite. In each experiment, we exposed different proportions of the susceptible clone and one of the resistant clones to parasite spores, with the same density of D. dentifera in each treatment.   

Results/Conclusions

We found substantial variation among treatments in the overall infection prevalence. As expected, the treatment containing only the susceptible clone had the highest infection prevalence, and the treatment with only the resistant clone had the lowest level of infection. A within-species dilution effect would be demonstrated by a non-linear relationship between the proportion of the resistant clone and the prevalence of infection. However, preliminary results indicate that the relationship is linear, which suggests that the presence of resistant conspecifics does not produce a within-species dilution effect in this system. This means that, while there is substantial variation in resistance among individuals in a population, the resistant genotypes may not protect the susceptible genotypes from infection.