Hidden Infections and Scarred Livers: Urban Schistosomiasis Runs Deeper Than Brazil’s Official Numbers Suggest

In a flooded neighborhood on the edge of Brazil’s Recife metropolitan region, nearly one in three residents is carrying a parasitic worm that official statistics say should be rare there. A new field study from Pernambuco state reveals that Schistosoma mansoni, the blood fluke responsible for intestinal schistosomiasis, is thriving in Novo Horizonte, a locality within the municipality of Jaboatão dos Guararapes that is officially classified as a low-endemicity area. The research, published in Acta Parasitologica, combined traditional stool examinations, molecular testing, abdominal ultrasound, and detailed socioenvironmental surveys to build one of the most complete portraits yet of how this neglected tropical disease persists in an urban setting.

The numbers are striking. Using the Kato-Katz technique, the standard coproscopic method recommended by the World Health Organization, researchers found a positivity rate of 23.68 percent among 266 residents who provided stool samples. When the team added the Hoffman, Pons and Janer sedimentation technique, six additional infections emerged. Crucially, they then applied real-time polymerase chain reaction to a subset of samples that had tested negative by both microscopic methods, and nearly 16.5 percent of those came back positive for parasite DNA. That molecular layer pushed the overall estimated prevalence to 28.94 percent, a figure the authors caution is likely still an underestimate, because qPCR was not applied to every negative sample and its greater sensitivity relative to parasitological techniques is well established.

This diagnostic gap matters far beyond academic precision. Under Brazilian Ministry of Health guidelines, the recommended response to schistosomiasis depends directly on the measured local positivity rate. In areas with rates between 15 and 25 percent, all positive cases and their household contacts should be treated with praziquantel. Above 25 percent, the guidance escalates to treating entire communities and prioritizing those areas for intensified control. A neighborhood that crosses that threshold on paper therefore triggers a fundamentally different public health strategy than one that appears quiet because insensitive tests missed low-burden infections. The Kato-Katz method, while practical and inexpensive, is known to perform poorly in individuals carrying few eggs, precisely the profile that dominates in low-transmission settings where control programs believe they are winning.

The study’s clinical findings add urgency. Of 55 infected residents who underwent upper abdominal ultrasound, 31 showed periportal fibrosis, the scarring around the liver’s portal veins that develops as the immune system walls off parasite eggs deposited in hepatic tissue. Fibrosis graded by the Niamey protocol ranged from peripheral ring echoes to advanced patterns. Consistent with the disease’s pathophysiology, individuals with fibrosis were significantly older, with a mean age of about 41 years compared with 21 among infected people without fibrosis, and they carried heavier parasite loads, averaging around 330 eggs per gram of stool. Fibrosis was also far more common in males, who made up roughly three quarters of the fibrosis group, echoing sex-related patterns reported across schistosomiasis research.

Perhaps most telling was the state of the portal vein. Infected residents with periportal fibrosis had a significantly larger portal vein diameter than those without, a measurement associated with progressing disease severity and, in advanced hepatosplenic cases, with the risk of life-threatening esophageal variceal bleeding. In this community-based sample the diameters remained within normal reference limits, and the researchers found no significant differences in spleen size or liver lobe measurements between groups, likely because only one participant showed the most advanced fibrosis pattern. That scarcity of severe disease is itself informative: it captures the pathology at a compensated, earlier stage, before the decompensated forms that fill referral hospitals, and it underlines why population-based surveys rather than hospital records are essential for understanding where transmission is heading.

One result stood out as a warning about early exposure. Although hepatic fibrosis typically requires years of chronic infection, and the relationship between age and fibrosis is generally treated as roughly linear, the team documented children and adolescents under 18 who already displayed pattern C peripheral fibrosis. The questionnaire data explain why. Residents described routine, involuntary contact with floodwater: roughly 98 percent of both infected and uninfected participants reported water accumulating on their streets, and almost no one had a practical means of avoiding exposure. In Novo Horizonte, infection risk is not tied to occupational fishing or farming but to simply walking through inundated streets during the rainy season, a hallmark of urban schistosomiasis.

