Mathematical modelling of the transmission dynamics of opisthrochiasis

The trematode liver fluke (flat worm), Opisthorchis viverrini, is prevalent in southeast Asia, causing the chronic hepatobiliary disease, opisthorchiasis. Long term infection can lead to the bile duct cancer, cholangiocarcinoma, which is typically fatal. We develop an ordinary differential equation (ODE) model of the transmission dynamics of O. viverrini through its life cycle in snails, fish, and humans; a second ODE model that includes potential transmission from reservoir hosts such as domestic cats and dogs; and a third partial differential equation (PDE) model that includes heterogeneity in age in humans. We calibrate these models to data collected from two communities in Khong Island in Southern Lao PDR, using maximum likelihood estimates and Bayesian sampling-resampling methods. We define basic reproduction numbers and type-reproduction numbers for these models to show that humans can maintain the transmission cycle through snails and fish, so interventions targeting humans with a sufficiently high coverage could eliminate transmission. Numerical simulations suggest that, as compared to improved sanitation and behaviour change campaigns, treating humans at least once a year increases the probability of achieving elimination and reduces the time to elimination. We finally develop a stochastic individual-based model that better captures heterogeneity in the intensity of infection in the human population and its subsequent impact on morbidity.