Schistosomiasis, sometimes called bilharzia or snail fever, remains one of the world’s most burdensome parasitic diseases. The worms responsible, Schistosoma mansoni and Schistosoma haematobium among them, spend part of their life cycle in freshwater snails and emerge as infectious larvae that penetrate human skin during contact with contaminated water. Adult worms then settle in the blood vessels around the intestines or the bladder, where female worms release thousands of eggs daily. S. mansoni eggs are shed in stool, while S. haematobium eggs exit in urine, and it is the detection of these eggs in laboratory examinations that reveals an active infection. Chronic disease can damage the liver, intestines, bladder, and kidneys, and in children it is associated with anemia and impaired growth and cognition.

Praziquantel, administered as a single oral dose of 40 milligrams per kilogram of body weight, is the backbone of global schistosomiasis control. Mass drug administration campaigns deliver the drug to entire at-risk communities, often school-aged children, on an annual or biannual schedule. Yet the drug has known limitations: it is highly effective against adult worms but does not act on immature stages, and cure rates vary widely between settings, parasite species, and individual patients. Understanding why those rates vary, and what factors predict treatment failure or rapid reinfection, is essential for programs deciding how often to treat and whom to prioritize.

The new study addressed this question in a setting where both major species coexist, a situation common across much of sub-Saharan Africa but underexplored in treatment trials. The researchers, led by Joule Ntwan Madinga and colleagues working with Congolese and Belgian institutions, randomly selected 388 residents of a village co-endemic for S. mansoni and S. haematobium. Rather than a single before-and-after assessment, they screened every participant every three weeks for nine consecutive weeks, using Kato-Katz thick smear examinations of stool to detect and quantify S. mansoni eggs and urine filtration to detect S. haematobium eggs. Anyone found positive at a visit was treated on the spot with praziquantel at the standard 40 milligrams per kilogram dose.

The baseline picture confirmed how intense transmission is in the village. Just over 60 percent of participants carried S. mansoni, and a nearly identical proportion carried S. haematobium, with 36.6 percent infected with both species simultaneously. More striking were the worm burdens implied by egg counts. People with mixed infections carried far heavier loads than those with single infections: S. mansoni egg counts averaged 200.7 eggs per gram of stool in co-infected individuals versus 70 in those infected with S. mansoni alone, and S. haematobium counts averaged 29 eggs per 10 milliliters of urine versus 9.6. Both differences were highly statistically significant. Co-infection, in other words, is not merely the sum of two independent infections but a marker of substantially greater exposure or susceptibility.

When the researchers examined treatment outcomes, the two species behaved very differently. A single round of praziquantel cured 94.6 percent of S. mansoni infections, with an egg reduction rate of 96.5 percent, meaning that among those not cured, egg output collapsed to a small fraction of pretreatment levels. S. haematobium proved considerably more stubborn: only 62.7 percent of infections were cured by one treatment, although egg reduction still reached 97.5 percent. After three rounds of treatment spaced across the nine-week follow-up, the cumulative cure rate for S. mansoni rose to 93.7 percent, while S. haematobium reached only 75 percent, with cumulative egg reduction rates of 97.5 and 95.6 percent respectively. The pattern suggests that repeated dosing closes much of the gap for intestinal schistosomiasis but leaves urinary schistosomiasis persisting in a quarter of patients.

Co-infection status emerged as a key modifier of these outcomes. Treatment efficacy was significantly higher in people with single infections than in those with mixed infections across every measure the team calculated, including cure rate, egg reduction rate, and their cumulative counterparts. Single infections achieved a cure rate of 76.7 percent compared with 58 percent for mixed infections, a difference with a p-value of 0.01. The authors note that this effect was driven primarily by S. haematobium, the species that already showed weaker cure rates on its own. Age mattered as well: adults and older participants cleared their infections more readily than children under 16, a finding consistent with the possibility that younger children in high-transmission settings carry infections that include immature worms less susceptible to the drug, or that their heavier and more recent exposure leaves them with larger worm populations at treatment.

Perhaps the most reassuring result concerns what happened after treatment. The team tracked reinfection over the follow-up period and found no significant association between a person’s pretreatment co-infection status and their likelihood of becoming reinfected. In other words, carrying both species before treatment makes the initial cure harder to achieve, but it does not appear to make a cured person more vulnerable to picking up new worms afterward. Reinfection in such settings is governed largely by ongoing exposure to contaminated water, which affects the community broadly regardless of an individual’s prior infection history. This distinction matters because it separates a pharmacological problem, which repeated or adjusted dosing might address, from an environmental one, which requires safe water, sanitation, and snail control.

The authors suggest that these findings could help tailor control activities in co-endemic areas. Where both species overlap, programs may need to anticipate lower cure rates, particularly for S. haematobium and for co-infected individuals, and consider more frequent treatment rounds to achieve cumulative cure. The high egg reduction rates even among uncured patients offer some comfort: praziquantel suppresses egg output dramatically, which likely reduces both disease progression and transmission even when worms survive. But the persistence of a quarter of S. haematobium infections after three treatments underscores that a one-size-fits-all annual dose may be insufficient in the hottest transmission foci.

The study also carries implications for how treatment success is measured. Programs typically evaluate mass drug administration using single-dose cure rates, which this research shows can substantially understate the drug’s cumulative effect, especially for S. mansoni, whose cure rate climbed from 62.7 percent after one round to 93.7 percent after three. Conversely, the persistent shortfall for S. haematobium suggests that species-specific monitoring is warranted in mixed-endemic villages. As global goals shift from controlling schistosomiasis toward eliminating it as a public health problem, understanding the fine-grained dynamics of mixed infections, drug efficacy, and reinfection in places like this Congolese village will be central to designing strategies that finally bend the transmission curve.

Subject of Research: Praziquantel efficacy and reinfection dynamics in mixed Schistosoma mansoni and Schistosoma haematobium infections in the Democratic Republic of Congo

Article Title: Impact of praziquantel treatment and early reinfection patterns in a mixed Schistosoma mansoni and Schistosoma haematobium infection focus in the Democratic Republic of Congo

Article References: Madinga, J. N., Roucher, C., Linsuke, S., Baloji, S., Hermy, M., Lutumba, P., & Polman, K. (2026). Impact of praziquantel treatment and early reinfection patterns in a mixed Schistosoma mansoni and Schistosoma haematobium infection focus in the Democratic Republic of Congo. PLOS Neglected Tropical Diseases, 20(10), e0014764. https://doi.org/10.1371/journal.pntd.0014764

Image Credits: AI Generated

DOI: 10.1371/journal.pntd.0014764

Keywords: schistosomiasis, praziquantel, Schistosoma mansoni, Schistosoma haematobium, co-infection, Democratic Republic of Congo, cure rate, reinfection, neglected tropical diseases, mass drug administration, Kato-Katz, urine filtration

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