Zimbabwe Parasite Resurgence After Three-Year Treatment Pause, National Survey Finds

Zimbabwe’s hard-won progress against schistosomiasis slipped back during a three-year pause in preventive chemotherapy, according to a nationwide survey published in BMC Global and Public Health. The 2021 cross-sectional assessment, which tested 11,190 people across the country, estimated national schistosomiasis prevalence at 7.0 percent (95 percent confidence interval 5.0 to 9.8 percent), and found that heavy-intensity infections had climbed back above the 1 percent threshold that defines elimination as a public health problem. The authors attribute the rebound to the suspension of annual deworming treatment between 2018 and 2021, a gap forced by the COVID-19 pandemic rather than a deliberate policy change.

The survey was designed and analysed with an unusually sophisticated statistical framework. Rather than sampling districts at random, the team used model-based geostatistics, building a predictive surface of Schistosoma haematobium prevalence from a 2018 impact survey and then simulating candidate survey designs to find one that could correctly classify 90 percent of Zimbabwe’s 1,961 wards into prevalence categories with minimal effort. That process settled on 200 schools as primary sampling units, selected with probability proportional to the predicted chance that S. haematobium prevalence exceeded 2 percent. Within each school, 58 children aged 9 to 14, stratified by sex, were recruited, and at a random subset of 25 sites, preschool children and adults were sampled alongside them.

Diagnosis followed standard WHO-aligned methods. Every participant provided mid-morning urine and fresh stool, with S. haematobium detected by urine filtration and intensity expressed as eggs per 10 millilitres, while S. mansoni and soil-transmitted helminths were assessed with duplicate Kato-Katz thick smears expressed as eggs per gram of faeces. Ten percent of slides were re-read by blinded senior technicians for quality control. Complete datasets were ultimately available from 9,778 school-aged children in 177 schools across eight provinces; the two urban provinces, Bulawayo and Harare, had no schools selected. Data from 725 preschool children and 750 adults were also collected at the 25 sub-sample sites.

The headline schistosomiasis numbers were sobering. Survey-adjusted prevalence was 4.0 percent for S. haematobium (95 percent CI 2.9 to 5.6 percent) and 3.8 percent for S. mansoni (95 percent CI 2.2 to 6.5 percent), with 0.5 percent of participants carrying both species. All outcomes were higher than the 2018 estimates, though they remained marginally within the confidence bounds of those earlier figures. More consequential was the heavy-intensity finding: crude prevalence of heavy infection with any Schistosoma species was 2.4 percent, and after design adjustment, 1.4 percent (95 percent CI 0.9 to 2.0 percent). That places Zimbabwe above the 1 percent elimination threshold, having moved from 0.4 percent (95 percent CI 0.3 to 0.6 percent) in 2018, when the country had been declared to have reached elimination as a public health problem for schistosomiasis in school-aged children.

The geographic pattern of the rebound was far from uniform. Most heavy-intensity infections were caused by S. haematobium, at 1.2 percent nationally, and were concentrated in northern Zimbabwe. S. mansoni showed particularly pronounced increases in Mashonaland Central and Masvingo provinces, the latter being the most striking because its observed infection prevalence had been below 2 percent in 2018. The fitted geostatistical models revealed short-range spatial autocorrelation for both species, with estimated ranges under 20 kilometres, meaning transmission foci are tightly clustered. For S. haematobium, environmental covariates did not improve model fit, whereas the S. mansoni model benefited from precipitation, temperature, land cover, socioeconomic and healthcare access variables.

Translated to ward-level classifications, the predictions paint a picture of where treatment must resume. Nearly all wards, 1,940 of 1,961 or 98.9 percent, were classified as exceeding 2 percent prevalence for any schistosomiasis species, an area the authors estimate is home to 17,909,432 people. Among these, 730 wards, or 37.2 percent, exceeded 10 percent prevalence and require ongoing annual preventive chemotherapy, covering an estimated 6.64 million people, or 36.9 percent of the national population. Sixteen wards, 0.8 percent, exceeded 50 percent prevalence and may need intensified interventions. The wards needing annual treatment form a largely contiguous block across the north and east of the country, with isolated higher-prevalence foci in the south flagged for further investigation because of model uncertainty there.

