Insects that love us to death
House flies are a species which has probably been responsible for more human misery than any other insect… (so said Luther West, a prominent early medical entomologist). But house flies don’t bite, sting, scrape, suck our blood, or inject their eggs beneath our skin like other insects of medical importance do. Like annoying little brothers, they are problems because they simply love being around us.
Flies are what scientists call anthropophilic: human loving. If there were no humans on Earth, there would be a lot fewer house flies. In fact, house flies are likely one of the most broadly distributed insects in the world simply because they tag alongside their human buddies wherever we go.
What do they like about us? They like our food, our shelter, and our domesticated animals’ poop. And they also like that we foolishly always try to slap them by bringing our hand down on top of them (come quickly from the side, or slap your hands together directly above a fly and you’ll get them almost every time).
Ask any house fly, and their least favorite human throughout history probably has to be Henry Ford. Replacement of horse-drawn carriages and their smelly, brown accessories with cars largely eliminated house flies from urban areas, and many of the human health threats associated with flies are diminished in countries like the U.S. where livestock has been moved out of major population centers. Sewer and septic systems also helped immensely.
This is because problems arise when large numbers of house flies have simultaneous and unrestricted access to feces (or other bodily fluids) and human food.
The flies aren’t trying to hurt us, they just can’t resist a fresh patch of body fluid. The pathogens take advantage of this behavior, and with every step a fly takes it accidentally picks up potentially deadly microbes.
And the list of pathogens that flies transmit is a long one, filled with notoriously nasty diseases. Typhoid, yaws, gangrene, trachoma, bowel infections, cholera, dysentery, diarrhea, anthrax all can use house flies as vectors. Indeed, it has even been shown that house flies can transmit antibiotic resistant bacteria, a timely and major concern faced by health officials.
But when considering this list of deadly pathogens transmitted by our little insect roadies, it is important to realize two things.
First, flies are seldom the primary vector of diseases; they just add to what is often a much bigger problem. If these diseases relied solely on fly transmission, their stories would be short ones.
Also, within more temperate regions, microbial transmission by house flies is almost always benign and is sometimes even beneficial- higher organisms like humans and animals need bacteria to survive. If all of these transmissions spread deadly pathogens, wellÉwe’d be dropping like flies.
In the end, when we stop leaving our poo lying around, disease transmission by flies drops dramatically. Sanitation is key in managing these diseases; manage the real problem and the flies will quiet down. Attempts to eradicate the flies associated with these diseases with things like insecticides may give short term benefits, but inevitably results in expensive failure.
Jonathan Lundgren is a research entomologist at the USDA-ARS research facility in Brookings, S.D. Although interested in all aspects of insect biology, he specializes on insect conservation and reducing crop pests through the use of beneficial insects.