Evaluation of the efficacy of insecticide-treated scarves to protect children from the trachoma vector Musca sorbens (Diptera: Muscidae): A phase II randomised controlled trial in Oromia, Ethiopia

Abstract

Background: The eye-seeking fly Musca sorbens can act as a vector for ocular Chlamydia trachomatis, causing trachoma, yet there has been very little research on control measures. We investigated whether insect repellent products, specifically insecticide-treated clothing, could provide personal protection to the user from eye-seeking flies.

Methods: We first conducted a series of phase I laboratory studies to inform our choice of field intervention. We then conducted a phase II randomised controlled trial testing the efficacy of permethrin-treated scarves (PTS) in reducing fly-face contact in Oromia, Ethiopia. Children aged 4-10 years in full health and with no known adverse reactions to permethrin or other insecticides were allocated to either arm using restricted randomisation. Intervention arm children wore Insect Shield® versatile wraps (as PTS) for 28 days. The primary outcomes, fly-eye, -nose and -mouth contact, were assessed on the first day (0/30/60/180 minutes), on day 7 and on day 28. All participants present per timepoint were included in analyses. This trial was registered with ClinicalTrials.gov (NCT03813069).

Findings: Participants were recruited to the field trial between 29/10/2019 and 01/11/2019, 58 were randomised to test or control arm. More fly (-eye, -nose and -mouth) contacts were observed in the PTS arm at baseline. After adjusting for baseline contact rates, across all timepoints there was a 35% decrease in fly-eye contacts in the PTS relative to control arm (rate ratio [RR] 0.65, 95% CI 0.52-0.83). Similar cross-timepoint reductions were seen for fly-nose and fly-mouth contacts (RR 0.69, 95% CI 0.51-0.92 and RR 0.79, 95% CI 0.62-1.01, respectively). All children were included on day 0. Two in the control arm were absent on day 7, one left the study and four were excluded from analysis at day 28. No adverse events occurred in the trial.

Interpretation: Musca sorbens flies are sufficiently repelled by PTS to reduce fly-eye contacts for the wearer, thus possibly reducing the risk of trachoma transmission. Permethrin-treated scarves may therefore an alternative to insecticide space spraying for protection from these flies.

Funding: Wellcome Trust.

Keywords: Eyeseeking flies; Insecticide-treated clothing; Musca sorbens; Personal protection; Trachoma; Vector-borne disease.

Conflict of interest statement

The authors have nothing to disclose.

© 2022 The Authors.

Figures

Figure 1

Trial profile. PTS=permethrin-treated scarf.

Figure 2

Stages in development of a new vector control product. The World Health Organization's (WHO) Vector Control Advisory Group categorises studies that test new vector control interventions into phases I-IV. In our phase I laboratory studies we designed a bioassay for testing repellent products against eye-seeking M. sorbens flies, screened a range of repellent products, then selected the field trial intervention product. Our randomised controlled field trial, testing the use of permethrin-treated scarves against eye-seeking flies, was a phase II study (entomological outcomes only). Phase III studies assess the efficacy of interventions against epidemiological outcomes and inform policymakers (image adapted from; TPP, target product profile; MoA, mode of action).

Figure 3

A field study participant wearing the intervention product, a permethrin-treated headscarf. These were Insect Shield® versatile wraps, 100% polyester and factory-treated with a proprietary permethrin formula using the “Insect Shield®” process, at a weight ratio of 0·52% w/w.

Figure 4

Preliminary laboratory trial results. Jungle Formula Kids® (JFK, orange markers) and permethrin-treated scarves (PTS; total amount permethrin 102 [P102, purple markers] and 204 [P204, pink markers] mg/scarf) were tested in a preliminary laboratory trial with six people. The product ‘test’ bioassays were run in-between control bioassays (‘before’ and ‘after’). Protective efficacy (PE) was calculated at each minute for both ‘test’ and control ‘after’ bioassays; PE was defined as the proportion of M. sorbens contacting the hand relative to that in the control ‘before’ bioassay. Points represent mean protective efficacy (+/- 95% confidence intervals), the red dashed line represents 30% PE, the threshold for use in the subsequent field trial.

Figure 5

Fly contacts experienced in the two study arms. Number of fly-eye, -nose and -mouth contacts (mean in 10-minutes, predicted and unadjusted values; 95% confidence intervals) in intervention (permethrin, orange bars) and control (purple bars) arms (each n=29). Contacts were measured at baseline (before scarves were given), then at each time-point: immediately (T0), 30, 60 and 180 minutes later (T30/T60/T180), seven days later (D7) and 28 days later (D28).