UC Berkeley

Schistosomiasis is still prevalent in seven provinces in China despite several decades of continuous effort. Suppression of transmission in low-infection areas proves to be extremely difficult, and re-emergence of the disease in areas it was previously controlled or eliminated further complicates the situation. New surveillance and control strategies based on a better understanding of current transmission patterns are therefore crucial to the achievement of disease elimination. This dissertation explores these questions with the focus on the hilly and mountainous regions in Sichuan Province.

The main contents of this dissertation are centered on the development and simulation of a stochastic individually-based model. Focused on two cohorts representing the post-equilibrium and re-emerging environments, respectively, survey protocols and data description on human demographics, infection status, snail density, and cercarial mouse bioassay are presented and compared. A health education intervention was also introduced as an example of how individual data could be used for hypothesis testing related to model component determination. Upon development of the model, Monte-Carlo simulations were conducted to account for randomness from multiple sources including parasite acquisition, worm development, egg shedding and infection testing.

Surveillance results in the re-emerging villages indicated low sensitivity of current techniques for low infection-intensity environment. The distribution of individual susceptibility to S. japonicum infection appeared to be log-normally distributed with a high skewness. Given the cercarial density and water contact magnitude distributions of the surveyed populations, a considerable fraction of people had susceptibility levels that would not lead to infection. For infection surveillance purposes, sampling individuals with high susceptibility has markedly greater efficiency than other indices focused on either water or cercarial exposure. Based on these findings, priorities should be given to the development of more sensitive environmental monitoring and infection testing methods and exploration of an appropriate and quantifiable index for susceptibility. Environmental and economic modifications in addition to the traditional control methods need to be conceptualized and implemented to further conquer the disease and achieve comprehensive elimination.