Spring has finally arrived. To celebrate the long-awaited warm weather, your friends decide to have a picnic in the park. But just as your friend brings her sandwich to her mouth, she finds herself staring at a fly perched on top of her bread. She swats it away and then glances at you. It’s probably fine to eat the sandwich anyway, right?
When a fly touches down, it begins exploring the new terrain for food. If it deems the substance meal-worthy, it will regurgitate saliva onto the surface, says Dana Nayduch, a molecular biologist at the US Department of Agriculture. Flies don’t have teeth, so they spurt saliva from a snout-like mouth to liquefy food, which can then be sucked in and swallowed. This process begins minutes or seconds after a fly lands. In addition to regurgitation, the fly might defecate as it explores the new surface.
What the fly transfers onto your food depends on its travels before landing on your lunch. The insect might have visited some unsavory spots, such as dumpsters and dog poop in a more urban area, or farms and animal carcasses in a more rural area. The fly could have also ingested bacteria or carried it along on its feet, wings, or mouth. In fact, Nayduch and her colleagues have identified more than 200 pathogens that have been found in adult house flies.
A range of factors, such as the fly’s travels, determine what exactly happens when it lands on your food. The outcome depends on the species of bacteria the fly harbors, the amount of bacteria, whether the bacteria is on or inside the fly, proximity to the source of the bacteria, and even the sex of the fly, Nayduch says.
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The Worst That Will Happen
The worst case scenario would occur if the fly was harboring harmful bacteria and that bacteria had time to grow in your food. For example, if the fly explored a garbage can or animal droppings, and then spent a significant amount of time in your sandwich or in the serving dish. Maybe you were distracted and didn’t see the fly land, or you decided to go back for seconds after the food was sitting out for a few hours. In cases like that, you could contract an infection from salmonella or E. coli, says William Kern, an entomologist at the University of Florida. This would lead to a very unpleasant few days, but the illness wouldn’t be life-threatening for healthy adults.
That’s the worst situation in the United States, but flies pose a larger threat in other parts of the world. In countries without established plumbing or sanitation systems, flies may harbor pathogens from human waste and pass on deadlier diseases like cholera, vibriosis, or dysentery, Nayduch says.
What Will Probably Happen
Probably nothing. If you aren’t sitting near an especially germ-infested area, and you swatted the fly away fairly quickly, you can feel more confident that you won’t consume harmful bacteria. Our bodies are also skilled at navigating the countless microbes we encounter on a daily basis. “Every day you touch doorknobs, money, and credit cards, and then you bite your nails or rub your eye and inoculate yourself with bacteria, but our immune system takes care of it,” Nayduch says. “So flies pose very little threat in a country like ours that has good sanitation and where filth is kept sequestered.”
What To Tell Your Friend
Contemplating a fly performing its bodily functions on your meal isn’t especially pleasant, but the actual health risks are very low. So tell your friend to go ahead and enjoy her lunch. And if she wants to be extra cautious, she can abide by Nayduch’s rule of thumb. “I’ll tell you what I do,” she says. “I just break off the piece where the fly landed, and then I eat the sandwich.”
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