[Fly Control] Maggots in Human Tissues

When fly larvae get in the tissues of people or animals, the condition is referred to as myiasis. Adult flies lay their eggs on or near a neglected or pus-filled wound and developing maggots feed in the affected tissues. Myiasis may be gross and scary, but, generally, it’s not life threatening. However, pest control personnel should still be concerned about the possibility for myiasis because if it happens in one of your health-care facility accounts, somebody’s going to be sued for not keeping flies out of the facility.

Myiasis can be “accidental,” when fly larvae occasionally find their way into the human body, or “facultative,” when fly larvae enter living tissue opportunistically after feeding on decaying tissue in malodorous wounds. Myiasis can also be “obligate,” in which the fly larvae must spend part of their developmental stages in living tissue. Obligate myiasis is the most serious form of the condition from a pathogenic standpoint and constitutes true parasitism. Fly larvae are not capable of reproduction and, therefore, myiasis under normal circumstances should not be considered contagious from person to person. Transmission of myiasis occurs only via an adult female fly.

ACCIDENTAL MYIASIS. Accidental enteric myiasis (sometimes referred to as pseudomyiasis) mostly is a benign event, but the larvae possibly could survive temporarily, causing stomach pains, nausea or vomiting. There is some question as to whether or not these cases should be classified as true myiasis because of lack of fly development after the ingested eggs hatch. Numerous fly species in the families Muscidae, Calliphoridae and Sarcophagidae may produce accidental enteric myiasis. Some notorious offenders are pomace flies and fruit flies (Drosophilia spp.); the cheese skipper, Piophilia casei; the black soldier fly, Hermetia illucens; and the rat-tailed maggot, Eristalis tenax. Other instances of accidental myiasis occur when fly larvae enter the urinary passages or other body openings. Flies in the genera Musca, Muscina, Fannia, Megaselia and Sarcophaga often have been implicated in such cases.

FACULTATIVE MYIASIS. Facultative myiasis may result in considerable pain and tissue damage as fly larvae leave necrotic tissues and invade healthy tissues. I once consulted in a case wherein fly maggots were found in a hospital patient’s eye socket. Numerous species of Muscidae, Calliphoridae and Sarcophagidae have been implicated in facultative myiasis. In the United States, the blow fly Phaenicia sericata has been reported causing facultative myiasis on several occasions. Another calliphorid, Chrysomya rufifacies, has been introduced into the United States from the Australasian region and is also known to be regularly involved in facultative myiasis. Other muscoid fly species that may be involved in this type of myiasis include: Calliphora vicina, Phormia regina, Cochliomyia macellaria and Sarcophaga haemorrhoidalis.

OBLIGATE MYIASIS. Several fly species must develop in the living tissues of a host. This is termed “obligate myiasis” and is caused by species affecting sheep, cattle, horses and many wild animals. In people, obligate myiasis is primarily due to the screwworm flies (Old and New World) and the human bot fly. Obligate myiasis rarely is fatal in the case of the human bot fly of Central and South America, but it has led to considerable pathology and death in the case of screwworm flies. Screwworm flies use livestock as primary hosts, but they do attack humans.

If, for example, a female screwworm fly oviposits just inside the nostril of a sleeping human, hundreds of developing maggots may migrate throughout the turbinal mucous membranes, sinuses and other tissues. Surgically, it would be extremely difficult to remove all the larvae. Fortunately, due to the sterile male release program, screwworm flies virtually have been eliminated from the United States and Mexico.

CONTRIBUTING FACTORS. Accidental myiasis in the gastrointestinal tract occurs generally from ingesting fly eggs or young maggots on uncooked foods or previously cooked foods that have been subsequently infested. Cured meats, dried fruits, cheese and smoked fish are the most commonly infested foods. Other cases of accidental myiasis may occur from the use of contaminated catheters, douching syringes or other invasive medical equipment, or sleeping with the body exposed.

