Praziquantel Cures 82% of Schistosoma mansoni in Northern Uganda, But Reinfection Returns in 6 Months
Every year, hundreds of millions of people swallow a single pink tablet to hold a parasitic worm at bay. In the fishing villages and farm fields of Northern Uganda, that tablet is doing its job. The parasite, though, keeps finding its way back. A 24-week follow up of schoolchildren in one of the country’s most infected regions paints a clear and uncomfortable picture of what “control” actually means once you leave the clinic.
This post covers what the study found, why it should worry anyone working on neglected tropical diseases, and what it says about how often we ought to be treating the people most exposed.
What schistosomiasis is, and why children take the hit
Schistosomiasis, also called bilharzia, comes from flatworms of the genus Schistosoma. Schistosoma mansoni is the main cause of intestinal schistosomiasis across sub-Saharan Africa. The burden is heavy: the disease racks up more than 1.4 million disability-adjusted life years a year, and the World Health Organization says over 251 million people needed preventive treatment in 2021.
Kids suffer the worst of it. The worms settle in the body and the immune system reacts to the trapped eggs, which drives inflammation and scarring. That shows up as belly pain, diarrhoea, blood in the stool, and an enlarged liver and spleen. Year after year, infection is linked to anaemia, stunted growth, and problems with thinking and learning that quietly chip away at a child’s education and future income. That is exactly why school-aged children sit at the center of every control program.
The one tool we really have is preventive chemotherapy: mass drug administration, or MDA, with praziquantel. It is cheap, safe, and usually works, and it is given once a year at 40 mg per kilogram of body weight.
The setting: Lango, in Northern Uganda
The Lango sub-region sits in Northern Uganda, wrapped around Lake Kyoga and its wetlands. Those wetlands are perfect homes for the small Biomphalaria snails that carry the parasite from one person to the next. People here fish, farm, and haul water from open sources, so contact with infested water is close to a daily habit.
Uganda’s national program calls for annual praziquantel for all school-aged children. Coverage in this region has been patchy, slowed by logistics and drug shortages. In the group studied, under 5% of children said they had taken praziquantel in the three months before the survey, even though the program was supposed to reach them.
How the study ran
Researchers tracked 802 primary school children, ages 5 to 16, across 20 schools in Lira District, Lira City, and Kole District. Each child handed in a stool sample, read with the modified Kato-Katz method, the standard way to count parasite eggs under a microscope.
They split the children into two groups:
- Infected group (279 children): got a single 40 mg/kg dose of praziquantel, then returned at 4 weeks to see who was cured. The ones who weren’t got a second dose.
- Uninfected group (523 children): re-tested at 4 weeks to catch “prepatent” infections. These are cases where a child was already infected at the start but the worms hadn’t matured enough to lay eggs yet. Those children were treated and pulled out of the analysis, so the researchers could measure real new infections instead of counting ones that had been there all along.
Everyone was followed for 24 weeks, about six months, to see who got reinfected or picked up a fresh infection.
Finding 1: infection is still common
At the start, 34.8% of children carried S. mansoni. Prevalence swung hard by location:
- Lira District: 44.3%
- Kole District: 27.9%
- Lira City: 18.6%
The nearer a community sat to Lake Kyoga, the higher the rates. That tells you transmission is shaped by geography and water contact, not just by whether a drug exists. Most infections were light, but roughly one in five was heavy. Children aged 9 to 12 had the highest rate of all.
Finding 2: does praziquantel still cure? Yes, and strongly
After one dose, 82.1% of children were cured. That lines up with recent East African reviews, which put cure rates between 70% and 89% depending on how intense transmission is.
Cure rates fell as infections got heavier: 91.4% of lightly infected children cleared the worm, versus 78% of moderate and 66.7% of heavy cases. The youngest kids, ages 5 to 8, did best at 92.9%, probably because they carried fewer worms to begin with.
The 49 children who didn’t clear the worm the first time took a second dose. That second dose worked for 93.9% of them and pushed the overall cure rate close to 95% in most groups. Only three children stayed infected after both doses and were sent on for more care.
One thing worth watching: even where cure rates were high, the drop in egg counts was uneven, as low as 15.8% in Lira District. In some heavily infected children, a few worms may survive and keep shedding eggs. The researchers note this could be an early sign of the drug losing some ground, though the high cure rate argues against true resistance. The sensible move is to keep watching both cure and egg reduction over time.
Finding 3: reinfection and new infection return within six months
This is the part that matters most. Of the children cured after treatment, 7.7% were reinfected within 24 weeks. Put the other way, more than 92% stayed cured half a year on. That is encouraging, but the parasite was plainly still moving through the population.
Among children who were genuinely uninfected at the start, once prepatent cases were pulled out, the true rate of new infection was 5.0% over six months. The study also caught 5.9% of children who looked negative at first but were actually in the prepatent stage. That is a neat demonstration of why a single baseline test can make treatment look better than it is.
What this means for control programs
A high burden at the start, plus reinfection and new infection showing up within months, points to one conclusion: transmission here is intense and it does not let up. Annual MDA cuts sickness, but it may not be enough to break the cycle.
The authors put three practical moves on the table:
- Treat more often. Biannual MDA, twice a year instead of once, has beaten annual schedules in other East African programs, including Zanzibar and Kenya.
- Use a second dose for non-responders. A test-and-re-treat approach rescued most of the initial failures. The catch is cost and logistics: testing adds expense, while simply re-treating everyone avoids the testing but wastes drugs. In high-transmission areas, just re-treating the non-responders may be the more workable path for now.
- Pair the drugs with WASH and snail control. Pills alone will not end transmission. Clean water, sanitation, hygiene, and efforts to cut snail numbers are what make the gains stick.
The limits worth naming
No field study is clean. The team flagged a few:
- Attrition ran highest, 30.4%, among the children needing the second dose, which could skew those results.
- The Kato-Katz test misses many light infections, so true prevalence is probably higher and measured incidence a touch overstated.
- They collected one stool sample per child rather than several on separate days.
- They did not map water-contact habits or snail density, so the “why” behind local hotspots stays partly unexplained.
- One district’s 100% incidence figure came from just two children and should not be read as a real trend.
The bottom line
Praziquantel remains a reliable weapon against S. mansoni in Northern Uganda. One dose clears the worm in most children, and a second dose cleans up most of what’s left. But curing people is not the same as stopping transmission. With reinfection and new infection reappearing within months, the once-a-year approach probably needs a rethink: more frequent treatment, smarter re-treatment of failures, and real investment in water and sanitation.
For the families on the shores of Lake Kyoga, that little tablet is a lifeline. The hard part now is throwing it often enough, and backing it with the basics, clean water and safe places to wash, that keep the parasite from simply swimming back.
