Reducing Filth Fly Populations and Transmission of Diarrhoeal Pathogens in Harsh Settings

Study Setting This study will take place in Maban County, Upper Nile State, South Sudan, which is home to 4 refugee camps. Many refugees originate from Sudan and others from Ethiopia, Eritrea, and Somalia. The 4 camps host 218,439 refugees (104,784 in Doro, 57,605 in Yusuf Batil, 34,914 in Kaya and 21,136 in Gendrassa), according to the UN High Commissioner for Refugees (UNHCR) population estimates from 30 November 2025.

The refugee camps shelters are constructed from a mix of locally sourced materials. Most are temporary shelters made from tarpaulin and plastic sheeting, but some use a mixture of wood, mud, thatch and corrugated iron. The latrines primarily comprise polypropylene sheeting suspended by wooden sticks, with an open roof. They are rudimentary and built to be easily replaceable in case of flooding. Camps are designed with a set ratio of latrines to family shelters.

According to recent District Health Information Systems data, the major causes of death among refugee children arriving to the camps are measles and malnutrition. Malaria remains the leading cause of morbidity and mortality, accounting for 66% of outpatient consultations, 30% of admissions and 50% of deaths. Diarrhoeal diseases affect all refugee age groups but are worse among those under 5s, attributable to 10% of deaths in this age group.

Age Eligibility Criteria: See "Eligibility"

Arms and Interventions: See "Arms and Interventions"

ITNs are the current standard of care in the refugee camps in Maban County. These are distributed every 2-3 years to protect the population from malaria. ITN coverage is expected to be equitable between intervention and control camps. ITNs will not impact filth flies as they are not hematophagous or nocturnal.

The control arm will not receive a placebo intervention; therefore this will be a non-blinded trial at the implementation level.

Intervention arm 1 (LARV): Dimilin® GR-2 larvicide is an effervescent granule containing 2% diflubenzuron (20g/kg), an insect growth regulator (IGR). Diflubenzuron interferes with chitin synthesis resulting in the inhibition of mosquito (Anopheles, Aedes and Culex spp.) or fly (Musca domestica and Stomoxys calcitrans) ecdysis. This intervention will interrupt filth fly development at the larval / pupal stage, to reduce overall population density. All eligible pit latrines (with at least 3 standing walls) assigned to this intervention arm will be treated with Dimilin® GR-2 at 0.5 kg/10m2; granules will be sprinkled onto damp sewage per pit latrine. If waste is desiccated, this intervention will be diluted in water and applied as a spray. This intervention will be re-applied to pit latrines and open defecation sites, every month during each 6-month implementation phase.

Intervention 2 (TRAP): iCatchi® ProMax fly traps. iCatchi® ProMax fly traps are designed for commercial and residential use. These traps are 10 L containers which hold 3 L of proprietary fly bait, formulated to maximize fly attractiveness to ensure high capture rates. The container design minimizes spillage and allows for easy set up and maintenance in different environments, including agricultural settings, waste management sites and urban areas. Each TRAP requires re-baiting every 8 weeks. TRAP will be placed on the ground outdoors and flies enter the large black container through rectangular holes between the bait bowl and larger trap chamber and are unable to exit; they will eventually die from starvation / dehydration. During each implementation phase, in camps assigned to this intervention arm, one iCatchi® ProMax fly trap will be placed outside (\<5 m) pit latrines at a set trap to pit latrine ratio and will be re-baited every 2 months.

To prevent filth fly contamination between camps, Doro and Kaya camps will receive the study interventions since they are more than 7 km apart (29.4 km), based on maximum filth fly dispersal range. Yusuf Batil and Gendrassa (2.6 km apart) will be the parallel control camps throughout the study period; Kaya is 11.6 km away from Yusuf Batil and 13 km from Gendrassa; Doro is 23.4 km from Gendrassa and 25.8 km from Yusuf Batil.

During each implementation phase, project staff will go pit latrine-to-pit latrine to supervise intervention deployment. At the beginning of each wash-out phase, project staff will go pit latrine-to-pit latrine to supervise removal of TRAP; the residual efficacy of LARV is 1 month, therefore this intervention will dissipate during the wash-out period.

Explanation for the Choice of Comparators:

According to the WHO Vector Control Advisory Group (WHO-VCAG), studies should always have a control group from which data collection occurs contemporaneously. Our trial design includes an untreated control arm, which is generally acceptable when compared against an intervention arm, and trial arms are balanced with respect to standards of care. Our study design will not withhold standard of care for any medical treatment or intervention, in alignment with WHO-VCAG guidance.

Criteria for Discontinuing or Modifying Allocated Interventions:

All participation is completely voluntary, and community members can withdraw at any time with impunity. Study staff may terminate subject participation at any time during the trial, as necessary, if a subject no longer meets the inclusion criteria and/or based on adverse event (AE) and/or severe adverse event (SAE) clinical assessment.

Strategies to Improve Adherence to Interventions:

Because trial interventions are implemented at the camp level, not the shelter level, we do not anticipate any issues of participant intervention adherence. The study team will ensure that interventions are deployed at the correct coverage per camp during each implementation phase. Throughout the entomological monitoring, the study team will also perform periodic spot checks that no modifications have been made to pit latrines and intervention re-treatment may be considered, as appropriate.

Relevant Concomitant Care Permitted or Prohibited During the Trial:

Our study design will not withhold standard of care for clinical management or intervention of any medical condition in either trial arm. Camps assigned to either trial arm will continue to receive balanced humanitarian aid and access to healthcare and other medical services.

