Deciphering the effect of food availability, growth and host condition on disease susceptibility in a marine invertebrate.

Food provisioning influences disease risk and outcome in animal populations in two ways. On the one hand, unrestricted food supply improves the physiological condition of the host and lowers its susceptibility to infectious disease, reflecting a trade-off between immunity and other fitness-related functions. On the other hand, food scarcity limits the resources available to the pathogen and slows the growth and metabolism of the host on which the pathogen depends to proliferate. Here, we investigated how food availability, growth rate and energetic reserves drive the outcome of a viral disease affecting an ecologically relevant model host, the Pacific oyster, Crassostrea gigas We selected fast- and slow-growing animals, and we exposed them to high and low food rations. We evaluated their energetic reserves, challenged them with a pathogenic virus, monitored daily survival and developed a mortality risk model. Although high food levels and oyster growth were associated with a higher risk of mortality, energy reserves were associated with a lower risk. Food availability acts both as an enabling factor for mortality by increasing oyster growth and as a limiting factor by increasing their energy reserves. This study clarifies how food resources have an impact on susceptibility to disease and indicates how the host's physiological condition could mitigate epidemics. Practically, we suggest that growth should be optimized rather than maximized, considering that trade-offs occur with disease resistance or tolerance.