Researchers have always suspected that houseflies carry diseases and bacteria; a new study meticulously investigates the bacterial composition of the individual body parts of over a hundred flies living across three continents. The findings shed light on the extent of the flies’ disease-carrying potential.

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Houseflies may carry more pathogens than previously believed, suggests new research.

The first author of the new research is Ana Carolina Junqueira, professor of genetics and genomics at the Federal University of Rio De Janeiro in Brazil. Prof. Junquiera is also one of the two corresponding authors of the new paper.

Prof. Junqueira and her international team of researchers examined the microbiomes of 116 houseflies and blowflies living in urban, rural, and natural environments in Brazil, the United States, and Singapore.

Some of the urban habitats they collected the flies from were a food market, a public hospital, a public park, and a landfill for household waste. They also collected flies from a protected area in the Amazonian rainforest, and from various animal farms.

The researchers extracted and sequenced DNA from the head, thorax, abdomen, legs, and wings of blowflies — scientifically known as Chrysomya megacephala — and houseflies, known as Musca domestica.

The scientists analyzed the bacteria, and their findings were published in the journal Scientific Reports.

“This is the first study that depicts the entire microbial DNA content of insect vectors using unbiased methods,” Prof. Junqueira says. “Blowflies and houseflies are considered major mechanical vectors worldwide, but their full potential for microbial transmission was never analyzed comprehensively using modern molecular techniques and deep DNA sequencing.”

Prof. Junqueira and colleagues found 33 different species of bacteria, many of which can also act as human pathogens.

In particular, they found a high incidence of Helicobacter pylori, a pathogen that can cause peptic ulcers in humans and is a risk factor for stomach cancer and MALT, a type of non-Hodgkin’s lymphoma.

H. pylori was found in 15 of the 116 flies. It is not yet fully understood how H. pylori is transmitted, the researchers explain. They also say that until now, flies have not been considered as a potential vector for the transmission of this pathogen.

We believe that [the study] may show a mechanism for pathogen transmission that has been overlooked by public health officials, and flies may contribute to the rapid transmission of pathogens in outbreak situations.”

Study co-author Donald Bryant, Pennsylvania State University

Blowflies and houseflies may carry numerous diseases because they breed and feed off of decaying matter and feces, where they can easily pick up bacteria and transport them further.

The study’s other corresponding author, Stephan Schuster, research director at Nanyang Technological University in Singapore, reports on the additional insights the study has brought into how flies can help bacteria to spread.

“The legs and wings show the highest microbial diversity in the fly body, suggesting that bacteria use the flies as airborne shuttles,” he says. “It may be that bacteria survive their journey, growing and spreading on a new surface.”

“In fact,” Schuster adds, “the study shows that each step of hundreds that a fly has taken leaves behind a microbial colony track, if the new surface supports bacterial growth.”

“It will really make you think twice about eating that potato salad that’s been sitting out at your next picnic,” says Prof. Bryant.

“It might be better to have that picnic in the woods, far away from urban environments, not a central park,” Prof. Bryant continues, as the flies collected from rural habitats carried fewer pathogens than those in urban environments.

Given how easily they can pick up bacteria, flies could be used to sample microbes from areas that would otherwise be difficult to access, suggest the researchers.

“[F]lies could be intentionally released as autonomous bionic drones into even the smallest spaces and crevices and, upon being recaptured, inform about any biotic material they have encountered,” Schuster says.