Fly larvae, commonly known as maggots, can infest human skin, a condition known as cutaneous myiasis. This parasitic infestation occurs when the larvae of certain fly species enter the skin and feed on the host’s living or necrotic tissue or body substances. Although this condition is rare globally, it is a relatively frequent occurrence in rural tropical and subtropical regions. Understanding the biological mechanisms behind myiasis, the types of infestation, and the necessary medical response is important for travelers and individuals in endemic areas.
Defining Myiasis and Modes of Entry
Myiasis is the medical term for the parasitic infestation of a living vertebrate animal, including humans, by the larvae of dipterous flies. The larvae grow inside the host, feeding on tissue that can range from dead tissue in a wound to healthy, living tissue. This condition is differentiated based on the fly species’ life cycle requirements: primarily obligatory versus accidental myiasis.
Obligatory myiasis is caused by fly species whose larvae require a living host to complete their development. In contrast, accidental or pseudomyiasis happens when fly larvae, which typically develop in decaying organic matter, are accidentally ingested or deposited on a host. The larvae or eggs gain access to human skin through several distinct modes.
One primary entry method involves direct skin penetration, where the larvae hatch and burrow into unbroken skin, often seen with species like the Tumbu fly. A more complex route involves carrier insects, such as mosquitoes, which transport eggs that hatch immediately due to the warmth of the skin when the carrier bites a human. Larvae or eggs can also be deposited directly into existing open wounds, sores, or mucous membranes.
Types of Human Cutaneous Infestation
Cutaneous myiasis, the infestation affecting the skin, is broadly categorized into three main clinical forms based on how the larvae develop and interact with the host’s tissue. These manifestations are directly related to the species of fly involved.
Furuncular myiasis is the most common form, characterized by the development of a painful, boil-like lesion, or furuncle, containing a single larva. The larvae of the human botfly (Dermatobia hominis) and the Tumbu fly (Cordylobia anthropophaga) are common causes of this type. The larva remains at one spot, creating a dome-shaped cavity with a small central pore, which is used for breathing.
Wound myiasis occurs when fly species, such as the New World screwworm fly (Cochliomyia hominivorax), lay their eggs directly into an open wound or lesion. Once hatched, the larvae feed on the necrotic or living tissue within the wound. This form is particularly dangerous as it can lead to extensive tissue destruction and severe secondary bacterial infections.
The third form is creeping or migratory myiasis, where the larva burrows and tunnels beneath the skin’s surface. Larvae from the Hypoderma genus, typically cattle parasites, can cause this in humans. As the larva moves, it leaves behind a visible, reddish, tortuous track, which can advance at a rate of up to 30 centimeters per day.
Recognizing Symptoms and Diagnosis
Recognizing the symptoms of cutaneous myiasis depends on the type of infestation. For furuncular myiasis, patients typically notice a localized, firm, tender swelling that resembles a persistent boil. A small opening, known as a punctum, is often visible at the center of the lesion, which may intermittently drain a serosanguineous (thin, bloody) fluid.
A unique symptom is the sensation of movement or a stabbing pain, often reported to intensify at night when the larva is most active. In cases of wound myiasis, the wound may become foul-smelling, and the maggots may be visible within the tissue. Migratory myiasis presents with a distinct, intensely itchy, linear, red track that changes location over hours or days as the larva tunnels.
Medical professionals confirm the diagnosis through visual inspection of the lesion and physical evidence, such as the central breathing hole or the visible movement of the larva. Obtaining a detailed travel history to endemic regions is also a key diagnostic clue. If the larva is removed, it is often sent for identification to determine the exact fly species, which helps guide the management and assess the risk of complications.
Treatment and Prevention
The treatment for myiasis focuses on the complete removal of the larva, which is necessary for resolving the infestation and preventing secondary infection. For furuncular myiasis, one common technique is occlusion, where a substance like petroleum jelly, mineral oil, or even bacon fat is placed over the breathing hole. This suffocates the larva, forcing it to migrate toward the surface, where it can be extracted using forceps.
Surgical removal is another primary method, involving a small incision under local anesthesia to extract the larva, especially if it is difficult to remove through occlusion. In some cases, systemic antiparasitic medications, such as oral or topical ivermectin, may be used to kill the larva or induce its migration before manual extraction. Individuals should seek professional medical help and avoid attempting self-removal, as incomplete removal can leave behind parts of the larva, leading to a severe inflammatory reaction.
Prevention strategies are important for people traveling to or living in endemic areas. Travelers should wear long-sleeved clothing and use insect repellent containing DEET to minimize fly contact. In regions where the Tumbu fly is prevalent, clothing should be thoroughly ironed after washing and drying outdoors, as the heat kills any eggs deposited on the fabric. Prompt and meticulous care of any open wounds is also necessary, as wounds are a major attractant and entry point for many myiasis-causing flies.