Seasonal variation in trachoma and bush flies in north-western Australian Aboriginal communities

Background: Among younger age groups trachoma (Chlamydia trachomatis) has been identified as a major cause of morbidity in Australian Aboriginal communities. North-western Australia has two seasons, referred to as the wet and the dry, and until recently most trachoma screening programmes were conducted during the dry season. This study compared the prevalence of trachoma between three Aboriginal communities, two in the west and one in the east Kimberleys with differences in adult bush fly (Musca vetustissima) populations between the wet and dry seasons. Methods: All preschool and school-aged children in each community were screened for trachoma in February and July 1996 using the World Health Organization method for clinical assessment of trachoma. Flies were trapped fortnightly from September through to May (inclusive) using a wind-orientated fly trap. Results: Two communities in the west Kimberleys had a significantly higher rate of trachoma during the wet season (14-59% in dry season compared with 46-69% in wet season). One community showed no difference but this was probably due to the reduced re-screening rate. Further-more, it was demonstrated that fly populations are so low during the dry season that they were untrappable; however, populations of bush fly significantly increased during the wet season (ranging from 6 to 146 flies per hectare per month). Conclusions: If bush fly populations are correlated with increased levels of trachoma, then measures aimed at augmenting public health plans for bush fly control may decrease the cross-infection rate. Additionally, based on the results of this study, wet season trachoma screening trips should be considered.