One man's meat is another man's poison: exploring the effect of mass drug administration against schistosomes on the aquatic food web.

Parasites are an integral but under-recognised component of ecosystems. In particular, within food webs they can alter chain length and connectance and, because they act across trophic levels, they can increase web stability. In aquatic food webs the parasitic trematodes are a particularly important group, with aquatic molluscs, commonly acting as their intermediate hosts. Within these molluscan hosts the asexual reproductive phase of the parasites' life cycle occurs, characterised by a rapid and substantial increase in parasite biomass, as the definitive host infective stages are produced. These trematodes are therefore important, because of the substantial effects they have upon their host molluscs, which are themselves an important link between the primary producers and the higher trophic levels, and because of the substantial output of infective stages from the molluscs. These infective stages form a significant component of the zooplankton, an essential food source for juvenile fish and other higher organisms. Interestingly, the parasites can also affect one another, as different species can compete for resources inside coinfected molluscan hosts, so that any change in abundance of one parasite species can have ripple effects upon its competitors. Among the trematodes, the Schistosomatidae are a significant family, within which the genus Schistosoma is notable, as it is species from this genus that are a serious threat to human health in Low to Middle Income Countries (LMIC), with over 220 million people infected each year. Due to the health and economic importance of this genus, mass drug administration is frequently used to control parasites. Such mass drug administration programmes stop, or very substantially reduce, the output of mollusc infective stages into the aquatic environment. The unexplored consequences of these substantial and repeated parasite perturbations, upon aquatic food web structure and / or function, will be the focus of this PhD. Specifically, we aim to determine:
1) What role schistosomes play in the aquatic food web of Lake Victoria.
2) To what extent mass drug administration of humans alters the food web structure and trophic dynamics.
3) How long-lasting any perturbations are within the food web.
4) Whether we can leverage this new understanding of parasites in food web ecology to improve aquatic ecosystem health, including decreasing schistosome transmission.