Predictive Risk Mapping of Schistosomiasis in Madagascar Using Ecological Niche Modeling and Precision Mapping

Abstract

Schistosomiasis is a neglected tropical disease (NTD) found throughout tropical and subtropical Africa. In Madagascar, the condition is widespread and endemic in 74% of all administrative districts in the country. Despite the significant burden of the disease, high-resolution risk maps have yet to be produced to guide national control programs. This study used an ecological niche modeling (ENM) and precision mapping approach to estimate environmental suitability and disease transmission risk. The results show that suitability for schistosomiasis is widespread and covers 264,781 km² (102,232 sq miles). Covariates of significance to the model were the accessibility to cities, distance to water, enhanced vegetation index (EVI), annual mean temperature, land surface temperature (LST), clay content, and annual precipitation. Disease transmission risk is greatest in the central highlands, tropical east coast, arid-southwest, and northwest. An estimated 14.9 million people could be at risk of schistosomiasis; 11.4 million reside in rural areas, while 3.5 million are in urban areas. This study provides valuable insight into the geography of schistosomiasis in Madagascar and its potential risk to human populations. Because of the focal nature of the disease, these maps can inform national surveillance programs while improving understanding of areas in need of medical interventions.

Keywords: disease mapping; ecological niche modeling; geographic information science; precision public health; schistosomiasis.

Figure 1

Geographic distribution of occurrence data (n = 231) in comparison to the predominant topographic characteristics of Madagascar. The model calibration area (M) is visualized as 40-km buffers (black).

Figure 2

Variable contribution for the weighted mean suitability model.

Figure 3

Environmental suitability of schistosomiasis in Madagascar (A). The estimated weighted sum of predictions (weighted mean) (B). Model uncertainty based on the coefficient of variation (CV). The filtered occurrence records (red) are superimposed, including the aquatic snails Biomphalaria and Bulinus.

Figure 4

Schistosomiasis disease exposure risk. The color scale from orange to dark red corresponds to medium, high, and very high exposure risk, while values from yellow to grey represent low–very low risk.

Figure 5

The environmental suitability of schistosomiasis in Madagascar based on a binary threshold value of 0.478. Level 2 classifications represent administrative boundaries according to the Database of Global Administrative Areas (https://gadm.org/) (access date: 2 November 2021). Yellow dots represent the study occurrence data (n = 231).