Outcomes: See "Outcome Measures"

Participant Timeline:

From enrolment to the end of the cross-over trial intervention phases.

Sample Size:

Baseline diarrhoeal disease prevalence data collected during the trial census was used to calculate sample sizes, targeting a 20% relative reduction in diarrhoeal incidence (episodes per person-time), with 80% power and FWE = 0.05 across three primary comparisons, for two pre-specified age cohorts (≤5 years old and ≥15 years old). Diarrhoeal disease prevalence was 7.20% (898/12476) and 4.55% (1231/27039) in ≤5 years old and ≥15 years old, respectively, at trial baseline (MENTOR Initiative, unpublished data). The design comprises a 3-period cross-over: two implementation periods (phases I-II) and one combined-intervention period (phase III), each consisting of repeated fortnightly measurements. Recruitment targets allowed for 20% loss-to-follow-up. Under these assumptions, the minimum fortnightly sample per camp in implementation phase I-II is estimated to be 650 and 325 individuals for the ≤5 cohort and 1202 and 601 individuals for the ≥15 cohort, in the intervention and control camps, respectively; in implementation phase III we will enroll 627 for the ≤5 cohort and 1147 for the ≥15 cohort in all camps. In the absence of an intervention effect, in phase I-II, per phase, this is expected to yield 608 cases in each intervention camp and 304 cases in each control camp in the ≤5 cohort and 710 cases in each intervention camp and 355 cases in each control camp in the ≥15 cohort. In phase III, this is expected to yield 587 cases per camp in the ≤5 cohort and 678 cases per camp in the ≥15 cohort.

Latrine level density of filth flies will be estimated using quadrat sampling. Targeting a mean difference in density with an effect size of 0.80, an intraclass correlation coefficient (ICC) of 0.15, a power of 80%, and a Bonferroni corrected α = 0.0167, a total of 43 latrines will be sampled per trial arm. In implementation phase I-II, each intervention camp will target 43 latrines; each control camp will target 22. In implementation phase III, each camp will target 22 latrines.

Recruitment:

All participants will be recruited from the 4 refugee camps (Doro, Kaya, Yusuf Batil and Gendrassa) in Maban County, Upper Nile State, South Sudan. Prior to trial commencement, a tiered and iterative approach will be taken to sensitize the communities to project objectives. Letters of introduction will be sent to each camp manager and local community leaders to seek permission to conduct the study. Camp managers, community leaders and local health staff will be invited to sensitization sessions, where the study team will inform them of the project, explain the procedures and timelines and address any questions or concerns. Due to the changing dynamics of this humanitarian emergency, the research team will continually re-assess camp security and accessibility before trial commencement and throughout each implementation phase. Because new issues can arise at any point, project staff will regularly attend camp meetings throughout the baseline year and implementation years to address any problems and to provide updates of study progress.

During trial baseline, project staff will perform a shelter-to-shelter census, where every shelter and pit latrine will be mapped with a Global Positioning System (GPS) and camp members will be informed of the study, procedures and timelines. Recruitment of participants ≤5 years old or ≥15 years old for the post-intervention cohorts will also occur shelter-to-shelter during the baseline census survey. Cohort recruitment per camp will continue until we reach our projected sample size, therefore, we anticipate no difficulty achieving recruitment targets. In each subsequent fortnightly survey, cohort monitoring will occur shelter-to-shelter by our trial clinical team. Our sample size calculation for both cohorts assumes a 20% loss-to-follow-up across 6 months, with individuals considered "lost-to-follow-up" if they miss at least 3 consecutive visits per implementation period. Because displaced populations can be mobile and the emergency in South Sudan is dynamic, to address issues of participant attrition between project years, each cohort will be re-enrolled at the beginning of each implementation phase. Despite the instability of this setting, we do not anticipate significant participant loss-to-follow-up per monitoring period, since the MENTOR Initiative has conducted several prior clinical trials in these camps, establishing strong local partnerships and a precedent for high study participant retention. Additional eligible individuals will be recruited per cohort, if we encounter greater than anticipated loss-to-follow-up.

Methods: Assignment of Interventions Allocation and Implementation:

Because this is an interventional non-randomized cross-over trial design, sequence generation and concealment for participant randomization are not required. Intervention implementation will occur after completion of trial baseline analyses and verification of underlying assumptions of study power. During implementation phase I, Doro camp will receive TRAP and Kaya camp will receive LARV. During implementation phase II, Doro camp will receive LARV and Kaya camp will receive TRAP. During implementation phase III, Doro and Kaya camps will receive both LARV and TRAP. Participants will be recruited to 2 prospective cohorts for monitoring all-cause diarrhoeal case incidence. Camps have been pragmatically allocated to study arms, to minimize intervention contamination from filth fly dispersal range.

Methods: Assignment of Intervent

• Impact of LARV or TRAP, or both interventions combined, on filth fly density - latrine level [Срок оценки: At trial baseline and then every fortnight during each of three 6-month intervention phases.]
• Impact of LARV or TRAP, or both interventions combined, on filth fly species composition - latrine level [Срок оценки: At trial baseline and then every fortnight during each of three 6-month intervention phases.]
• Impact of LARV or TRAP, or both interventions combined, on filth fly species density - shelter level [Срок оценки: At trial baseline and then every fortnight during each of three 6-month intervention phases.]
• Impact of LARV or TRAP, or both interventions combined, on filth fly species composition - shelter level [Срок оценки: At trial baseline and then every fortnight during each of three 6-month intervention phases.]