Epidemiology of trachoma
Transmission: The role of flies.
Trachoma is caused by the eye to eye transmission of infection with Chlamydiaceae. Traditionally eye seeking (synanthropic) flies are considered a major factor in the spread of trachoma [Jones, 1975] and this indeed seems likely in many areas. Studies in The Gambia using fly traps found that only 2 species of fly, Musca sorbens and M. domestica, were caught from the eyes of children. Significantly, although M. sorbens comprised < 10% of the total number of flies caught with attractant traps it was responsible for > 90% of the fly - eye contacts. As would be expected, all fly species were more numerous in the wet season than the dry season. The median number of eye contacts with children was 3 per 15 minutes, the number of contacts being greatest in those children with potentially infective eye or nasal discharges. Surprisingly, chlamydial DNA was demonstrated on only 2 of 395 flies caught from the eyes of children with a current active trachoma infection. These flies were M. sorbens [Emerson et al., 2000] confirming their potential role as vectors of trachoma. To determine the actual importance of flies as vectors of trachoma in The Gambia, intervention studies were performed in two pairs of villages; one pair in the wet season and one in the dry season. In the test village in each pair, insecticide was sprayed for 3 months to control the fly population, the remaining village acting as an unsprayed control. Fly populations were monitored and the impact on new cases of trachoma (and also diarrhoeal disease) was assessed. There were 75% fewer new cases of trachoma in the test villages versus the controls: (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% CI 0.09-0.64], p=0.003) [Emerson et al., 1999]. These are excellent studies which yielded a remarkable result considering that eye-seeking flies had not hitherto been considered to be a major route of trachoma transmission in The Gambia [Bailey et al., 1989] compared with, for example, the Nile Delta.
Bailey, R., Osmond, C., Mabey, D. C. W., Whittle, H. C. & Ward, M. E. (1989). Analysis of the household distribution of trachoma in a Gambian village using a Monte Carlo simulation procedure. International Journal of Epidemiology 18, 944-955.
Emerson PM, Bailey RL, Mahdi OS, Walraven GE, Lindsay SW (2000) Transmission ecology of the fly Musca sorbens, a putative vector of trachoma. Transactions of the Royal Society of Tropical Medicine and Hygiene 94, 28 - 32.
Emerson, P. M., Lindsay, S. W., Walraven, G. E., Faal, H., Bogh, C., Lowe, K., Bailey, R. L. (1999). Effect of fly control on trachoma and diarrhoea. Lancet 353, 1401 - 1403.
Jones, B. R. (1975). The prevention of blindness from trachoma. Transactions of the Ophthalmological Society of the UK 95, 16 - 33.