Climate change stressors will place different selective pressures on both parasites and their hosts, forcing individuals to modify their life history strategies and altering distribution and prevalence of disease. Few studies have investigated whether parasites are able to detect host stress and respond by varying their reproductive schedules to favor their survival and fitness. This study compared both host and parasite life history parameters in unstressed and drought-stressed environments. We used the human parasite Schistosoma mansoni and its aquatic snail intermediate host Biomphalaria alexandrina, which live in areas predicted to face increased drought pressure. Schistosoma mansoni develops within the snail and then releases cercariae, the infective stage to humans and other vertebrates. To determine how drought stress impacts parasite and host life history we used four treatment groups consisting of unstressed-uninfected, unstressed-infected, stressed-uninfected, or stressed-infected snails. Snails were monitored weekly for mortality, reproduction (egg pouches), and growth (shell length) as well as parasite reproductive output (number of cercariae released from the host snail). We predicted that 1. Drought-stressed snails would have lower growth and higher reproduction than unstressed snails, and 2. Parasites in drought-stressed snails would produce more cercariae than parasites in unstressed snails.
Results/Conclusions
Unstressed snail hosts exposed to the parasite demonstrated a significant reproductive burst during the pre-patent period (fecundity compensation, p<0.005), but that response was absent in a drought-stressed environment. Overall, drought-stress reduced host reproduction. There were no significant differences in snail growth among treatment groups. We found that infections in drought-stressed snails had significantly higher parasite reproductive outputs than infections in unstressed snails (p<0.005). This study documents fecundity compensation by a parasite, in which the parasite can detect and respond to host stress by altering its life history to increase fitness. This finding suggests that climate change may alter the infection dynamics of this debilitating human parasite.