The sight of a fly on food often arouses reluctance for fear that it may have contaminated our meal. But is it really like that? Should we be so worried as to avoid eating at least the part of the food on which the fly landed?
The fear that flies can contaminate food means that, usually, one does not have much pleasure at the idea of eating something where one of these insects has stopped. But what is true? The literature is full of analyzes that, at least on paper, prove those who are afraid right.
First of all, yes, it’s true, technically flies “vomit” on our food when they land on it, because, not having a very complex digestive structure (for example, they don’t have teeth) they “spit” digestive enzymes onto the food which then allow the insect to feed itself. As if we spit saliva on food before eating it.
And this actually makes a bit of an impression. But the point is the rest: the short life of a fly, notoriously, passes between excrement and decomposing organic material in generaltherefore his spitting is not exactly healthy, and theoretically can dirty what was previously clean.
Furthermore, to become an adult, the fly really needs live bacteria.
Some bacteria resist digestion and the subsequent proteolytic processes that occur during metamorphosis and are transported from one growth stage to the next – as stated in a 2017 review article – Adult houseflies ingest bacteria directly from septic substrates or indirectly through self-cleaning. Ingested bacteria also address digestion in adults; however, some microbes not only survive, but they proliferate and exchange genetic material
Scientists confirm that in fact flies, traveling even for kilometers, can disperse pathogens as well as their antibiotic resistance and virulence genes.
Taken together, these aspects of adult fly biology underlie their role in the epidemiology and ecology of infectious diseases
the authors explain
But is it really dangerous to eat food if a fly has landed on it?
Research led by the Department of Veterinary Medicine of the University of Milan collected 101 house flies from 3 different sites of a commercial dairy and grew the identified bacterial culture on external surfaces and internal parts of house flies, identifying 26 pathogens that cause mastitis, 5 food sources and 5 organisms capable of alter the milk.
However, this does not mean that if a fly lands on our food, eating it means getting sick, especially if we have a healthy body with a competent immune system. The possibility that the event could actually cause us an infection depends on some factors, first of all the quantity of flies that remained on our food and for how long.
If a single fly has brief contact with something freshly cooked, even a doctor would say it’s no big deal and there’s no need to throw away the food. Of course, if dozens of flies have feasted up there, perhaps common sense dictates throwing everything away.
Many researchers then observe that in the poorest countries, where very serious gastrointestinal diseases circulate, the quantity of flies is not under control, but it is also true that this depends on precarious sanitary conditions.
Anyway, .
(…) the larvae can be used advantageously to clear manure of pathogens before it is used as a fertilizer – the researchers note – Furthermore, housefly defense effectors such as antimicrobial peptides serve as an untapped resource with the potential to generate new classes of microbicidal therapies
Relative risks and the role of the immune system
While flies can indeed transfer pathogens to foods they land on, the likelihood of contracting an infection is related to various factors, such as the quantity of flies and the length of exposure. A single quick contact on freshly prepared food may not pose significant risks to a healthy person with a competent immune system. The situation is different in which many flies have “feasted” on the same food for a long period; in this case, common sense suggests avoiding consuming the food. Furthermore, in countries with serious sanitation problems, flies can contribute to the spread of dangerous gastrointestinal diseases, but the environmental context plays a fundamental role in such cases.
An unexpected silver lining: the potential of flies in medicine
Not all “evils” brought by flies are necessarily harmful. Researchers are exploring the use of fly larvae to reduce the presence of pathogens in manure, making it safer as a fertilizer. Furthermore, antimicrobial peptides produced by houseflies represent a potential resource for developing new microbicidal therapies. The unique biology of these insects could therefore have useful applications not only in the agricultural field but also in the medical field, showing how nature knows how to surprise and prove precious even where we least expect it.
Sources: Annals of Entomological Society of America / National Library of Medicine / JDS Communications
