Host nutrition alters the variance in parasite transmission potential

The environmental conditions experienced by hosts are known to affect their mean parasite transmission potential. How different conditions may affect the variance of transmission potential has received less attention but is an important question for disease management, especially if specific ecological contexts are more likely to foster a few extremely infectious hosts. Using the obligate-killing bacterium Pasteuria ramosa and its crustacean host Daphnia magna, we analysed how host nutrition affected the variance of individual parasite loads, and therefore transmission potential. Under low food, individual parasite loads showed similar mean and variance, following a Poisson distribution. In contrast, among well-nourished hosts, parasite loads were right-skewed and over dispersed, following a negative binomial distribution. Abundant food may therefore yield individuals causing potentially more transmission than the population average. Measuring both the mean and variance of individual parasite loads in controlled experimental infections may offer a useful way of revealing risk factors for potential highly infectious hosts.