Causes and consequences of intra- and inter-host heterogeneity in defence against nematodes

Despite strong natural and artificial selection for increased resistance to nematode parasites, there is considerable heterogeneity between hosts in human, livestock and wildlife populations, with a minority of hosts carrying the majority of parasites. In addition, levels of defence may vary across the lifespan of individuals due to changes in their physiological state and infection history. Such variation influences nematode transmission dynamics and the evolution of parasite life-history strategies. Therefore, identifying sources of between- and within-individual variation in resistance and predicting their consequences is crucial for understanding the epidemiology of nematode parasitic diseases. In this review, several key sources of variation are identified, using examples from mouse models, immuno-epidemiological studies of human populations and observational and experimental studies of wildlife and livestock. The mutual applicability of approaches used across these study systems is emphasized, with the assertion that the concerted efforts of researchers from a range of disciplines will enable us to better understand the proximate and ultimate causes of variation in defence against nematode parasites. This will facilitate predictions of the epidemiological and evolutionary consequences of this variation, with the potential to improve disease treatment and management.