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Canyon Flies And Face Flies Are Now Transmitting Parasitic Worms

A Giant Hairy Scorpion native to Arizona climbing on a cactus.

Musca autumnalis, better known as the “face fly,” is a very common fly species that is known to be a frequent home-invading pest throughout the United States. Face flies frequently invade homes in large numbers during the fall and winter seasons in order to establish a warm shelter for overwintering. These flies gravitate into wall voids, tight attic spaces and other inaccessible indoor areas, and they get their common name from their habit of landing on people’s faces where they feed on tears and other fluids secreted from the mucous membranes in the nose, eyes and mouth. Unlike cosmopolitan face flies, the group of fly pests commonly known as “canyon flies” can only  be found in the southwestern region of the country. Much like face flies, however, canyon flies feed on mucus, tears, sweat and other bodily fluids.

Since face flies breed on manure, they spread bacteria into the eyes of humans, cattle and horses, which often results in conjunctivitis, or “pink eye.” In addition to causing pink eye, face flies are known to transmit a parasitic disease known as thelaziasis to horses and cattle. Less is known about the breeding habits of the nine documented canyon fly species found in the southwest, but at least one canyon fly species, Fannia thelaziae, has been documented as transmitting thelaziasis to humans on at least ten occasions in the southwest. All ten of these cases involved the nematode eye-worm species known as Thelazia californiensis. These flies transmit several eye worm-parasite species to animals in the country, including Thelazia gulosa.

During 2016, an Oregon woman visited the doctor after experiencing odd sensations in her eye. Doctors discovered several parasitic worms in her eye that were later revealed to be Thelazia gulosa parasites. While common in cattle, this particular eye-worm species had never been found infecting a human, and last year, T. gulosa parasites were found in the eye of a southern California woman. This woman contracted the parasites after jogging through a fly swarm. So far these are the only two documented human cases of T. gulosa infection, and it is not known whether face flies or canyon flies transmitted the parasites in these cases. Because of these two alarming cases, medical researchers are worried that flies may start transmitting eye-worms to humans throughout the country.

Have you ever been pestered by flies landing on your face?

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