# Field- and laboratory-based studies on correlates of Chlamydia trachomatis transmission by Musca sorbens: Determinants of fly-eye contact and investigations into fly carriage of elementary bodies

## TL;DR

This study shows that Musca sorbens flies can carry the trachoma-causing bacterium and may contribute to its spread, especially in households with infected individuals.

## Contribution

The study provides new evidence on how Musca sorbens flies transmit Chlamydia trachomatis and identifies factors influencing fly-eye contact.

## Key findings

- Ct-carrying flies cluster near households with Ct-infected residents.
- Fly-eye contact is linked to trachoma, lower body weight, and higher body temperature in children.
- Ct was found on fly bodies but rarely on legs or heads, suggesting transmission via thorax, wings, and abdomen.

## Abstract

Musca sorbens (Diptera: Muscidae) flies are thought to be vectors of the blinding eye disease trachoma, carrying the bacterium Chlamydia trachomatis (Ct) between the eyes of individuals. While their role as vectors has been convincingly demonstrated via randomised controlled trials in The Gambia, studies of fly-borne trachoma transmission remain scant and as such our understanding of their ability to transmit Ct remains poor. We examined fly-eye contact and caught eye-seeking flies from 494 individuals (79% aged ≤9 years) in Oromia, Ethiopia. Ct-carrying flies (harbouring Ct DNA) were found to cluster spatially in and nearby to households in which at least one resident had Ct infection. Fly-eye contact was positively associated with the presence of trachoma (disease), lower human body weight and increased human body temperature. Studies of laboratory-reared M. sorbens indicated that Ct is found both externally and internally following feeds on Ct culture, with scanning electron microscopy revealing how Ct bodies can cling to fly hairs (setae). Testing for Ct on field-caught M. sorbens found fly ‘bodies’ (thorax, wings and abdomen) to consistently test positive for Ct while legs and heads were infrequently Ct-positive. These studies strongly support the role of M. sorbens as vectors of trachoma and highlight the need for improved understanding of fly-borne trachoma transmission dynamics and vector competence.

Trachoma is a blinding eye disease caused by infection of the eye by the bacterium Chlamydia trachomatis (Ct). Trachoma is responsible for vision impairment or blindness in 1.8 million people and is the leading infectious cause of blindness worldwide. The presence of flies around the eyes of young children is a common feature of life in trachoma endemic communities worldwide. This fly is most often Musca sorbens, an understudied species thought to transmit Ct.

In order to understand more about Musca sorbens flies and their relationship with Ct, the researchers studied 247 households in the Oromia region of Ethiopia, where there is known to be a lot of trachoma. The authors found evidence of Ct on flies and that flies in households containing children with Ct infection were much more likely themselves to be carrying Ct, adding to evidence that M. sorbens flies contribute to transmission of the bacterium. They also found that children who weigh less and currently have trachoma were more likely to have flies land on their face. Should further studies clearly elucidate a role for M. sorbens as vectors of trachoma, controlling fly populations or preventing fly-eye contact may become an important aspect of trachoma disease control.