A neglected tropical disease is retreating in Benin as its water disappears
Sixteen years of PCR-confirmed data show Buruli ulcer incidence down nearly 80%. Over the same period open water surfaces shrank 25–30% and mean air temperature rose by more than 1°C.
Reported cases of Buruli ulcer — a skin and soft-tissue disease caused by Mycobacterium ulcerans that mainly affects rural populations in west and central Africa — fell across the continent from 5,871 in 2004 to 1,573 in 2023 [s1]. A decline that steep in a neglected tropical disease invites an obvious suspicion: that surveillance weakened rather than the disease.
A nationwide study in Benin set out to distinguish between those two explanations, and its answer is that the decline appears real [s1]. Its proposed explanation is less comfortable than good news usually is: the disease may be retreating because the water it depends on is disappearing.
The data
Researchers used records from Benin's National Buruli Ulcer Control Programme at four specialised health-care centres in the south of the country, covering 16 years from 2008 to 2024 [s1]. The data were collected prospectively at the time of patient care, and the analysis was restricted to 2,568 patients with PCR-confirmed disease [s1].
That restriction is the study's central methodological choice. Buruli ulcer is often diagnosed clinically, and clinical case counts can fall for reasons that have nothing to do with transmission — fewer active case-finding campaigns, staff turnover, reporting fatigue. Requiring laboratory confirmation throughout removes one large source of that ambiguity, though it cannot remove all of it: if patients stopped presenting to these four centres, confirmed cases would fall regardless.
Annual incidence trends were examined by age, sex, municipality and clinical form of the lesion [s1]. Environmental and climatic variables — water surface area, temperature, urbanisation and land use — were integrated with the patient data using open-access geospatial databases, and population denominators came from Benin's National Institute of Statistics and Demography [s1]. In parallel, a large-scale environmental survey looked for M. ulcerans DNA in aquatic ecosystems [s1].
What they found
Incidence fell by nearly 80% over the study period, from 1·37 per 10,000 inhabitants (SD 0·21) in 2010–16 to 0·24 per 10,000 in 2024 [s1]. The steepest decrease was in children aged 15 and younger [s1].
Two epidemiological signatures of the disease also faded. Historical differences in incidence by age and by sex largely disappeared over the period [s1] — the pattern flattened as the overall number came down.
The environmental survey returned a striking null. M. ulcerans DNA was undetectable at 122 aquatic sites sampled between 2021 and 2024 [s1].
And the decline coincided with two measured environmental changes: a 25–30% reduction in open water surfaces, and a rise of more than 1°C in mean air temperature [s1].
How much weight the environmental explanation can carry
The authors state the association carefully — that environmental changes, particularly climate warming and water surface reduction, might have contributed to the trend [s1]. That is the right level of confidence for what is an ecological association across a single country over 16 years.
The reasoning is at least mechanistically plausible. Buruli ulcer transmission has long been linked to aquatic environments, and this study's own environmental sampling failed to detect the pathogen's DNA in 122 water bodies [s1]. If the habitat contracts by a quarter to a third [s1], a decline in environmentally acquired infection is not a surprising consequence.
But an ecological correlation over one time series cannot separate the environmental hypothesis from everything else that changed in southern Benin across the same 16 years — urbanisation, water and sanitation coverage, land use, health-seeking behaviour, and case management. The study integrates urbanisation and land use as variables [s1], which helps, but a nationwide observational design with one declining outcome and several co-declining exposures cannot establish which one is doing the work.
The negative environmental sampling result is also open to more than one reading. It is consistent with the pathogen having become rare in these water bodies. It is equally consistent with the pathogen being present at densities below the detection threshold of the method, or with the sampled sites not being the relevant reservoirs.
Why a disease disappearing is not simply good news
The authors describe this as, to their knowledge, the first comprehensive country-wide evidence of a sustained decline in Buruli ulcer incidence in Africa [s1], and their concluding recommendation is not celebration but continued surveillance — to confirm whether foci are genuinely disappearing and to detect potential re-emergence early [s1].
There is a practical reason for that caution beyond scientific conservatism. Control programmes for neglected tropical diseases are funded roughly in proportion to their visible burden. A disease whose case counts fall by 80% is a disease whose programme becomes a candidate for defunding — at exactly the moment when the surveillance capacity to detect its return is most valuable and least justifiable on current numbers.
If the driver really is environmental, that also means the decline is contingent rather than earned. A disease pushed down by shrinking surface water and rising temperature has not been eliminated by any intervention that could be maintained. Its trajectory is tied to conditions that could shift again.
What to watch
The immediate question is whether the same pattern shows up elsewhere in the endemic belt. Benin is one country with an unusually well-documented control programme and 16 years of PCR-confirmed records — which is precisely why the study could be done there and has not been done in most affected countries. Whether the continental decline from 5,871 to 1,573 cases [s1] reflects a similar environmental story across west and central Africa, or a mix of real decline and eroding surveillance, remains open.
Sources
- PCR-confirmed Buruli ulcer incidence over a 16-year period in Benin: a nationwide, retrospective, observational study, The Lancet Global Health, 24 July 2026
Sources
- PCR-confirmed Buruli ulcer incidence over a 16-year period in Benin: a nationwide, retrospective, observational study — The Lancet Global Health , July 24, 2026
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