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

Schistosomiasis is caused by trematode worms in the genus Schistosoma. The parasites require freshwater snails to complete part of their life cycle. Infected humans or animals release parasite eggs into water, where the eggs hatch and infect compatible snails. The parasites multiply inside the molluscs and eventually emerge as cercariae, which can penetrate human skin during contact with contaminated water. Schistosoma mansoni is associated primarily with intestinal disease, while S. haematobium causes urogenital schistosomiasis. Other species, including S. bovis and S. rodhaini, generally circulate among livestock and wildlife. The distinction matters because cercariae from different species can look alike under a microscope, while their public-health implications differ. In the Ugandan study, researchers therefore used molecular methods to identify parasite DNA rather than relying solely on appearance. Their approach connected ecological observations with human health and animal infection, providing a One Health view of a disease system shaped by people, wildlife, livestock, snails and changing water bodies.

From June 2023 through October 2024, the team visited nine sites every two months, collecting snails during standardized 30-minute sampling periods. Two collectors used long-handled scoops with a 2-millimetre mesh, recording the location and examining the animals in a field laboratory. They identified 2,524 Biomphalaria snails at seven sites and 422 Bulinus forskalii snails at one site. The distribution was strongly associated with the history of forest clearance. Three sites cleared earlier contained 74 percent of all Biomphalaria collected, while more recently encroached sites contained the remaining 26 percent. The abundance of these snails was significantly higher in older cleared areas, with a statistical value of p = 0.002. No snails were found at a forest-edge site that retained tree cover. Across the monitoring period, 0.4 percent of Biomphalaria and 1.9 percent of Bulinus shed schistosome cercariae. The highest monthly shedding rates occurred in December 2023, reaching 3.1 percent among Biomphalaria and 3.7 percent among Bulinus.

The researchers also detected a physical transformation in the water environments where the snails lived. Average water temperature across the monitoring sites was 23.5 degrees Celsius, with a mean conductivity of 235.0 microsiemens and a mean pH of 6.9. Water at sites cleared more than a decade earlier averaged 24.4 degrees Celsius, compared with 22.8 degrees Celsius at recently cleared sites. That difference was statistically significant, with p = 0.0001 and a 95 percent confidence interval of 0.811 to 2.34 degrees. The contrast is consistent with the loss of the forest canopy’s cooling and shading effects. As trees are removed, sunlight reaches shallow water directly, while agricultural activity, settlement and altered drainage can create or enlarge swamps, marshes and pools. These habitats may provide food, shelter and stable conditions for snails that were previously limited by cooler, shaded environments. The study does not claim that deforestation alone caused every infection, but its findings show a close geographic and environmental association between forest conversion, snail colonization and parasite transmission.

Molecular testing revealed an unexpectedly complex parasite community. DNA from cercariae shed by two Biomphalaria pfeifferi snails at one site identified S. rodhaini, a species associated with rodents. Tissue from two other infected snails at the same site identified Biomphalaria sudanica carrying S. mansoni, the human parasite responsible for intestinal schistosomiasis. No coinfections were detected in the examined Biomphalaria snails. At another site, genetic analysis of a cercaria-shedding B. forskalii identified S. bovis, a parasite typically associated with cattle. The analyses used different molecular tools, including polymerase chain reaction, DNA sequencing and high-resolution melting assays. For S. rodhaini, researchers compared partial sequences from mitochondrial cox1, nuclear internal transcribed spacer DNA and 18S ribosomal DNA. These tests are important because species that appear similar during microscopic inspection may follow different transmission routes and pose different risks to humans, livestock or wildlife.

The environmental signal was accompanied by evidence of human infection. In August 2024, health workers and community teams screened school-aged children living within 200 metres of sites where infected snails had been found. Near the site containing S. mansoni-positive snails, 41 children provided stool samples for examination using the Kato-Katz method, which detects and estimates parasite eggs in feces. Twenty-three children tested positive, producing a prevalence of 56.1 percent. Infection intensity ranged from 12 to 900 eggs per gram, with an overall mean of 109 eggs per gram. The children were between six and 14 years old. By contrast, urine screening of 41 children near the site where Bulinus snails carried S. bovis found no evidence of urogenital schistosomiasis. The absence of S. haematobium infection in that small group does not eliminate future risk, particularly if infected people move into the area and introduce parasite eggs into water where compatible snails are present. The study team treated children found to have S. mansoni with praziquantel through local health services.

