Cutaneous myiasis is skin infestation by the larvae (maggots) of certain fly species.

There are three main types of skin infestation by fly larvae:

• Furuncular (pimple- or boil-like) myiasis
• Wound myiasis
• Migratory myiasis

The disorders vary depending on the species of fly involved. Infestation usually occurs in tropical countries. People in the United States who have myiasis usually have recently arrived from or visited a tropical country.

Furuncular Myiasis: Many of the common sources are known as bot flies. The most well-known species of flies that cause furuncular myiasis come from South and Central America and sub-Saharan and tropical Africa. Many of the flies do not lay eggs on humans. Instead, the flies lay their eggs on other insects (such as mosquitoes) or on objects (such as drying laundry) that may come into contact with people's skin. Eggs hatch into larvae, which burrow into the skin and develop into mature larvae. The mature larvae are up to ¹/₂ to 1 inch (about 1.3 to 2.5 centimeters) long, depending on the species. If people are not treated, the 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 sharp, stabbing pain. At first, people have a small red bump that may resemble a common insect bite or a beginning pimple (furuncle). Later, the bump enlarges, and a small opening may be visible at the center. The opening may drain clear, yellowish fluid, and sometimes a small portion of the larva is visible.

Because larvae require oxygen, blocking the skin opening may cause them to leave or at least come closer to the surface. When they are closer to the surface, it is easier to pull them out. To block the skin opening, some people apply petroleum jelly, nail polish, or even bacon or a paste of tobacco. Larvae that die before being removed are harder to get out and often cause an intense inflammatory reaction. Another option for removal is squeezing the skin. Sometimes doctors inject a local anesthetic, make a small incision, and pull the larva out with forceps. The drug ivermectin, given by mouth or applied to the skin, also may kill the larva or cause it to leave.

Wound Myiasis: Open wounds, typically in homeless people, alcoholics, and others in poor social circumstances, may become infested with fly larvae. The most common flies are green or black blowflies. Unlike the maggots of common houseflies, most larvae that cause wound myiasis invade healthy as well as dead tissue. Doctors remove the larvae by flushing the wounds and pulling the larvae out. Doctors also cut away any dead tissue.

Migratory Myiasis: The most common sources are flies that typically infest horses and cattle. People can become infested if they have contact with infested animals. Less often, the flies lay eggs directly on people. Larvae do not stay in one spot. They burrow under the skin, causing itchy lesions that may be mistaken for cutaneous larva migrans (see Parasitic Skin Infections: Cutaneous Larva Migrans). The treatment is the same as for furuncular myiasis.

Last full review/revision September 2008 by James G. H. Dinulos, MD