Soil-transmitted helminths told a milder story. National prevalence of infection with any STH species was 1.4 percent (95 percent CI 1.0 to 2.1 percent), nearly double the 2018 estimate of 0.7 percent (95 percent CI 0.6 to 0.9 percent) but still low in absolute terms. Ascaris lumbricoides accounted for 135 of 163 infections, or 82.8 percent, with hookworm at 0.1 percent and Trichuris trichiura at 0.2 percent. Prevalence of moderate or heavy-intensity STH infection remained well below the 2 percent elimination threshold at 0.2 percent (95 percent CI under 0.1 to 1.2 percent), based on just five moderate-intensity infections and no heavy ones. Roughly two-thirds of surveyed schools, 115 of 177, recorded no STH infection at all. Nevertheless, geostatistical classification placed 33.3 percent of wards, home to an estimated 5,781,584 people or 32.1 percent of the population, in the category requiring ongoing biennial treatment.

The authors argue that the interruption itself, not some other shift, drove the schistosomiasis rebound. The suspension stemmed from pandemic-related public health and social measures, including school closures in 2020, and the seven rounds of integrated PC delivered from 2012 to 2018 had produced a consistent declining trend. A 2010 baseline survey before treatment began had shown high transmission potential in the provinces now rebounding, with over 20 percent prevalence of any Schistosoma infection nationally and Mashonaland Central and Masvingo above 40 percent. Reactive treatment rounds were carried out in seven of 91 districts in 2020 after district surveillance flagged atypical reports of STH and schistosomiasis-related complications, with mean coverage of 77.2 percent for praziquantel and 70.5 percent for albendazole, which the authors note likely softened the impact in those areas. The pattern is consistent with transmission models and with field reports of resurgence following treatment interruptions in Zanzibar and Kenya.

The study’s methodological contribution may prove as important as its epidemiological findings. By incorporating prior prevalence data, environmental covariates and spatial correlation structures, the geostatistical framework allowed ward-level classification based on exceedance probabilities rather than point prevalence alone, prioritising conservative assignment so that a lower treatment category was recommended only when there was at least 70 percent certainty that prevalence fell below the relevant threshold. These gains in resolution were achieved without increasing survey size, which the authors suggest makes the approach scalable and cost-effective for other endemic countries transitioning from district-level to community-level programme planning under the WHO 2030 roadmap for neglected tropical diseases.

The authors also lay out the study’s limits candidly. The design deliberately oversampled areas of predicted higher prevalence, leaving some regions with sparse data and less precise estimates; in parts of southern Midlands and western Matabeleland, models predicted high S. haematobium prevalence driven largely by covariates despite few detected cases. Environmental covariates came from global public datasets and may miss local drivers such as water-contact behaviour and snail ecology. Adults were sampled only at 25 sites, limiting generalisability beyond school-aged children, and the survey covered just two months in late 2021, so seasonal variation in infection prevalence is not captured. Even so, the authors conclude that the findings reaffirm the critical importance of continued surveillance after elimination targets are met, and raise difficult questions about the optimal frequency of preventive chemotherapy in settings where transmission persists at low levels, a question made more urgent by recent cuts to global health funding that could delay or halt mass drug administration cycles elsewhere.

Subject of Research: A national geostatistical reassessment of schistosomiasis and soil-transmitted helminth prevalence in Zimbabwe after a three-year interruption of preventive chemotherapy.

Article Title: A national geostatistical survey for schistosomiasis and soil-transmitted helminths in Zimbabwe following interruption of preventive chemotherapy

Article References: Midzi, N., Mutsaka-Makuvaza, M. J., Phiri, I., Mwabvu, R., Mubaiwa, K., Greenblatt, E., Ower, A., Palacio, K., Dhanani, N., Fronterre, C., Fleming, F. M., Pullan, R. L., & Timothy, J. W. (2026). A national geostatistical survey for schistosomiasis and soil-transmitted helminths in Zimbabwe following interruption of preventive chemotherapy. BMC Global and Public Health, 4(1), Article 92. https://doi.org/10.1186/s44263-026-00301-x

Image Credits: AI Generated

DOI: 10.1186/s44263-026-00301-x

Keywords: schistosomiasis, soil-transmitted helminths, Zimbabwe, preventive chemotherapy, geostatistics, COVID-19, neglected tropical diseases, recrudescence, school-aged children, ward-level mapping, praziquantel, elimination as a public health problem