Responses of the putative trachoma vector,Musca sorbens, to volatile semiochemicals from human faeces
Authors: Robinson Ailie , Bristow Julie , Holl Matthew V. , Makalo Pateh , Alemayehu Wondu , Bailey Robin L. , McLeod David , Birkett Michael A. , Caulfield John C. , Sarah Virginia , Pickett John A. , Dewhirst Sarah , Chen-Hussey Vanessa , Woodcock Christine M. , D’Alessandro Umberto , Last Anna , Burton Matthew J. , Lindsay Steve W. , Logan James G. 
Published: August 2019

Background

The putative vector of trachoma,Musca sorbens, prefers to lay its eggs on human faeces on the ground. This study sought to determine whetherM. sorbensfemales were attracted to volatile odours from human faeces in preference to odours from the faeces of other animals, and to determine whether specific volatile semiochemicals mediate selection of the faeces.

Methodology/Principal findings

Traps baited with the faeces of humans and local domestic animals were used to catch flies at two trachoma-endemic locations in The Gambia and one in Ethiopia. At all locations, traps baited with faeces caught more femaleM. sorbensthan control traps baited with soil, and human faeces was the most successful bait compared with soil (mean rate ratios 44.40, 61.40, 10.50 [P<0.001]; 8.17 for child faeces [P=0.004]). Odours from human faeces and some domestic animals were sampled by air entrainment. Extracts of the volatiles from human faeces were tested by coupled gas chromatography-electroantennography with laboratory-reared femaleM. sorbens. Twelve compounds were electrophysiologically active and tentatively identified by coupled mass spectrometry-gas chromatography, these included cresol, indole, 2-methylpropanoic acid, butanoic acid, pentanoic acid and hexanoic acid.

Conclusions/Significance

It is possible that some of these volatiles govern the strong attraction ofM. sorbensflies to human faeces. If so, a synthetic blend of these chemicals, at the correct ratios, may prove to be a highly attractive lure. This could be used in odour-baited traps for monitoring or control of this species in trachoma-endemic regions.

Author summary

Musca sorbens, also known as the Bazaar Fly, visits people’s faces to feed on ocular and nasal discharge. While feeding,M. sorbenscan transmitChlamydia trachomatis, the bacterium that causes the infectious eye disease trachoma. Around 1.9 million people worldwide are visually impaired or blind from this disease. Although it is believed thatM. sorbenstransmits trachoma, very few studies have looked at ways to control this fly. A large-scale trial has shown that control of fly populations with insecticide reduces active trachoma disease prevalence. Odour-baited traps for the suppression of disease vector populations are an attractive option as there is no widespread spraying of insecticide, however, highly attractive baits are critical to their success. Here we demonstrate that the preference of these flies for breeding in human faeces is probably mediated by odour cues, and we isolate chemicals in the odour of human faeces that cause a response in the antennae ofM. sorbens. These compounds may play a role in the specific attractiveness of human faeces to these flies, perhaps by being present in greater amounts or at favourable ratios. These may be developed into a chemical lure for odour-baited trapping to suppressM. sorbenspopulations.