Flies Are Even More Disgusting Than We Thought, Acting As 'Airborne Shuttles' For Disease

Housefly
A new study shows that flies carry diseases that could be harmful to humans. Amrita Bhattacharyya on Flickr

The flies in your home, at your picnic, and at the local food court may be even grosser than you thought. A new study finds that they are crawling with bacteria, and even host diseases that could be harmful to humans.

Researchers at Penn State collected 116 blowflies and houseflies from different habitats in Brazil, Singapore and the United States. They sampled from urban sites such as a food market and a hospital emergency entrance, rural sites including farms, and natural sites such as the Amazon Rainforest. In addition to the wild-caught flies, they also sampled control flies from a colony of blowflies that had been captive-bred for 20 generations. They found that the bodies of the animals, especially the wild-caught ones, were covered in bacteria—including bacteria that can be harmful to humans.

Most bacteria found on the flies wouldn’t be able to infect a human host, but Helicobacter pylori can live in your gut for years and then form ulcers and could, according to some reserach, increase the risk of stomach cancer. The researchers found virulent strains of the bacteria on several blowflies. However, they also noted that the chance that you could be adversely affected by this microbe is highly dependent on how susceptible you are to infection. Half of the world’s population already has this bacteria in their gut, but it’s not clear how many people got the infection from flies.

After collecting the wild-caught flies, the researchers sequenced the genomes of the life found on their respective bodies. They found that the wings and legs of the animals had the most microbial diversity, or different types of microbes living on them. Surprisingly, the flies collected from urban environments were covered with even more pathogens than those collected from stables. The research was published in the journal Scientific Reports.

It shouldn’t be too surprising that flies might carry pathogens. Flies aren’t known for their attraction to cleanliness, and prefer to lay their eggs on rotting carcasses and in feces. However, they do keep themselves clean enough. If you’ve ever seen a fly rubbing its legs together and scrubbing its face, that fly was actually cleaning itself, just as a cat or mouse would.

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