The research has been conducted at the Radboud University Medical Center in Nijmegen (Dr. Federica Giardina) in the Biostatistics group, department of Health Evidence, in collaboration with Dr. Poppy Lamberton at the University of Glasgow in the School of Biodiversity, One Health and Veterinary Medicine. The SchiSTOP project revolves around the development of an agent-based model (ABM) for the transmission dynamics of schistosomiasis. The ABM consistes of four main building blocks: the human population, the parasites living in the human host, the parasites living in the contaminated water environment, and the snail population. Briefly, the model includes: i) human demography (births, deaths, aging, and migration), ii) infection dynamics in human and intermediate hosts, including diagnostic schemes to assess infection in humans, iii) maturation, reproduction, and death of worms within the human host, iv) control interventions, such as MDA. Mechanisms regulating transmission have been considered at three different levels: worm (density dependent fecundity), human host (host-acquired immunity) and intermediate host (snail carrying capacity). Importantly, water contact data have been used to derive a new age-exposure function. We extracted frequency of water contacts adjusted for body surface and time of day of contact reported for three age groups (0-9, 10-19, 20+ years old) from an extensive data collection on direct water contact observations in a village in Northern Senegal. To better estimate exposure in the third and highest age-group, various age-exposure profiles from Kenya were used to inform the age at which we expected to show a decline in exposure. We calibrated the models to different settings and performed simulation studies to investigate the role played by the different regulating mechanisms and the extent to which they can explain different observed epidemiological patterns. After identifying the best model structures, we used the models to estimate the probability of reaching the WHO targets under different control strategies.
We find that regulating mechanisms alternative to the commonly assumed density dependence in worm fecundity can reproduce stable pre-control settings, including low endemicity settings. The dynamics in the snail population plays an important role in explaining sustained low transmission. All modelling scenarios agreed that treating school aged children (SAC) only with 10 years of annual MDA will not allow to reach the elimination of schistosomiasis as a public health problem (EPHP) in high or moderate endemicity settings, nor interruption of transmission in the low endemicity settings. Expanding MDA to all individuals older than 2 years of age, makes predictions much more optimistic.
The work performed by Federica Giardina (PI) and Veronica Malizia (PhD student) resulted in published papers, and more manuscripts submitted or in preparation. The results have been presented at national and international conferences, and during two invited talks. Also, a workshop on infectious disease models and a schistosomiasis expert meeting was organized on the topic of the action. SchiSTOP has also had the opportunity to be showcased in the Horizon magazine.
