The importance of abiotic context in the relationship between host traits and contribution to pathogen transmission

Background/Question/Methods

A host’s contribution to vector-borne pathogen transmission increases with the probability of infection (susceptibility), the probability of infecting new vectors (infectiousness), and vector performance on a host. These parameters vary both within and among host species, and have been correlated with host lifespan or metabolic rate in several observational studies. Often, these correlations are assumed to arise from genetic variation in host traits, but host traits can be highly plastic. If host traits are filtered by the environment, then observational approaches may confound genetic and plastic variation. For example, short-lived hosts may be more susceptible only when occupying relatively resource-rich environments. This could lead to incorrect predictions of pathogen transmission in novel environments. In experimentally controlled conditions, we factorially manipulated host species and resource supply, and assessed the relative importance of host identity and environment in explaining variation in three host epidemiological parameters: susceptibility, infectiousness, and vector performance. We also tested the influences of host lifespan (annual/perennial), and metabolic rate. Finally, we compared the strength of relationships between host traits and the epidemiological parameters in various scenarios of environmental filtering. 

Results/Conclusions

Across 23 grass host species of an aphid-borne Barley Yellow Dwarf Virus, host susceptibility, infectiousness, and vector production varied both within and across hosts, and were highly plastic in response to resource supply. Resource (nitrogen) supply was as or more important than host identity or lifespan in determining two out of the three epidemiological responses: host infectiousness and vector production. For host susceptibility, host identity and lifespan were more important than resource supply. In extant grasslands, resource supply filters host community composition such that perennial hosts persist mostly in resource-poor habitats, and annuals dominate resource-rich habitats. When annuals were observed at high nitrogen and perennials at low nitrogen, host susceptibility, infectiousness, and vector performance increased from perennials to annuals and with host metabolic rate (photosynthetic capacity). Correlations remained significant but weakened when hosts were observed across nitrogen treatments. Except for the relationship between host lifespan and susceptibility, correlations became insignificant when annuals were observed at low nitrogen and perennials at high nitrogen. By filtering the host community and cresting substantial plastic variation in host traits, environmental gradients may contribute to observed patterns of covariation in host traits and epidemiological parameters.