Are Flies Dirty and Do They Cause Disease?

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Fly. Getty Images

The standard house fly (musca domestica) annoys people, but they also serve as transmission vectors for bacterial infections, sometimes in ways that aren't immediately obvious.

Types of Flies

The house fly measures about a quarter-inch long and shows four distinctive stripes on its thorax, behind its head. But the house fly is just one of dozens of different kinds of flies that enter homes and food-preparation areas.

These flying insects sometimes called "filth flies," are divided into two groups.

Large filth flies generally feature stout bodies, short legs and feed on manure, dead animals, and food waste. This kind of insect includes house flies as well as blow flies, stable flies, and flesh flies.

Small filth flies have slender bodies with long legs; they prefer to dine on drain sludge, rotting fruits and vegetables and other decaying plant matter. Fruit flies, drain flies, and fungus gnats are common representatives of the type.

How Flies Transmit Disease

Flies are born into a food source. House flies, for example, lay eggs in the garbage or animal droppings. The eggs hatch into maggots, eating the food around them. The maggots eventually turn into pupae, within which they affect their final transition into a winged adult fly.

After they've fully pupated, the mature flies continue their search for food.

Their preferred foodsources—generally, moist and decaying organic matter—naturally contain bacteria, many of which are harmful to humans. When the fly eats, it regurgitates part of its stomach onto its new meal; the stomach acids digest the food and then the fly drinks it up.

Because flies eat garbage, manure, and other bacteria-laden materials, and because they spit up their stomach contents on surfaces for which humans may make frequent contact, these winged devils are capable of transmitting more than 60 different serious illnesses, including:

  • Typhoid fever
  • Dysentery
  • Cholera
  • Anthrax
  • Tuberculosis
  • Leprosy

Even if the fly didn't eat on a surface that humans contact, the fly may have left droppings that contain these bacteria.

Controlling Fly Populations

Experts at Penn State's Department of Entomology recommend four complementary approaches for managing fly populations:

  1. Sanitation. Eliminate the food source that flies crave by keeping trash bagged or in bins. Keep rotting organic material, including compost piles, well managed. Isolating the preferred food for flies is the single most significant step you can take to keep them at bay.
  2. Exclusion. To keep flies out of your house, look for ways they can enter. Pay careful attention to broken screens, gaps around pipes and conduits, and vents. Roof vents, for example, don't always include a screen with mesh tight enough to keep flying insects out of an attic or crawl space.
  3. Non-chemical traps. Devices like sticky traps, ultraviolet light traps, and baited fly traps will catch flies so they can't spread.
  4. Chemical traps. Pesticides containing a synthetic pyrethroid, applied by a licensed pest-control specialist, will last for several days or a week or two if applied to the exterior of your house. Pyrethroids are sensitive to sunlight and will break down quickly. Inside the house, you can use an aerosol defogger to get them. Take careful steps to avoid poisoning yourself or your pets by keeping the pesticides away from food and water sources.

    Sources:

    The Pennsylvania State University College of Agricultural Sciences. House Flies. http://ento.psu.edu/extension/factsheets/house-flies 

    Illinois Department of Public Health. The House Fly and Other Filth Flies. http://www.idph.state.il.us/envhealth/pcfilthflies.htm

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