Why is the housefly an overlooked potential vector for disease?

The next time a fly lands on your food, you might consider discarding that bite. A new review suggests that the mud that flies regurgitate may be very well contaminated with the pathogen.

When you think of disease-transmitting insects, you probably imagine a blood-sucking mosquito or tick. But recent findings suggest that your average non-biting housefly (Housefly) may pose a greater risk to human health than is often believed.

House flies contain an organ at the beginning of their intestine that stores food before digestion. This organ is also a great hiding place for microbes and parasites.

When a fly lands on your food, there is a good chance that the insect will vomit some of its plant’s contents and some digestive enzymes. Without teeth, this is how a fly breaks down its meal so that it can be sucked up through its straw mouth.

In addition to expelling enzymes, the fly can also vomit viruses and bacteria from its crops that have been previously harvested from other food sources, such as wounds, saliva, mucus, or poop.

A recent review of this overlooked route of transmission was initially inspired by the COVID-19 pandemic when the author, entomologist John Stoffolano, read a book entitled Side effects: animal infections and the next human pandemic.

As Stoffolano flipped pages, he realized that the house flies he had worked on for more than half a century had been largely ignored as transmitters of disease.

“I’ve been working on [non-biting] has been going on since I was a student in the 60’s. AND [non-biting] the flies were largely ignored, ”says Stoffolano of the University of Massachusetts Amherst.

“Blood-sucking flies are in the spotlight, but we should pay attention to those that live among us, because they get their nutrients from humans and animals that shed pathogens in their tears, faeces and wounds.”

Since flies are attracted to dirt, as are dead animals and their droppings, non-biting insects likely transfer pathogens from one animal to another as they buzz around.

According to one recent study, more than 200 different pathogens have been detected in adult house flies, including some bacteria, viruses, worms, and fungi.

In 2020, scientists demonstrated in laboratory experiments that house flies can even transmit SARS-CoV-2 by mechanically transmitting live virus to new hosts on legs, wings or mouthparts.

But it’s not just about the mechanical gearbox that we have to worry about. In the 1990s, studies showed that Escherichia coli bacteria can multiply in and on the mouth organs of house flies.

In retrospect, Stoffolano believes this is because the flies continually throw away the contents of their crops during feeding and grooming (where the insects smear with vomit).

For example, in 2021, a study showed that house flies were infected Chlamydia tachomatis it can keep this pathogen alive on its crop for 24 hours – plenty of time to fly away and return to its new host.

Another study found that pathogens can remain in the crop for at least 4 days.

While scientists continue to work to understand these dirty creatures, remember, however, that the risk is low if food is not left outside for too long.

“While there is no doubt that flies can transmit bacteria, viruses, and parasites from waste to our food, it is unlikely that a single application will trigger a chain reaction leading to disease in the average healthy person,” wrote University of Sydney entomologist Cameron Webb. 2015.

However, many studies to date that have examined the inside of flies for pathogens have not identified what proportion of the flies they clipped. Steffano says scientists should investigate the crop because it contains more fluid in which microbes and possibly parasites can bathe.

Scientists should also note that some fly species have higher yields and therefore may be able to carry more pathogens, posing a greater risk when these insects wander.

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

The study was published in Insect.

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