One in the eye for flies - a new approach to trachoma prevention

Trachoma and diarrhoea are both associated with poor sanitation. But how important are flies in the transmission of these two diseases? Could fly control reduce the incidence of trachoma and diarrhoea? Researchers from the Medical Research Council Laboratories in The Gambia and the London School of Tropical Medicine and Hygiene tested this strategy in Gambian villages.

The study showed that the incidence of both diseases is significantly reduced by the use of insecticide. Fly control could be a useful addition to the World Health Organisation's (WHO) strategy to eliminate trachoma by 2020.

Trachoma is the world's most common infectious cause of blindness. It is associated with poverty and largely disappears with access to improved sanitation. Although flies can carry the micro-organism responsible for trachoma, the contribution of flies to disease transmission has not been quantified. However, flies are known to transmit diarrhoea, which causes the deaths of 3.3 million children worldwide each year.

The WHO's 'SAFE' strategy to eliminate trachoma by 2020 involves Surgery, Antibiotic treatment, Facial cleanliness and Environmental improvement. However, the mechanisms of transmission of the infection are poorly understood and there are doubts about the sustainability of antibiotic treatment due to poor compliance and prohibitive costs. A realistic method of reducing transmission rates would greatly strengthen the initiative.

This study looked at two pairs of villages in the wet and dry seasons. In each pair, a fly control strategy using insecticide (deltamethrin) was carried out in one village while the other acted as a control. The researchers found that:

• Flies are more common in the wet season than the dry season. Insecticide spraying significantly reduces fly numbers in both seasons.
• One species, M. sorbens makes over 90 percent of fly-eye contacts, despite forming only ten percent of the fly population. The remaining contacts are made by M. domestica. Both are also strongly attracted to faeces and food.
• Trachoma prevalence was lower in the intervention village than in the control village in both seasons.
• Fly control produces a 75 percent reduction in new cases of trachoma. This compares favourably with the effects of face-washing.
• Children suffer 58 percent less diarrhoea in the dry season than the wet season. Episodes of diarrhoea decline with increasing age.
• Diarrhoea prevalence was reduced by around a quarter in the intervention villages in both seasons.

Implications for health policy include:

• Insecticide spraying to control flies could improve trachoma-elimination strategies and would also decrease diarrhoea-related illness.
• However, fly control with insecticide is unlikely to be a sustainable routine public health measure.
• Development and assessment of cost-effective measures for fly control should be a priority.

Source(s):
'Effect of fly control on trachoma and diarrhoea' by P. Emerson et al., The Lancet 353 (1999)

Funded by: UK Department for International Development

id21 Research Highlight: 1 June 2001

Further Information:
Robin Bailey
London School of Hygiene and Tropical Medicine
Keppel Street
London
WC1E 7HT
UK

Contact the contributor: robin.bailey@lshtm.ac.uk

London School of Hygiene and Tropical Medicine, UK

Other related links:
Visit the website of the WHO trachoma elimination programme.

Sightsavers International runs blindness prevention programmes in many countries.

The Journal of Community Eye Health is available free on-line.