Predicting the environmental suitability and population at risk of podoconiosis in Africa

Abstract

Podoconiosis is a type of tropical lymphedema that causes massive swelling of the lower limbs. The disease is associated with both economic insecurity, due to long-term morbidityrelated loss of productivity, and intense social stigma. The geographical distribution and burden of podoconiosis in Africa are uncertain. We applied statistical modelling to the most comprehensive database compiled to date to predict the environmental suitability of podoconiosis in the African continent. By combining climate and environmental data and overlaying population figures, we predicted the environmental suitability and human population at risk of podoconiosis in Africa. Environmental suitability for podoconiosis was predicted in 29 African countries. In the year 2020, the total population in areas suitable for podoconiosis is estimated at 114.5 million people, (95% uncertainty interval: 109.4-123.9) with 16.9 million in areas suitable for both lymphatic filariasis and podoconiosis. Of the total 5,712 implementation units (typically second administrative-level units, such as districts) defined by the World Health Organization in Africa, 1,655 (29.0%) were found to be environmentally suitable for podoconiosis. The majority of implementation units with high environmental suitability are located in Angola (80, 4.8%), Cameroon (170, 10.3%), the DRC (244, 14.7%), Ethiopia (495, 29.9%), Kenya (217, 13.1%), Uganda (116, 7.0%) and Tanzania (112, 6.8%). Of the 1,655 environmentally suitable implementation units, 960 (58.0%) require more detailed community-level mapping. Our estimates provide key evidence of the population at risk and geographical extent of podoconiosis in Africa, which will help decision-makers to better plan more integrated intervention programmes.

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“…The data sources and development of this model have been described elsewhere. 8 Elevation and annual precipitation were the strongest predictors within the model, with the highest suitability predicted in areas with 1000– 1500 mm annual precipitation and elevation >1000 m above sea level. Other environmental predictors included soil characteristics (clay and silt fractions) and soil acidity of the topsoil, the mean land surface temperature, distance to the nearest body of water and enhanced vegetation index, a measure of vegetation cover.…”

“…Given the paucity of routine data on podoconiosis, population-based surveys combined with environmental modelling have become the mainstay of ongoing global efforts to estimate the burden and map the distribution of the disease. 8 Surveys in Cameroon, Ethiopia and Rwanda have found a prevalence of between 0.06 and 4.05% at the national level, and higher within barefoot populations. 9–11 Predictive models informed by empirical data from these surveys have revealed strong environmental associations, with the potential ecological niche mostly restricted to remote rural areas and characterised by annual precipitation levels and elevation and a lesser influence of vegetation, topography, hydrology and soil factors.…”

“… 12 Extrapolation of this niche across the African continent suggests that 114.5 million people in Africa live in areas suitable for the disease. 8 …”

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“…For other endemic countries, prevalence estimates are based on small focal studies [ 9 ]. The East African region has the highest environmental probability for podoconiosis occurrence and is home to 81.7% of the 114.5 million people at risk of developing podoconiosis [ 12 ]. Geographical overlap between podoconiosis and lymphatic filariasis has also been highly predicted among East African countries [ 12 ].…”

“…The East African region has the highest environmental probability for podoconiosis occurrence and is home to 81.7% of the 114.5 million people at risk of developing podoconiosis [ 12 ]. Geographical overlap between podoconiosis and lymphatic filariasis has also been highly predicted among East African countries [ 12 ]. People living with podoconiosis experience reduced quality of life as a result of restricted community participation, reduced productivity, discrimination, marginalization, low marriage prospects, psycho-social problems (depression, stress, and anxiety) and chronic poverty [ 13 – 15 ].…”