Trachoma and flies. Individual vs environmental risk factors

The risk of active trachoma in children appears to be higher in association with flies in the environment. However, a measure of fly density that could consistently be related to an increased risk of trachoma is unknown. In a survey of six villages in a hyperendemic area of Tanzania, a comparison was made between the number of flies on the faces (face-fly scores) of children and the number of household flies around the main doorways (household-fly scores). The risk of trachoma associated with each measure was evaluated after adjusting for the age and sex of the child. A multiple logistic regression model demonstrated that the presence of flies on the face was consistently associated with increased risk of trachoma, and that number of flies on the face is a superior predictor in terms of ease of measurement and strength of association than is number of household flies.