When you think of a disease-carrying insect, you probably think of a blood-sucking mosquito or tick. But recent findings suggest that your average housefly doesn’t bite (house fly) may pose a greater threat to human health than is often recognized.
Houseflies contain an organ at the beginning of their gut known as a crop, which stores food before digestion. This organ is also an ideal hiding place for microbes and parasites.
When a fly lands on your food, there’s a good chance the bug will regurgitate some of the contents of its harvest and some digestive enzymes. Toothless, this is how the fly breaks up its food so it can be sucked up through its straw-like mouth.
In addition to spitting out enzymes, the fly may also regurgitate viruses and bacteria from its culture, which had previously been picked up from other food sources, such as wounds, saliva, mucus, or poop.
A recent review of this overlooked route of transmission was initially sparked by the outbreak of the COVID-19 pandemic, when the author, entomologist John Stoffolano, read a book called Spillover: Animal Infections and the Next Human Pandemic.
As Stoffolano turned the pages, he realized that the houseflies he had been working on for more than half a century had been largely ignored as carriers of disease.
“I’ve been working [non-biting] has been flying since he was a graduate student in the 1960s. I [non-biting] flies have been largely ignored,” says Stoffolano of the University of Massachusetts Amherst.
“Blood-feeding flies have taken center stage, but we need to pay attention to those that live among us because they get their nutrients from people and animals that shed pathogens in their tears, droppings and wounds.”
Because flies are attracted to dirt, such as dead animals and their feces, non-biting insects are likely to spread pathogens from one animal to another as they buzz.
According to a recent study, more than 200 different pathogens have been found in adult houseflies, including some bacteria, viruses, worms and fungi.
In 2020, researchers showed in laboratory experiments that houseflies could even carry SARS-CoV-2, mechanically transporting the live virus to new hosts on their legs, wings or mouthparts.
But we don’t just have to worry about the mechanical transmission. In the 1990s, a study found that Escherichia coli bacteria can proliferate in and on the mouthparts of houseflies.
In retrospect, Stoffolano thinks this happens because the flies are constantly regurgitating the contents of their crop during feeding and grooming (where the insects are smeared with vomit).
In 2021, for example, a study found that houseflies were infected Chlamydia tachomatis could keep this pathogen alive in its culture for 24 hours, plenty of time to fly and regurgitate into a new host.
Another study found that pathogens can remain in the culture for at least 4 days.
While scientists continue their work to understand these nasty creatures, keep in mind, however, that the risks are low if food is not left out for too long.
“While there is little doubt that flies can carry bacteria, viruses and parasites from waste to our food, a single landing is unlikely to set off a chain reaction leading to illness for the average healthy person.” wrote University of Sydney entomologist Cameron Webb in 2015.
However, many studies to date that have examined the inside of flies for pathogens have not specified which part of the fly they dissected. Steffano says researchers should examine the culture because it contains more liquid for microbes and possibly parasites to bathe in.
Researchers should also note that some species of flies have larger crops and therefore may be able to carry more pathogens, posing a greater risk when these insects roam.
“It’s the little things that cause the problems,” says Stoffolano. “Our health depends on paying more attention to these flies that live with us.”
The study was published in insect.