Fly vomit on your food ‘far greater risk to health’ than realized — worse than being bitten!


AMHERST, Mass. — Common houseflies are notoriously annoying and elusive, but they’re nothing more than a non-biting nuisance, right? It appears that the flies living among us are more of a threat than most realize, according to researchers from the University of Massachusetts, Amherst. Scientists warn that “synanthropic” flies (non-biting flies typically seen living beside humans) could be carrying diseases in their vomit.

Up until now, epidemiologists have understandably focused the majority of their attention on flies of the biting variety. When an insect bites a human, it can potentially spread diseases by transferring infected blood. Surprisingly, however, study authors say what non-biting flies vomit appears to be a far greater risk to human health.

“I’ve been working on synanthropic flies since I was a graduate student in the 1960s,” says study author John Stoffolano, professor of entomology at UMass Amherst’s Stockbridge School of Agriculture, in a university release. “And synanthropic flies have largely been ignored. Blood-feeding flies have taken the limelight, but we should pay attention to the ones that live among us because they get their nutrients from people and animals that shed pathogens in their tears, feces and wounds.”

Researchers point to the common housefly as an example. Over the course of a typical day, a housefly buzzes around your home, helping itself to various readily available foods. These food sources often include not-so-appetizing dishes such as roadkill, animal dung, rotting garbage, and the sewer. Whenever a housefly feeds, it fills its crop.

“The crop is like a gas tank,” Prof. Stoffolano explains, “a place to store food before it makes its way into the digestive tract where it will get turned into energy for the fly.”

Here’s how a fly leaves disease on your food via vomit

Since the crop is intended for storage, not digestion, it lacks an adequate number of digestive enzymes or antimicrobial peptides that would neutralize the majority of potentially harmful pathogens present in the fly’s meals. So, in a roundabout way, the crop serves as a place to store disease-producing pathogens.

Let’s imagine a fly takes flight just after enjoying some fresh dog feces from the nearby sidewalk. Its crop is filled with leftovers. As the fly buzzes by, it rids itself of excess water in its crop by “bubbling,” or regurgitating water out. This creates a “misting” effect, potentially contaminating everything it contacts.

home fly sitting on delicious ham sandwich
Home fly sitting on delicious ham sandwich (Credit: University of Massachusetts Amherst )

Eventually, that same fly makes its way into your home and lands on a sandwich you’re making in the kitchen. Before the fly actually eats some of your food, it vomits up a portion of what’s left in its crop onto your bread. Even worse, the research team notes a fly’s crop is known to promote the development of antibacterial resistance, meaning what gets regurgitated onto your food may even be resistant to conventional treatments or drugs.

All in all, study authors say this is an important health topic that is woefully understudied. Prof. Stoffolano goes so far as to say modern science hasn’t even figured out the “basics” yet. For example, how robust are the immune systems of different synanthropic flies? Or, do flies incubate and encourage the proliferation of harmful pathogens in their guts, or are they simply disease carriers? Do crop volumes vary according to species? There are also questions regarding male flies versus female flies.

“It’s the little things that cause the problems,” Prof. Stoffolano concludes. “Our health depends on paying closer attention to these flies that live with us.”

The study is published in the journal Insects.

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