Investigating the spatial and temporal epidemiology of Schistosoma species infection within school-aged-children along the southern shoreline of Lake Malawi

Reed, Amber and Jewell, Christopher and Stothard, J. Russell and Fronterre, Claudio and Stanton, Michelle (2024) Investigating the spatial and temporal epidemiology of Schistosoma species infection within school-aged-children along the southern shoreline of Lake Malawi. PhD thesis, Lancaster University.

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Abstract

Schistosomiasis (Bilharzia) is a focal water-borne neglected tropical disease (NTD) caused by trematodes of the genus Schistosoma. In Africa schistosomes utilise freshwater intermediate snail host species of Bulinus and Biomphalaria. As with many NTDs, fine-scale data on schistosomiasis is sparse, needing application of advanced quantitative methods to better infer spatial distributions and demographic associations. This thesis applies quantitative methods to interpolate available epidemiological data from Lake Malawi to explore in greater detail the spatial and temporal epidemiology of intestinal schistosomiasis (IS) and urogenital schistosomiasis (UGS), and their snail hosts. Through applying spatial and dynamical modelling methods to a newly emerging focus of IS, concurrent within an area for UGS, peak age infection prevalence was shown to be 11 years of age for IS and IS/UGS co-infection, considerably younger than that previously reported. Using remote sensing, geostatistical analyses provided insight into snail species abundance along the shoreline, noting environmental associations, revealing substantial heterogeneities but identifying snail hotspots. To better understand transmission by replicating the previous age profiles, an age-structured SEIRS (Susceptible-Exposed-Infectious-Recovered-Susceptible) transmission model was constructed; with age-related immunity and the effect of exposure to snail populations considered, followed by an optimisation of the model parameters. From this, “snail exposure” was judged not to be simply due to proximity to “snail habitat”. These findings add quantitative insight to the Lake Malawi shoreline setting and may later support World Health Organisation guideline development in criteria for interruption of schistosomiasis transmission. Future work should consider information from longitudinal cohorts, rather than cross-sectional studies, when attempting to model and quantify fine-scale transmission heterogeneities and putative impacts of current control interventions.