Several fly species lay eggs on dead animals or rotting flesh. Accordingly, the flies may mistakenly oviposit in a foul-smelling wound of a living animal, resulting in facultative myiasis. The developing maggots then invade healthy tissue. Facultative myiasis most often is initiated when flies oviposit in necrotic, hemorrhaging or pus-filled lesions. Wounds with watery alkaline discharges (pH 7.1 to 7.5) have been reported to be especially attractive to blow flies.

Facultative myiasis frequently occurs in semi-invalids who have poor (if any) medical care. Often, in the case of the very elderly, their eyesight is so weak that victims do not detect the myiasis. In clinical settings, facultative myiasis is most likely to occur in incapacitated patients who recently have had major surgery or those having large or multiple uncovered (or partially covered) festering wounds. However, not all human cases of facultative myiasis occur in or near a wound. In the United States, larvae of the blow fly, P. sericata, have been reported from the ears and nose of healthy patients with no other signs of trauma in those areas.

Obligate myiasis is essentially a zoonosis; humans are not the ordinary host but may become infested. Human infestation by the human bot fly is very often via a mosquito bite — the eggs are attached to mosquitoes and other biting flies; however, human screwworm fly myiasis is a result of direct egg laying onto a person, most often in or near a wound or natural orifice. Egg-laying activity of screwworm flies occurs during daytime.

PREVENTION, MANAGEMENT & TREATMENT. Prevention and good sanitation can avert much of the accidental and facultative myiasis occurring in the industrialized world. Exposed foodstuffs should not be unattended for any length of time to prevent flies from ovipositing therein. Covering (and preferably refrigerating) leftovers should be done immediately after meals. Washing fruits and vegetables prior to consumption should help remove developing maggots, although visual examination should be done during food preparation. Other forms of accidental myiasis may be prevented by protecting invasive medical equipment from flies and avoiding sleeping nude, especially during daytime.

To prevent facultative myiasis, extra care should be taken to keep wounds clean and dressed, especially on elderly or helpless individuals. Daily or weekly visits by a home health nurse can go a long way to prevent facultative myiasis in patients who stay at home. In institutions containing invalids or otherwise helpless patients, pest control personnel should be diligent to control entry of flies into the facility. This might involve such things as keeping doors and windows screened and in good repair, thoroughly sealing all cracks and crevices, installing air curtains over doors used for loading and unloading supplies and installing UV fly traps in areas accessible to the flies but inaccessible to patients. In addition, scatter baits for adult flies may be placed outside the facility according to label instructions. Prevention of obligate myiasis involves avoiding sleeping outdoors during daytime in screwworm-infested areas, and using insect repellents in Central and South America to prevent bites by bot fly egg-bearing mosquitoes.

Treatment of accidental intestinal myiasis is probably not necessary (although there may be rare instances of clinical symptoms), since in most cases there is no development of the fly larvae within the highly acidic stomach environment and other parts of the digestive tract. They are killed and merely carried through the digestive tract in a passive manner. Treatment of other forms of accidental myiasis, as well as facultative or obligate myiasis, involves removal of the larvae. Some doctors recommend debridement with irrigation. Others have suggested surgical exploration and removal of larvae under local anesthesia. Doctors usually take great care not to burst the maggots upon removal.

Interestingly, human bot fly larvae have been successfully removed using “bacon therapy,” a treatment method involving covering the punctum (breathing hole in the patient’s skin) with raw meat or pork. In a few hours the larvae move out into the meat and are then easily extracted. Maggot infestation of the nose, eyes, ears and other areas may require surgery if larvae cannot be removed via natural body openings. Since blow flies and other myiasis-causing flies lay eggs in batches, there could be tens or even hundreds of maggots in a wound — something no pest controller wants to happen in his/her health-care facility account!

All photos are © Jerome Goddard.

The author is a medical entomologist for the Mississippi Department of Health, Jackson, Miss.