Scientists say fly vomit may pose greater risk to humans than insect bites

Houseflies may pose a larger risk to human health than commonly believed, according to scientists.

Research from the University of Massachusetts Amherst, US, published in theInsects journal, suggests that these insects could be carriers of disease-producing pathogens, which they transmit to humans through what is known as “fly vomit”.

Daily, flies will typically feed on a variety of foods including roadkill, animal faeces, food waste and other rubbish.

Each time a fly feeds, it fills its “crop”. The crop is a part of a fly’s anatomy where food is stored.

This food is later transported to the digestive tract where it is turned into energy for the fly.

As the crop is primarily for storage, not digestion, very few digestive enzymes or antimicrobial peptides are present. If present, these enzymes would neutralise pathogens.

Without these, the crop “inadvertently also becomes a place to store disease-producing pathogens”, experts explained.

As the fly moves onto its next feeding station, it will rid itself of excess water in its crop by a process known as “bubbling” or regurgitating the water out. This is commonly known as fly vomit.

For example, if the insect lands on your sandwich and regurgitates some of this water before filling its crop, any illness-causing pathogens in the crop could also be transmitted.

The research found that while male and female flies visited faecal matter in equal quantities, female flies – especially those developing eggs – were more likely to feed on animal carcasses.

“A female with developing eggs not only visits carcasses more frequently, but she also produces more regurgitation/defecation than females with eggs ready to be laid,” the study said, adding: “Thus, females make better anal disseminators of pathogens and parasites.”

John Stoffolano, a professor of entomology at the university’s School of Agriculture, said previously, most research has focused on biting flies that spread diseases by transferring infected blood from one host to the next.

He fears that non-biting flies, known as synanthropic flies, could pose a greater risk to human health.

“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,” Stoffolano said.

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