UC Berkeley

Schistosomiasis is the second most debilitating parasitic disease of humans in the world, trailing only malaria in terms of its harmful effects on global human health. Schistosomiasis is caused by infection with trematode parasites of the genus Schistosoma that are transmitted via intermediate host snails and thrive in areas of the world which lack adequate access to safe water, sanitation, and hygiene. One of many neglected tropical diseases (NTDs), schistosomiasis is slated for elimination in the coming decades due to the availability of cheap and effective drugs to cure the disease. Like other diseases caused by helminthic parasites, schistosomiasis transmission is hypothesized to be limited by positive density dependent mate limitation, giving rise to a transmission breakpoint below which elimination becomes inevitable. This breakpoint represents a logical target for elimination campaigns, but there has been little effort devoted to empirically estimating the breakpoint population size of schistosomes. Furthermore, there has been limited investigation of the implications of breakpoints for elimination strategies.

This dissertation seeks to fill these research gaps, using mathematical modeling and statistical analysis to investigate the breakpoint dynamics of schistosomiasis for the first time. Chapter 1 contextualizes schistosomiasis transmission, providing background on associated pathology and global burden, the schistosome life cycle, and the influence of positive density dependence on schistosomiasis transmission. Chapter 2 explores the dynamics of schistosome aggregation, a key determinant of the strength of positive density dependence, and its implications for the breakpoint. Chapter 3 proposes a novel intervention, the use of prawn aquaculture as a biocontrol agent and a source of revenue in impoverished areas, to combat schistosomiasis. Chapter 4 introduces a modeling framework and a novel, individual-based model of schistosomiasis to estimate schistosomiasis transmission breakpoints and to simulate intervention strategies including mass drug administration, snail control, snail habitat reduction, sanitation, and education to identify strategies that most effectively suppress transmission below the breakpoint. Chapter 5 concludes with commentary on the future of schistosomiasis control and elimination, some reflections on the strengths and limitations of the work, and future directions for the ideas introduced here.