Snail Surveillance Study Maps Schistosomiasis Hotspots Across East Africa

A sweeping new analysis of freshwater snail populations across East Africa has revealed that a small but persistent fraction of the region’s intermediate host snails continue to carry the parasitic worms responsible for schistosomiasis, one of the world’s most devastating neglected tropical diseases. The systematic review and meta-analysis, published in BMC Infectious Diseases, pooled data from seventeen studies encompassing more than 72,000 snails collected at sites spanning the region, and its findings carry significant implications for the World Health Organization’s ambitious goal of eliminating schistosomiasis as a public health problem by 2030. With mass drug administration campaigns running at scale across much of the continent, the study provides a rare quantitative window into the reservoir of infection that persists in the environment itself, hidden within the shells of the snails that sustain the parasite’s life cycle.

Schistosomiasis, also known as bilharzia, is caused by trematode flatworms of the genus Schistosoma, whose eggs are shed in the urine or feces of infected people. When those eggs reach freshwater, they hatch into free-swimming miracidia that must find and penetrate a suitable snail host within hours to survive. Inside Biomphalaria snails, Schistosoma mansoni larvae multiply asexually before emerging as infectious cercariae; inside Bulinus snails, the same role is played by Schistosoma haematobium, the species responsible for urogenital schistosomiasis. These cercariae then seek out human hosts in the water, burrowing through the skin and maturing into adult worms that can live for years, producing thousands of eggs daily. Breaking this cycle at the snail stage has long been recognized as a critical lever for control, yet reliable, up-to-date data on how many snails actually carry infection has been scattered across dozens of small field studies.

To close that gap, a team of researchers led by Sisay Desale and Zewudu Mulatie of Wollo University in Ethiopia, working with colleagues in Ethiopia and Iran, conducted a review following the PRISMA 2020 reporting guidelines. The team searched five electronic databases for studies published between January 2020 and December 2025, a window that refers strictly to publication dates; the underlying snail sampling periods of the included studies stretched much further back, from 2009 through 2025. After screening, seventeen studies qualified for inclusion, contributing twenty-seven separate datasets. The researchers pooled study-level proportions using the Freeman-Tukey double-arcsine transformation to stabilize variance in proportions near zero or one, combined with a DerSimonian-Laird random-effects model implemented in Stata 17, and calculated exact binomial confidence intervals for individual estimates.

The headline number from the analysis is an overall pooled prevalence of 8.88 percent, with a 95 percent confidence interval running from 5.61 to 12.14 percent. But the authors are careful, and unusually candid, about what that figure does and does not mean. The statistical heterogeneity across datasets was extreme, with Higgins’ inconsistency index, I², reaching 100 percent and a p-value below 0.001. Under such conditions, the DerSimonian-Laird weighting scheme effectively assigns nearly equal weight to every dataset regardless of sample size, meaning the pooled estimate approximates the unweighted average of the twenty-seven dataset proportions rather than a true sample-weighted regional prevalence. The crude prevalence across all snails examined was considerably lower: 1,615 infected snails out of 72,473 examined, or 2.23 percent. Because of this discrepancy, the authors emphasize subgroup estimates over the overall pooled figure, a methodological caution that other meta-analysts would do well to emulate.

The subgroup findings are where the biological story emerges. Infection prevalence was significantly higher in Biomphalaria snails, at 13.44 percent, than in Bulinus snails, at 2.24 percent, and higher for Schistosoma mansoni than for Schistosoma haematobium, with p-values below 0.001 for both comparisons. This asymmetry matters for control planning: intestinal schistosomiasis caused by S. mansoni appears to be sustained by a substantially larger environmental reservoir than the urogenital form carried by Bulinus species. Geography also mattered. Ethiopia reported the highest pooled prevalence of any country in the analysis, at 16.79 percent, a finding consistent with the country’s extensive freshwater habitats and the intensity of schistosomiasis transmission documented in human surveys there. Estimates also varied by study design, diagnostic method, and publication year, though the authors stress that these exploratory meta-regression findings should be treated as hypothesis-generating rather than definitive, because residual heterogeneity remained at I² equal to 100 percent in every model they fitted.

