# Associations between podoconiosis and pedogenic factors globally – A systematic review

## TL;DR

This review explores the link between podoconiosis, a soil-related tropical disease, and factors like soil minerals and climate to better understand its causes.

## Contribution

This is the first global systematic review linking pedogenic factors to podoconiosis, identifying key soil minerals and elements associated with the disease.

## Key findings

- Nine studies found a correlation between podoconiosis and alkalic volcanic bedrock regions.
- Six studies linked pedogenic factors like altitude and rainfall to podoconiosis occurrence.
- Soil minerals such as phyllosilicate clays, quartz, and trace elements like iron and zirconium were associated with endemic regions.

## Abstract

Podoconiosis is a non-infectious neglected tropical disease that causes progressive swelling of the lower limbs in an estimated 4 million people globally. Podoconiosis has been associated with prolonged exposure to certain soils, however no specific causative component has been identified. We conducted a systematic review of articles to investigate links between podoconiosis and mineral, geochemical and climate factors affecting soil genesis (pedogenesis).

A systematic search was conducted across five electronic databases: Embase, Global Health, Medline, GeoRef and Web of Science. Searches were from database inception to January 2024. Primary quantitative research from any region was included, qualitative studies and those not in English were excluded. The AXIS tool was used to assess study quality and risk of bias. Twenty-seven studies were included and narrative synthesis was used to interpret data from tissue samples, soil samples, and remote sensed bedrock and pedogenic factors. Nine studies found a correlation between podoconiosis occurrence and regions with underlying alkalic volcanic bedrock, and six linked pedogenic factors (altitude and rainfall) with disease occurrence. Several studies linked specific soil mineralogy and geochemistry with endemic regions, including an abundance of phyllosilicate clay minerals, quartz, and trace elements, notably iron, beryllium and zirconium.

This systematic review is the first to be conducted on the literature linking pedogenic factors with podoconiosis globally. Study quality was low for some of the earlier (1970s) studies. Several soil mineralogical and geological variables were linked with podoconiosis prevalence, particularly kaolinite, smectite, micas, quartz, iron oxides and trace elements. However, it remains unclear whether these are covariates or direct contributors to the pathogenesis of the disease and what the role of other peculiarities of soils (complex mineral-organic or material-climate combined factors) might be. Studies on the enrichment of minerals and elements during pedogenesis should be prioritised in future research.

PROSPERO International prospective register of systematic reviews registration number CRD42024499266.

The disabling leg swelling condition, podoconiosis, is unusual in having never been linked to a biological agent such as a bacterium, virus, parasite or fungus. For five decades, the occurrence of podoconiosis has been connected with highland areas in the tropics where red clay soils predominate, and long-term barefoot exposure to these soils has been postulated as necessary to developing the condition. However, no clear factor or trigger within the soil has yet been clearly documented as ‘causing’ podoconiosis. It is vital for the prevention and treatment of podoconiosis to understand its aetiology better. We collected all research articles in which information on bedrock, soil or soil-generating (pedogenic) factors was collected with reference to podoconiosis. We identified 27 studies spanning 51 years, and extracted and synthesised information from all these. The findings suggest important routes of future research, in particular exploring the enrichment of certain elements and minerals within soils and the study of soil organic matter.