Fly control yields fewer trachoma cases.

A concerted effort to kill off houseflies and their cousins, bazaar flies, in two West African villages has reduced the incidence of trachoma, a bacterial disease that causes blindness. The experiment indicates that these common flies carry trachoma and that controlling them can stifle the spread of the disease.

The scientists sprayed the insecticide deltamethrin in and around one village in Gambia every other day in the rainy season for 2 weeks, while leaving another village unsprayed. For the next 10 weeks, the researchers treated the test village twice a week. In the dry season, the scientists carried out a similar experiment on two other villages. The spraying induced no ill effects in the residents.

Researchers used traps to measure fly populations. In both seasons, the average number of flies tallied in the sprayed areas was less than half that in the unsprayed areas.

Cases of trachoma, caused by the bacterium Chlamydia trachomatis, declined in both treated villages but not in the untreated sites, the scientists report in the April 24 Lancet. In the wet season, only 11 of 295 villagers had trachoma after 3 months of spraying, compared with 37 of 271 in the untreated village. In the dry season, 19 of 189 people in the treated area had the disease, compared with 32 of 169 in the unsprayed village.

Flies were long suspected to carry trachoma bacteria from person to person, but until this study, "nobody had successfully demonstrated it," says coauthor Robin L. Bailey of the London School of Hygiene and Tropical Medicine. The researchers also demonstrated that captured flies harbor the microbe.

Doctors believe that flies spread the bacteria by alighting on fluid in and around the eyes, mainly of children. Motherchild proximity may therefore explain why more women than men get trachoma, Bailey says.

The disease is an inflammation that causes repeated scarring of the inside of the eyelid. The eyelashes turn inward, scratching the cornea over time. Repeat infections can leave the cornea opaque, causing blindness, usually in late middle age, Bailey says. Antibiotics can readily cure a case of trachoma, but people in areas with little access to clean water face a lifetime of reinfection. A World Health Organization campaign aims at eradicating the disease by promoting sanitation, face washing, antibiotic dosing, and eyelid surgery, when needed.