Elsevier

The Lancet

Volume 353, Issue 9162, 24 April 1999, Pages 1401-1403
The Lancet

Articles
Effect of fly control on trachoma and diar rhoea

https://doi.org/10.1016/S0140-6736(98)09158-2Get rights and content

Summary

Background

Domestic flies are accepted vectors of diarrhoea, but their role in trachoma transmission has never been quantified and no study has shown that fly control decreases the prevalence of trachoma. We assessed the effect of fly control on public health in a pilot study in Gambian villages.

Methods

We studied two pairs of villages—one pair in the 1997 wet season, and one pair in the 1998 dry season. For each pair, deltamethrin was sprayed for 3 months to control flies in one village whilst the other was used as a control. Fly populations were monitored with traps. We surveyed trachoma at baseline and at 3 months, and collected daily data on diarrhoea in children aged between 3 months and 5 years.

Findings

Fly control decreased numbers of muscid flies by around 75% in the intervention villages compared with controls. Trachoma prevalence was similar at baseline (wet season, prevalence in intervention village 8·8% vs control 12·2%; dry season, 18·0% vs 16·0%), but after 3 months of fly control there were 75% fewer new cases of trachoma in the intervention villages (wet season 3·7% vs 13·7%; dry season 10·0% vs 18·9%; rate ratio and relative risk of pooled data 0·25 [adjusted 95% Cl 0·09–0·64], p=0·003). There was 22% less childhood diarrhoea in the wet season (14% vs 19%, period prevalence ratio 0·78 [0·64–0·95], p=0·01), and 26% less diarrhoea in the dry season (6% vs 8%; 0·74 [0·34–1·59], p=0·60) compared with controls.

Interpretation

Muscid flies are important vectors of trachoma and childhood diarrhoea in The Gambia. Deltamethrin spray is effective for fly control and may be useful for reducing trachoma and diarrhoea in some situations, but further research on sustainable fly–control methods is needed.

Introduction

Flies act as mechanical vectors of many pathogens, but they are commonly overlooked in public health interventions. Childhood diarrhoea causes 3·3 million deaths worldwide per year, with an estimated 1 billion cases of diarrhoea annually in children under 5 years.1 Flies are known vectors of diarrhoea-causing pathogens, particularly those requiring a minute inoculum such as shigella.2, 3, 4, 5 Chlamydia trachomatis causes trachoma, and has been found on flies fed on heavily infected laboratory culture media.6 There is an association between flies and trachoma,7, 8, 9 but as yet there are no field-based or epidemiological studies that show that flies are direct vectors of trachoma.

Trachoma is the main cause of preventable blindness worldwide.10 Such blindness is more common in women than men11 and results from repeated ocular C trachomatis infections over many years. In les-developed countries, young children are the reservoir of infection, and transmission is clustered within villages,10, 11, 12 but the mechanisms by which infection is spread are poorly understood. An intervention that lowers the frequency of transmission is likely to lower the prevalence of trachoma-related blindness in the future.

The WHO initiative for the global elimination of trachoma by the year 2020 (GET 2020) aims to control trachoma by eyelid surgery, antibiotic treatment, facial cleanliness, and environmental improvement (the SAFE strategy). However, the implementation and sustainability of antibiotic therapy is beset by poor compliance with topical application of ophthalmic tetracycline eye ointment13 and by the prohibitive cost of oral azithromycin. Trachoma is associated with poverty, and largely disappears under improved environmental conditions and access to better sanitation.14 Nonetheless, the prospect of rapid global improvement remains bleak. The SAFE strategy would be strengthened by the inclusion of realistic and sustainable methods to reduce trachoma transmission.

We undertook a pilot study to investigate the role of domestic flies in the transmission of trachoma and diarrhoea.

Section snippets

Study population

Our study took place in four villages over 3 months in the Sanjal area of The Gambia.15 One pair of villages was studied from September, 1997, to December, 1997 (the wet season), the other pair was studied from January, 1998, to April, 1998 (the dry season). The two pairs of villages were selected from 23 available small villages with populations of less than 400 in the Sanjal district on the basis of trachoma prevalence, lack of immediate access to primary health-care, homogeneous ethnic

Entomology

Muscid flies (Musca sorbens-bazaar fly: M domestica- housefly) were more abundant in the wet–season control village than in the dry–season control village the median (range) of the adjusted geometric mean numbers of flies per trap per day was 9·80 (2·29–37·3) versus 5·98 (2·41–38·3), respectively, for M sorbens; and 10·2 (2·5–102·9) versus 4·24 (1·14–12·05) respectively, for M domestica. Spraying resulted in 76% fewer M sorbens (2·15 [0·19–5·62], p=0·02) and 57% fewer M domestica (4·44

Diarrhoea

All resident children aged between 3 months and 60 months were recruited for diarrhoea surveillance, 263 children in total. Data were obtained for over 90% of possible child-days of observation, with no significant differences between villages or age groups. The distribution of numbers of children under 60 months per family was not significantly different in the four villages (table).

Children in the dry season had 58% less diarrhoea than those studied in the wet season (period prevalence

Discussion

In our pilot study the control of muscid flies lowered the transmission of trachoma and the period prevalence of diarrhoea in children. However, our study was done in only four small settlements and may have been affected by bias—for example, realisation that there were fewer flies at certain times might have made the mothers report less diarrhoea and led the nurse to make differential observations of trachoma. We cannot exclude this possibility, but it does not seem likely to have affected our

References (19)

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