Myiasis involves the larvae (maggots) of two-winged flies (dipterous flies). Three types of cutaneous infestation exist, depending on the species involved:
Other organs sometimes are involved (eg, nasopharynx, gastrointestinal tract, genitourinary tract). Infestation usually occurs in tropical countries, so most cases in the US occur in people who have recently arrived from endemic areas.
Many of the common sources are known as bot flies. Dermatobia hominis, native to South and Central America, is the most common cause in travelers returning to the US. Other species include Cordylobia anthropophaga (in sub-Saharan Africa), various Cuterebra species (in North America), and Wohlfahrtia species (in North America, Europe, and Pakistan). Many of the flies do not lay their eggs on humans but on other insects (eg, mosquitoes) or objects (eg, drying laundry) that may contact skin. Eggs on the skin hatch into larvae, which burrow into the skin and develop through successive stages (instars) into mature larvae; mature larvae may be 1 to 2 cm long, depending on the species. If the infestation is untreated, larvae eventually emerge from the skin and drop to the ground to continue their life cycle.
Typical symptoms include itching, a sensation of movement, and sometimes lancinating pain. The initial lesion may resemble an arthropod bite or bacterial furuncle but may be distinguished by the presence of a central punctum with serosanguineous drainage; sometimes a small portion of the end of the larva is visible. D. hominis lesions are more common on the face, scalp, and extremities, whereas C. anthropophaga lesions tend to occur in areas that are covered by clothing and appear on the head, neck, and back.
Because larvae require atmospheric oxygen, occlusion of the skin opening may cause them to depart or at least come closer to the surface, facilitating manual removal. The numerous occlusive methods include use of petrolatum, nail polish, bacon, or a paste of tobacco. However, larvae that die during occlusion are difficult to remove and often trigger an intense inflammatory reaction. Larvae may be extracted through a small incision. Ivermectin, oral (200 mcg/kg, 1 dose) or topical, may kill the larvae or induce migration.
Open wounds and mucous membranes, typically in the homeless, alcoholics, and other people in poor social circumstances, may be infested by fly larvae, most often from green or black blowflies. Unlike larvae of common houseflies, most agents of wound myiasis invade healthy as well as necrotic tissue.
Treatment of wound myiasis is usually with irrigation and manual debridement.
The most common flies are Gasterophilus intestinalis and Hypoderma species. These flies typically infest horses and cattle; people acquire them via contact with infested animals or, less often, via direct egg-laying on their skin. Larvae of these flies burrow under the skin, causing pruritic, advancing lesions, which may be mistaken for cutaneous larva migrans; however, fly larvae are much larger than nematodes, and the lesions created by fly larvae last longer.