Fruit Bats Emerge as a New Ecologic Clue in Buruli Ulcer Transmission
Buruli ulcer, caused by Mycobacterium ulcerans, has long been tied to aquatic environments in West and Central Africa. A research letter from Cameroon now adds an unexpected mammalian signal to that story: investigators detected M. ulcerans DNA in two clinically healthy fruit bats in Akonolinga, while 111 pooled aquatic macroinvertebrate samples from the same river system were negative. The finding does not prove a reservoir host, and it does not establish viable bacteria or active transmission. It does, however, widen the field of inquiry at a time when Buruli ulcer incidence in Akonolinga has already shifted sharply downward.
The local context matters. Buruli ulcer cases in Akonolinga fell by about 80% after 2011, and earlier environmental work had detected M. ulcerans DNA in aquatic invertebrates from the Nyong River in 2008 and 2012. By 2024, those same sites no longer yielded detectable bacterial DNA in the aquatic pools sampled for this study. The ecological signal appears to have moved, or at least to have become harder to find in the water-associated compartment that once anchored surveillance efforts.
Two Positive Bat Samples, No Visible Disease
Field sampling in June 2024 focused on fruit bats captured with mist nets along the Nyong River. Investigators sampled 46 bats across five taxa: Eidolon helvum, Epomops franqueti, Micropteropus sp., Hypsignathus monstrosus, and Nanonycteris sp. Oral and rectal swabs were collected before release, and none of the bats had cutaneous lesions compatible with Buruli ulcer. Two samples were positive for M. ulcerans DNA, representing 4.2% of the bat cohort.
One positive result came from a rectal swab from an adult female Micropteropus sp. bat, with PCR cycle threshold values of 28.8 for IS2404 and 33.3 for the ketoreductase B domain. The other came from an oral swab of a juvenile male E. helvum bat, with cycle threshold values of 33.85 and 35.76, respectively. Both positive detections required amplification of IS2404 and the ketoreductase B domain, a dual-target approach that strengthens specificity for the organism’s mycolactone-producing machinery. In practical terms, the signal was real at the molecular level, even if its biologic meaning remains unsettled.
No lesions were seen. No animals appeared sick. The pathogen was present, but only as nucleic acid in healthy-looking hosts.
Aquatic Pools Were Negative Where Earlier Surveys Were Positive
The environmental arm of the investigation was equally striking. Researchers collected 413 aquatic macroinvertebrates and grouped them into 111 taxonomically homogeneous pools: Heteroptera, Coleoptera, Araneae, and Odonata. Every pool tested negative for M. ulcerans DNA. Earlier surveys from the same sites had detected the bacterium in roughly 5% to 10% of aquatic invertebrate samples, so the absence of any positive pool in 2024 suggests a genuine shift in the local ecologic profile, not just a minor sampling fluctuation.
Quality control was clean throughout. Negative controls stayed negative, positive controls amplified as expected, and PCR inhibition was not detected. That matters because environmental mycobacterial surveillance can be vulnerable to false negatives or technical noise. Here, the laboratory signal supports the field observation: the aquatic compartment no longer looked like the dominant reservoir it once did.
Exposure, Carriage, or Transit Through the Gut
The central question is what the bat detections actually mean. Oral and rectal positivity could reflect transient exposure through food or water, passage through the digestive tract, or more persistent carriage. Because both positive bats belonged to frugivorous taxa, ingestion of contaminated fruit or plant material remains plausible. Experimental data make that idea biologically credible: plant-derived carbohydrates, especially glucose polymers, can stimulate M. ulcerans growth in vitro. A bat feeding on contaminated fruit could therefore encounter the organism through a route that would never appear in classic aquatic sampling.
Longitudinal fecal sampling would help separate transient passage from sustained colonization. So would dietary investigation and broader environmental sampling of fruiting trees, plant surfaces, and other nonaquatic substrates. Without that, the bat signal stays intriguing but incomplete.
Why Fruit Bats Are Worth Watching
The absence of cutaneous lesions does not weaken the hypothesis; if anything, it fits with prior experimental work showing that fruit bats can recover from M. ulcerans lesions. It also aligns with recent work on GPR84, a receptor linked to inflammatory disease severity in Buruli ulcer and altered or lost in several fruit bat lineages. Those findings offer a mechanistic reason to study bats as permissive hosts, although no natural-carriage role for GPR84 has been demonstrated.
Mobility adds another layer. Fruit bats move across large distances and could, in principle, spread bacterial material between ecologic niches. That possibility meshes with molecular epidemiologic observations showing identical or closely related M. ulcerans genotypes in geographically separated endemic areas. A highly mobile mammal reservoir would fit a pathogen that appears capable of persisting across wide spatial scales even as human incidence declines in some settings.
A Changing Ecology, Not a Finished Story
The clearest message from Akonolinga is that the ecology of M. ulcerans is not fixed. In Benin, a similar decline in Buruli ulcer incidence coincided with disappearance of bacterial DNA from previously positive aquatic environments and an aging case population. Australia offers another model, where mammals are clearly implicated in the organism’s ecology. Cameroon may now be showing its own version of that transition: a fading aquatic signal, a new mammalian detection, and a transmission chain that may no longer be centered in the same habitat compartment.
These data are too limited to assign bats a reservoir role, and they cannot speak to bacterial viability or human infectivity. They do justify broader longitudinal surveillance across Cameroon and other endemic regions, especially surveillance that can track both aquatic and terrestrial interfaces over time. The study narrows the list of possibilities without closing the case. For Buruli ulcer, the next clues may be moving through the air, across fruiting trees, or out of sight of the river entirely.