One of the most intriguing results concerns diagnostic methodology, and it runs counter to conventional expectations. Cercarial shedding, the traditional method in which snails are induced to release parasites under light or heat, yielded a pooled prevalence of 9.98 percent, while polymerase chain reaction, or PCR, alone yielded 10.83 percent. Most strikingly, studies that combined shedding with PCR reported a pooled prevalence of only 3.35 percent. In the meta-regression, PCR alone did not differ significantly from shedding alone. Molecular methods are generally assumed to detect more infections than shedding, since they can identify pre-patent infections in which parasites are present but not yet producing cercariae. The fact that the combined-method stratum produced the lowest estimate suggests that methodological differences here are entangled with other variables rather than reflecting a clean diagnostic comparison.

The authors themselves flag exactly why that interpretation is hazardous. The PCR-alone stratum comprised four datasets drawn from a single Ugandan study of snails collected in 2009 and 2010, meaning the diagnostic method is fully confounded with country, host genus, and sampling period. In other words, the apparent similarity between PCR and shedding estimates, and the low figure for combined methods, cannot be attributed to the diagnostic techniques themselves; they may simply reflect the particular snails, places, and years those datasets happened to cover. This is a textbook illustration of why meta-regression on heterogeneous observational data must be read with care, and the study’s transparency about these limitations strengthens rather than weakens its conclusions.

What can be said with confidence is that infected intermediate host snails, especially Biomphalaria species harboring S. mansoni, were documented across the East African sites covered by the included studies, and that the published evidence base describes an uneven and temporally fragmented picture rather than a uniform snapshot of current transmission. Sampling periods span sixteen years and are distributed unevenly across countries, host genera, and diagnostic methods, so the estimates describe what has been published rather than what is happening in every water body today. That distinction is crucial for policymakers: a pooled prevalence drawn partly from snails collected in 2009 cannot be read as the 2025 infection pressure in a given village pond, but it does demonstrate that the environmental reservoir of infection has been continuously present across the region throughout the period in which mass drug administration has been scaled up.

The study’s practical recommendations follow directly from this logic. The authors call for coordinated, site-specific strategies that combine mass medicine delivery with targeted snail removal, molecular xenomonitoring, environmental management, and sensitive snail surveillance. Molecular xenomonitoring, in which snail pools are tested for parasite DNA using PCR or loop-mediated isothermal amplification, offers a way to map transmission intensity at fine spatial resolution without relying on the seasonal and species-limited window that cercarial shedding provides. Paired with mollusciciding and habitat modification, such surveillance could identify the specific water bodies where infected Biomphalaria populations persist and direct interventions where they will break transmission most effectively. As the WHO’s 2030 elimination deadline approaches, this analysis makes clear that the final mile of schistosomiasis control will be won or lost not only in clinics and classrooms where drugs are distributed, but in the shallow, warm freshwater habitats where snails and parasites continue their ancient partnership, largely out of sight and, until now, largely out of reliable statistical view.

Subject of Research: Prevalence of Schistosoma mansoni and S. haematobium infection in freshwater snail intermediate hosts in East Africa

Article Title: Prevalence of Schistosoma mansoni and S. haematobium in snail intermediate hosts in East Africa: a systematic review and meta-analysis of studies published between 2020 and 2025

Article References: Desale, S., Siyadatpanah, A., Eshetu, B., Aseme, A. H., Sebsibe, S., Erkihun, Y., Kassa, Y., Gessese, T., Alebachew, M., Achame, M. S., Feyisa, M. S., & Mulatie, Z. (2026). Prevalence of Schistosoma mansoni and S. haematobium in snail intermediate hosts in East Africa: a systematic review and meta-analysis of studies published between 2020 and 2025. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14535-5

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14535-5

Keywords: schistosomiasis, Schistosoma mansoni, Schistosoma haematobium, intermediate host snails, Biomphalaria, Bulinus, East Africa, meta-analysis, molecular xenomonitoring, neglected tropical diseases, WHO 2030 roadmap, Ethiopia