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