The socioenvironmental picture is bleak in ways that map directly onto transmission. Fewer than 11 percent of infected residents lived on paved streets. About three quarters reported open septic tanks leaking sewage onto their roads, and more than 95 percent described standing water and flooding near their homes. Most infected individuals had lived in the area for at least five years, and duration of residence was one of the few socioenvironmental factors showing a statistically significant association with infection. Nearby water bodies harbor the Biomphalaria snails that serve as intermediate hosts, releasing the cercariae that penetrate human skin in a matter of minutes. In such an environment, every heavy rainfall converts streets into transmission sites.

History shows how hard this cycle is to break. An epidemiological assessment from 2011 recorded a 15.7 percent positivity rate in Novo Horizonte, which fell after collective treatment with praziquantel. More than two decades later, the parasite is back at roughly twice that level, demonstrating that mass drug administration alone, without durable sanitation infrastructure, produces only temporary suppression. Notably, among residents who tested negative by all three diagnostic methods, a full third nonetheless showed periportal fibrosis on ultrasound. The researchers suggest many were treated in earlier campaigns for infections that were never formally diagnosed, leaving behind fibrotic scars from cleared or subclinical infections, a hidden reservoir of morbidity that standard surveillance cannot see.

The broader Brazilian context makes these findings more consequential. National control program activity has been declining, and Pernambuco illustrates the trend: in 2024, of 240,436 coproscopic examinations scheduled across endemic municipalities, fewer than 43 percent were completed. Jaboatão dos Guararapes reached only about 30 percent of its scheduled exams, yielding an official municipal positivity of 3.2 percent, which qualifies it as low endemicity even as this study documents a moderately positive hotspot within its borders. Meanwhile the state recorded 1,528 deaths from schistosomiasis between 2015 and 2024, including 111 in the final year, and it carries the country’s highest hospitalization rates for the disease. Where screening coverage falls, deaths rise, and low official positivity rates may simply reflect too few tests rather than genuinely low transmission.

The study team, led by researchers at the Federal University of Pernambuco and the Oswaldo Cruz Foundation’s Aggeu Magalhães Institute, argues that frequent parasitological surveys, strengthened active case finding, and adoption of more sensitive molecular diagnostics are needed to interrupt transmission before severe hepatic disease takes hold. All infected participants received their results and referrals for praziquantel treatment under Brazilian program guidelines, and all participants or their guardians provided written informed consent, with ethical approval from the university’s research committee. The researchers also acknowledge limitations: younger residents were less likely to submit stool samples, and infrastructure constraints meant not every negative sample could undergo qPCR or ultrasound. Even so, the message from Novo Horizonte is difficult to ignore. In Brazil’s urbanizing coastal Northeast, a neglected tropical disease is quietly rewriting the maps that guide its control, and the communities most exposed to flooded, unsanitized streets are paying the price while official statistics lag behind the worms.

Subject of Research: Urban transmission and clinical burden of Schistosoma mansoni infection in Pernambuco, Brazil

Article Title: Epidemiological and Clinical-Laboratorial Profile of Schistosomiasis in an Endemic Urban Area in Pernambuco, Brazil

Article References: da Silva Pereira, M. E., Júnior, W. L. B., da Silva Fernandes, D. L. R., Domingues, A. L. C., de Lorena, V. M. B., & de Souza Gomes, E. C. (2026). Epidemiological and Clinical-Laboratorial Profile of Schistosomiasis in an Endemic Urban Area in Pernambuco, Brazil. Acta Parasitologica, 71(5), Article 206. https://doi.org/10.1007/s11686-026-01384-0

Image Credits: AI Generated

DOI: 10.1007/s11686-026-01384-0

Keywords: schistosomiasis, Schistosoma mansoni, neglected tropical diseases, Brazil, urban transmission, Pernambuco, periportal fibrosis, Kato-Katz, qPCR diagnosis, ultrasound, public health, praziquantel