The next time a fly lands on your food, consider throwing that bite away. A new review suggests that the dung that flies shed could very well be infected with a pathogen.
When you think of a disease-carrying insect, chances are you picture a blood-sucking mosquito or tick. But recent findings suggest that your average non-biting housefly (Musca domestica) may pose a greater threat to human health than is commonly believed.
Houseflies contain an organ at the beginning of their gut known as a crop, which stores food before digesting it. This organ is also a great place for microbes and parasites to hide.
If a fly lands on your food, chances are the insect will regurgitate some of its crop contents and some digestive enzymes. With no teeth, this is how the fly breaks down its meal so it can be sucked up through its straw-like mouth.
In addition to spitting enzymes, the fly may also be able to gag from its crop viruses and bacteria that were previously picked up from other food sources, such as wounds, saliva, mucus or poop.
A recent review of this overlooked transmission route was initially prompted 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 flipped through the pages, he realized that the houseflies he’d been working on for over half a century had been largely ignored as pathogens.
“I’ve been working on [non-biting] has been flying since I was a graduate student in the 1960s. And [non-biting] flies have been largely ignored,” says Stoffolano of the University of Massachusetts Amherst.
“Blood-feeding flies have attracted attention, but we need to pay attention to those who live among us because they get their nutrients from humans and animals that release pathogens in their tears, feces and wounds.”
Because flies are attracted to dirt, such as dead animals and their droppings, non-biting insects are likely to spread pathogens from one animal to another as they buzz around.
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 lab experiments that houseflies can even carry SARS-CoV-2, mechanically transporting the live virus to new hosts on their legs, wings or mouthparts.
But it’s not just the mechanical transmission that we should be concerned about. In the 1990s, a study found that Escherichia coli bacteria can multiply in and on the mouthparts of houseflies.
In retrospect, Stoffolano now thinks this happens because flies constantly spit out the contents of their crop during feeding and grooming (where insects smear themselves on themselves).
In 2021, for example, a study showed that house flies are infected with Chlamydia tachomatis can keep this pathogen alive in their crop for 24 hours – enough time to fly away and regurgitate on a new host.
Another study found that pathogens can remain in the crop for at least 4 days.
However, as scientists continue their work to understand these nasty creatures, keep in mind that the risks are low if food isn’t left out for too long.
“While there’s little doubt that flies can bring bacteria, viruses and parasites from waste to our food, a single touch is unlikely to set off a chain reaction leading to illness for the average healthy person,” wrote entomologist Cameron Webb of the University of Washington. Sydney in 2015.
Nevertheless, many studies to date that have examined the insides of flies for pathogens have not specified what part of the fly they dissected. Steffano says researchers should examine the crop because it has more moisture for microbes and potentially parasites to bathe in.
Researchers should also keep in mind that some fly species have larger crops and therefore may carry more pathogens, posing a greater risk when those 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 is published in Insect.