ARTICLES THE LANCET • Vol 363 • April 3, 2004 • www.thelancet.com 1093 Summary Background Eye-seeking flies have received much attention as possible trachoma vectors, but this remains unproved. We aimed to assess the role of eye-seeking flies as vectors of trachoma and to test provision of simple pit latrines, without additional health education, as a sustainable method of fly control. Methods In a community-based, cluster-randomised controlled trial, we recruited seven sets of three village clusters and randomly assigned them to either an intervention group that received regular insecticide spraying or provision of pit latrines (without additional health education) to each household, or to a control group with no intervention. Our primary outcomes were fly-eye contact and prevalence of active trachoma. Frequency of child fly-eye contact was monitored fortnightly. Whole communities were screened for clinical signs of trachoma at baseline and after 6 months. Analysis was per protocol. Findings Of 7080 people recruited, 6087 (86%) were screened at follow-up. Baseline community prevalence of active trachoma was 6%. The number of Musca sorbens flies caught from children’s eyes was reduced by 88% (95% CI 64–100; p<0·0001) by insecticide spraying and by 30% (7–52; p=0·04) by latrine provision by comparison with controls. Analysis of age-standardised trachoma prevalence rates at the cluster level (n=14) showed that spraying was associated with a mean reduction in trachoma prevalence of 56% (19–93; p=0·01) and 30% with latrines (–81 to 22; p=0·210) by comparison with the mean rate change in the controls. Interpretation Fly control with insecticide is effective at reducing the number of flies caught from children’s eyes and is associated with substantially lower trachoma prevalence compared with controls. Such a finding is consistent with flies being important vectors of trachoma. Since latrine provision without health education was associated with a significant reduction in fly-eye contact by M sorbens, studies of their effect when combined with other trachoma control measures are warranted. Lancet 2004; 363: 1093–98 See Commentary page 1088 Introduction Trachoma, caused by conjunctival infection with Chlamydia trachomatis, is responsible for 15% of all blindness worldwide, and is the leading cause of preventable blindness. 1 The disease is endemic in the poorest countries, where an estimated 6 million people are blind and 148 million have active infections. 2 Despite this toll, trachoma has mostly been a forgotten disease of forgotten people, causing dependency and impeding development. 3,4 However, attitudes are now changing. A WHO global alliance and the International Trachoma Initiative have made major efforts to rid the world of blinding trachoma by 2020. These efforts are based on the SAFE strategy, which consists of trichiasis surgery for those at immediate risk of blindness, antibiotic distribution (supported through donation of patented pharmaceutical by Pfizer), promotion of facial cleanliness, and environmental improvement. 2 SAFE implies more than a drug distribution programme, and the complete strategy needs to be implemented to reduce transmission and to clear current infections. 5,6 Implementation of the full strategy is hampered by gaps in the knowledge about transmission, which hinders policy makers and programme managers, who need to make evidence-based decisions. We did this study to respond to the need for a strengthening of the evidence base for environmental control of trachoma and to build on the findings of our pilot study 7 and other evidence associating the presence of flies on the face with active trachoma. 8–10 The pilot study compared insecticide spraying with no intervention in two pairs of villages and showed a 61% (95% CI 23–80) reduction in trachoma prevalence associated with fly control but lacked statistical power 11 and was unmasked. 12 We aimed to test the hypothesis that eye-seeking flies are trachoma vectors. The Bazaar fly Musca sorbens is the most likely trachoma vector since it is strongly attracted to human eyes. 13,14 Studies on its ecology have shown that it breeds preferentially in exposed human faeces, but not latrines. 15,16 We tested whether latrine provision, without additional health education, would result in fewer M sorbens contacting eyes, and hence reduce trachoma. Methods Study population, recruitment, and randomisation We did our study in the North Bank and Central River divisions of The Gambia between September, 1999, and September, 2001. These divisions were known to have a high trachoma endemicity. 17 Since fly control is best achieved at the community level, rather than by household or individual, the trial was randomised by cluster, each of which consisted of one or more closely neighbouring rural communities, to give a total population of 300–550. Clusters were at least 1·5 km apart but were not matched since this would have reduced the interpretability and statistical power Role of flies and provision of latrines in trachoma control: cluster-randomised controlled trial Paul M Emerson, Steve W Lindsay, Neal Alexander, Momodou Bah, Sheik-Mafuji Dibba, Hannah B Faal, Kebba O Lowe, Keith P W J McAdam, Amy A Ratcliffe, Gijs E L Walraven, Robin L Bailey Medical Research Council Laboratories, PO Box 273, Banjul, The Gambia (R L Bailey FRCP, S-M Dibba, P M Emerson PhD, Prof K P W J McAdam FRCP, A A Ratcliffe DSc, G E L Walraven MD); School of Biological and Biomedical Sciences, University of Durham, Durham, UK (P M Emerson, Prof S W Lindsay PhD); National Eye Care Programme, Ministry of Health, The Gambia (M Bah MSc, H B Faal FRCOphth, K O Lowe); and London School of Hygiene and Tropical Medicine, London, UK (N Alexander PhD) Correspondence to: Dr Paul Emerson, School of Biological and Biomedical Sciences, University of Durham, South Road, Durham DH1 3LE, UK (e-mail: p.m.emerson@durham.ac.uk) Articles For personal use. Only reproduce with permission from The Lancet.