Are Flies Dirty? More Details Here

Filthy Flies Landing on Our Food

Fly. Getty Images

Nobody likes it when a fly lands on their plate at a picnic. Flies are naturally associated with dirty things like trash cans. Fortunately, our immune systems are powerful, and if you're privileged enough to live in the developed world, the chance that a housefly will pass on deadly disease is small. In other words, when a fly does land on your plate at a picnic, there's probably little to worry about; this common occurrence, although unsavory, is not dangerous.

Nevertheless, flies are filthy little creatures that fly from poop to food to poop again spreading little-regurgitated flecks of bacteria-saturated poop mixed with digestive enzymes. In developing nations that lack access to proper sewage treatment facilities and have open sewers (including parts of India and Africa), bacteria cause life-threatening dysentery and bloody diarrhea.

Various species of filth flies typically carry bacteria on their legs and feed on all types of organic matter like decaying vegetable matter, meat, feces, blood, pus and so forth. In unsanitary conditions, such as open market spaces in developing nations, flies carry more and different types of bacteria than flies in cleaner conditions. Furthermore, more flies are found in unsanitary conditions, increasing our chance for exposure to bacteria. Thus, a vendor selling meat in a market setting without access to running water or proper waste disposal is a likely source of severe (and possibly bloody) diarrhea.

On a related note, some public-health researchers in Africa recommend that the best way to limit communicable disease in open-market settings is to provide a sanitary infrastructure to food vendors like a common area for food preparation and waste disposal as well as bathrooms.

Although flies carry various types of bacteria on their legs, most of the time bacteria found on the legs of flies cause gastrointestinal illness.

Two principal bacterial pathogens carried by flies are E. Coli and shigellosis

A mild form of E. Coli infection is traveler's diarrhea. At one time or another, many of us have experienced traveler's diarrhea and remember the cramping, painful, copious bouts of watery diarrhea. Treatment for traveler's diarrhea is often supportive and targets symptoms (i.e., pain medications and rehydration). Furthermore, antibiotics (ciprofloxacin) can also be given.

A more serious form of E. Coli infection is 0157:H7. Like infection with shigella, with E. Coli 0157:H7, bloody diarrhea is a hallmark of infection. Furthermore, young children are at particular risk for hemolytic uremic syndrome (HUS). With hemolytic uremic syndrome, the destruction of red blood cells is so extensive that it clogs the kidneys and causes acute renal failure. Although a more prevalent cause of HUS is E. coli, shigella can also cause HUS. 

Acute renal failure caused by HUS is treated with a combination of fluid support, dialysis, blood transfusions and so forth.  Antibiotics are not typically administered to treat HUS.

So, in theory, the fly that landed on your egg salad at the company picnic could cause your kidneys to fail.

Fortunately, we live in the United States where sanitary conditions prevail. Although the sight of flies feasting on our food is revolting, the disease spread by flies is typically a reflection of more general sanitary conditions. In the United States, we have robust plumbing and sanitation systems which protect us from pestilence. 

To recap: The worst a fly can do at a picnic is usually a bout of diarrhea that goes away. And to be sure, bacteria carried by flies are by no means the only cause of bacterial food-borne illness. For example, staph, Bacillus cereus, Clostridium botulinim and Listeria monocytogenes are all found in food and cause gastrointestinal illness.


Mundie, OK, Kuria, E. Hygienic and Sanitary Practices of Vendors of Street Foods in Nairobi, Kenya. AJAFND Online. 2005;5.

WHO. Fact sheets on environmental sanitation.

Adeyemi, O, Dipeolu, OO. The numbers and varieties of bacteria carried by filth flies in sanitary and unsanitary city area. Int J Zoonoses. 1984; 11:195-203.

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