The Role of Host Age at Exposure, Host–Parasite Genetics and Host Size‐Parasite Length Mechanics in Shaping the Outcome of Parasitic Infections in Three Zooplankton Taxa

ABSTRACT Understanding disease emergence and spread is a key challenge of evolutionary ecology, and there is growing evidence that host age at exposure is pivotal for improving our understanding. Previous studies of the Daphnia ‐ Pasteuria system found that younger hosts were more susceptible and produced more transmission stages than older ones, with the effects of earlier infection being more virulent. However, it is unknown whether these host age effects are general or specific to a particular host–parasite system, and if they are general, whether they exhibit the same pattern in magnitude and direction. Using three species of Daphnia ( D. magna , D. similis and D. curvirostris ) and two isolates of the pathogenic yeast Metschnikowia bicuspidata , we investigated the impact of host age at exposure on host susceptibility, parasite transmission and parasite‐induced host mortality (virulence). We found that host age at exposure affected the probability of becoming infected; although the three Daphnia species differed in the pattern and strength of infection. We also found that in all Daphnia species, older hosts produced more transmission stages than younger ones. Our results further show that host and parasite genetics, coupled with a mechanistic interaction between host body size and parasite spore length, strongly influence host and parasite traits. These results highlight the generality and diversity of host age effects and reinforce the need to incorporate them into epidemiological modelling in order to improve our response to an epidemic.