Host susceptibility is altered by light intensity after exposure to parasites

Translating research advances to natural systems using experimental laboratory studies is often difficult because of the variability between the natural environment and experimental conditions. Because environmental conditions have a large effect on an organism's physiology, responses to stressors like nutrient limitation, temperature, oxygen deprivation, predation, and parasite/pathogen infection are likely to be context dependent. Therefore, it is essential to examine the impact the study environment has on the experimental outcome. Here, we explored the effect of light exposure on susceptibility to parasite infection. The Biomphalaria glabrata / Schistosoma mansoni study system is a well-established model for studying schistosomiasis. It has been general practice to maintain the vector, B. glabrata, in dark conditions after exposure to miracidia of the human pathogen S. mansoni. We evaluated susceptibility of B. glabrata to S. mansoni under 3 different light conditions during the prepatent period, light (125 lx) on a 12-12 cycle, dim light (3 lx) on a 12-12 cycle, and no light (24 hr at 0 lx). We hypothesized that stress due to photoperiod disruption (24 hr of darkness) would result in compromised immune function and lead to higher susceptibility to infection. Prevalence of infected snails differed significantly between the light conditions, and higher susceptibility was observed in the full light and complete dark conditions compared with the low light conditions. The dim conditions are representative of current methods for evaluating susceptibility in this system. Our results indicate that light exposure during the prepatent period can affect infection outcomes, and environmental conditions must therefore be considered when assessing fitness and immune response due to interactions between host genotype and environment.