Satellite imagery helped place these findings in a longer environmental timeline. Comparing land-cover data from 2000 and 2021, the researchers found that central Bugoma Forest had declined by 18.0 percent, while surrounding shrubland had fallen by 57.0 percent. Cropland and grassland expanded as forest was converted for subsistence and commercial agriculture. Field observations recorded settlement growth, sugarcane cultivation, cattle grazing, charcoal production and the cutting of poles for housing. The Kyangwali refugee settlement, which began in a cleared portion of forest in 1967, expanded after 2012 as displaced populations arrived from the Democratic Republic of the Congo and South Sudan. Internal migration and agricultural encroachment also contributed to land conversion. The researchers describe these changes as plausible drivers of new aquatic habitats and increased contact between people, animals and contaminated water. Their satellite analysis used the Global Land Cover 2000 dataset and the ESA WorldCover 2021 product, with the higher-resolution imagery resampled to a common one-kilometre scale before comparison.

The study’s central message is that schistosomiasis surveillance must track ecological change as well as human illness. Detecting infected snails can identify a transmission focus while it is still geographically limited, allowing health teams to investigate nearby communities and consider treatment or other interventions. The authors recommend finer-scale monitoring by Uganda’s Ministry of Health and stronger attention from the National Forestry Authority, particularly as forest loss continues and population movement connects previously separated disease systems. They also emphasize the limits of the evidence. Children were screened only around two villages, and molecular identification was performed on a small number of infected snails, meaning additional parasite species or transmission sites may have been missed. The researchers did not demonstrate every step of transmission from the detected snails to the surrounding population. Even so, the combination of infected intermediate hosts, high local prevalence of intestinal schistosomiasis and substantial landscape change presents a compelling public-health warning. In Bugoma, the disappearance of forest cover is not only altering biodiversity and water temperature; it may also be redrawing the map of a neglected tropical disease.

The study’s repeated sampling is important because both snail abundance and cercarial shedding can vary over time. A single visit might miss infected snails or mistake a temporary absence for a stable pattern. By returning every other month for 18 months, the investigators could compare sites across changing environmental conditions and identify a December 2023 peak in shedding. This does not establish a fixed seasonal cycle, but it demonstrates why surveillance based on occasional collection may underestimate transmission potential.

The findings also distinguish ecological suitability from confirmed human transmission. More Biomphalaria snails were found where forest clearance was greater, and some carried S. mansoni, but snail abundance alone is not a measure of disease risk. Transmission additionally depends on contamination of water with parasite eggs, compatibility between parasite and snail, survival of the parasite in the environment and frequency of human or animal contact with water. The detection of S. bovis and S. rodhaini therefore broadens the surveillance question: the same modified wetlands may support parasite cycles involving people, livestock or wildlife, even when a particular human infection is not detected.

The child survey provides a strong signal but remains a preliminary snapshot. It involved school-aged children living near selected sites and used a limited sample, so the reported prevalence should not be treated as a district-wide estimate. It does, however, justify extending screening and snail investigations beyond the sampled villages, especially along connections among farms, settlements, streams and permanent swamps. Molecular testing of cercariae is likewise valuable for targeting follow-up, because morphology cannot reliably separate several schistosome species. Together, these approaches can help health authorities identify which water bodies require urgent attention and whether control efforts should address human infection, animal reservoirs, environmental exposure or several of these pathways at once.

Subject of Research: Deforestation-driven expansion of schistosomiasis transmission in Western Uganda

Article Title: Longitudinal malacological monitoring, with a parasitological survey, reveals new schistosomiasis transmission foci in deforested sites in Western Uganda

Article References: Oguttu, D. W., Nkolokosa, C., Kiberu, D., Odongo, M., Besigye, F., Juhasz, A., Barungi, W., Huyse, T., Kabatereine, N. B., Tolo, C. U., Elliott, A. M., Webster, B. L., & Stothard, J. R. (2026). Longitudinal malacological monitoring, with a parasitological survey, reveals new schistosomiasis transmission foci in deforested sites in Western Uganda. BMC Environmental Science, 3(1), Article 21. https://doi.org/10.1186/s44329-026-00061-x

Image Credits: AI Generated

DOI: 10.1186/s44329-026-00061-x

Keywords: schistosomiasis, deforestation, Bugoma Forest, Uganda, Biomphalaria, Bulinus, snail surveillance, One Health, land-use change, Longitudinal, malacological, monitoring