Germ-Transmitting Flies

Most people agree that flies are nasty. But why? And are all flies equally nasty? Let me start with the second question. In my opinion, the answer is "no." They aren’t all nasty, but "filth flies" certainly are. Other fly species breed in water, mud, vegetation, etc., and aren’t as dirty in their habits.

What are filth flies? There are thousands of species of flies in the world, but only a few are referred to as "filth flies." Although other species or groups could also share the designation, "filth flies" are generally house-flies, fleshflies and blowflies. They are the domestic, non-biting flies commonly seen in and around human dwellings.

Houseflies are about 5 to 8 mm long with a dull gray thorax and abdomen (not shiny). They have four longitudinal dark stripes on their thorax. Houseflies breed in feces, decaying organic material and foodstuffs.

Fleshflies look like houseflies but are generally larger (11 to 13 mm long). They have three dark longitudinal stripes on their thorax, a checkerboard pattern of gray on the abdomen and sometimes a reddish-brown tip on the abdomen. Fleshflies breed in decaying meat or animal excreta.

Blowflies (also known as green or bluebottle flies) are about the same size as fleshflies, although some of the bluebottle flies (genus Calliphora) are larger and more robust. Blowflies, with the exception of the cluster fly, are metallic bronze, green, black, purplish or blue colored. Most blowflies breed in dead animal carcasses, although some breed in decaying vegetable matter and some are parasites of frogs, earthworms and even mammals.

transmitting Disease. Filth flies don’t bite, so there can be no biological transmission of disease agents (as is the case in malaria, dengue, sleeping sickness, etc.). So then, how are they medically important? Flies — as well as many other insects — are covered with numerous hairs, setae and spines that can pick up germs and other contaminants (see the figure below). These germs can be physically transported from one place to another — a phenomenon called mechanical disease transmission. If a fly has been walking around on dog feces and subsequently lands on your piece of cake…well, you get the picture.

Filth flies have been reported to carry organisms causing diseases such as typhoid, diarrhea, amoebic dysentery, cholera, giardiasis, pinworm and tapeworm. Certainly not all these organisms are on all flies; it’s just an indication of the wide variety of germs that flies can possibly transmit. One of the hazards from filth flies is when they feed on liquids containing disease germs. Later, both vomit drops and fly excreta can transmit infection. Experiments have shown that salmonella can multiply in the mid- and hind-gut of flies and be passed in excreta for up to a week.

Prevention and Control. Filth flies are best controlled by a combination of good sanitation, mechanical exclusion, ultraviolet light traps and chemical control. Good sanitation means finding and eliminating breeding sites — like emptying and steam-cleaning dumpsters on a regular schedule. Mechanical exclusion methods include air curtains and properly fitted doors and screens being used. UV light traps work well indoors and are now marketed in a variety of designs and models (some don’t even look like fly traps). If filth flies reach extremely high population numbers in an establishment, fogging with pyrethrins will provide instant relief, but long-term control must include finding and eliminating the breeding sites.