Q. Several patients have complained of a red eye response when gnats get in their eyes. Is this a possible vector for infection or just an inflammatory response to the gnat?

A: Most likely, the insect in question is not your run-of-the-mill bothersome bug, but more precisely, the eye gnat.

“An eye gnat is like a tiny house fly that has spongy mouthparts and laps up its food source, which is mucus,” says eye gnat expert James Bethke, Floriculture and Nursery Farm Advisor at the University of California Cooperative Extension San Diego.

Eye gnats (Hippelates spp.) are actually small flies—about 1/16 to 1/8 inch in length—with clear wings. Their bodies range from shiny black to dull gray, with yellow, orange, or dark brown and orange legs. Eye gnats are prevalent in the South, and they have been reported to cause problems along the Border States from California to Florida and as far north as North Carolina.¹ In the Southwest, eye gnats are primarily restricted to the arid desert regions of California and Arizona.

 Although eye gnats are non-biting insects, the adult females are strongly attracted to moisture around the eyes and nose of humans and animals. In mid-summer, large numbers of eye gnats persistently hover around the face, causing considerable annoyance.¹ They swarm around the head and dart at the eyes, mouth, nose or wounds of humans and other animals. If brushed off, they quickly return.

Female eye gnats get protein for egg production from exposed mucous or sebaceous secretions, pus and blood of humans or animals. They do not pierce the skin of the host to obtain this material; rather, they scrape at the skin with large, curved spurs on their hind legs to produce a pool of mucous material for soaking up with their spongy mouthparts.¹            

Although they are primarily an annoyance, eye gnats can spread disease organisms that cause acute bacterial conjunctivitis (pink eye), anaplasmosis, and bovine mastitis.¹

Part of Mr. Bethke’s recent study, the Jacumba Eye Gnat Research Project, looked into whether eye gnats can indeed cause infection. “The cases I have seen in our study area can be explained by other sources of infection and cannot be directly correlated with eye gnats,” Mr. Bethke says.

Optometrists who have treated this condition agree.

“I have always treated this as an inflammatory condition,” says Alabama optometrist Ernie Bowling. “I don't recall seeing any infections from this, as most patients have irrigated their eyes before they ever make it to the office. ”

Adds San Diego optometrist Brian Chou, "Eye gnats seem to be more a nuisance than a vector of serious eye disease.” Still, studies have implicated eye gnats as a potential vector for bacterial conjunctivitis, he adds.^2,3

In late summer of 1981, a widespread outbreak of acute bacterial conjunctivitis occurred in southeast Georgia, affecting primarily grade-school children.³ This outbreak was similar to previously described seasonal conjunctivitis in the South.³ As suggested in earlier studies, the eye gnat may serve as a mechanical vehicle in the transmission of the suspected causative agent, a possible Haemophilus species.³ The 1981 Georgia outbreak illustrated that "gnat sore eyes" are not obsolete and apparently can occur unpredictably during warm months in the Southern states. Physicians and other health care workers should be aware of the unique clinical and epidemiologic features of this acute seasonal conjunctivitis, the study concluded.            

Q: How should I treat "gnat sore eyes"?

A: In cases of bacterial conjunctivitis, the clinician can prescribe topical antibiotics to reduce its contagiousness and facilitate resolution, and if significant inflammation is also present, an antibiotic-corticosteroid combination is appropriate, Dr. Chou says.

“Fortunately, most eye irritation following exposure to eye gnats is mild, self-limiting, and resolves without special intervention,” Dr. Chou says. “In these more common situations, reassurance and frequent artificial tears can suffice."

For Dr. Bowling, treatment depends on the presentation. Typically, Dr. Bowling treats the affected eye with Lotemax (loteprednol etabonate, Bausch + Lomb) q.i.d. He also recommends an over-the-counter antihistamine like Benadryl (diphenhydramine, Johnson & Johnson) if the edema is severe, in addition to cold compresses. “Fortunately, the presentation resolves rather quickly,” Dr. Bowling says.

1. Pest Notes; County of San Diego; Department of Agriculture, Weights and Measures. August 2004.
2. Payne WJ Jr, Cole JR Jr, Snoddy EL, Seibold HR. The eye gnat Hippelates pusio as a vector of bacterial conjunctivitis using rabbits as an animal model. J Med Entomol. 1977 Jan 31;13(4-5):599-603.
3. Buehler JW, Holloway JT, Goodman RA, Sikes RK. Gnat sore eyes: seasonal, acute conjunctivitis in a southern state. South Med J. 1983 May;76(5):587-9.