Flies can be more than pesky picnic crashers, they carry hundreds of different species of harmful bacteria and may help to spread disease, a study warns.
Researchers at the Pennsylvania State University in the US have warned the public to avoid eating food during picnics.
In a study of the microbiomes of 116 houseflies and blowflies, they found that these flies carry hundreds of different species of bacteria, many of which are harmful to humans.
"We believe that this 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," said Donald Bryant, from the Pennsylvania State University.
The researchers were able to investigate the microbial content of individual fly body parts, including legs and wings.
The legs appear to transfer most of the microbial organisms from one surface to another, said Stephan Schuster, from the Pennsylvania State University.
"The legs and wings show the highest microbial diversity in the fly body, suggesting that bacteria use the flies as airborne shuttles," said Schuster.
"It may be that bacteria survive their journey, growing and spreading on a new surface. In fact, 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," he said.
Blowflies and houseflies - both carrion fly species - are often exposed to unhygienic matter because they use feces and decaying organic matter to nurture their young, where they could pick up bacteria that could act as pathogens to humans, plants and animals.
The study, published in the journal Scientific Reports, also indicates that blowflies and houseflies share over 50 per cent of their microbiome, a mixture of host-related microorganisms and those acquired from the environments they inhabit.
Surprisingly, flies collected from stables carried fewer pathogens than those collected from urban environments.
The researchers found 15 instances of the human pathogen Helicobacter pylori, a pathogen often causing ulcers in the human gut, largely in the blowfly samples.
The known route of transmission of Helicobacter has never considered flies as a possible vector for the disease, said Schuster.
"It will really make you think twice about eating that potato salad that's been sitting out at your next picnic," Bryant said.
"It might be better to have that picnic in the woods, far away from urban environments, not a central park," he said.