Myiasis is a parasitic infestation that occurs when the larval stage, or maggot, of a fly species invades the living tissue of a host. These infestations are generally localized and treatable when addressed promptly by medical professionals. Understanding the specific organisms and their unique methods of infestation is the first step toward effective diagnosis and prevention.
Identifying Organisms That Cause Infestation
The most frequent causes of myiasis are the larvae of two primary fly species: the human botfly and the Tumbu fly. The human botfly, Dermatobia hominis, is the most common agent causing furuncular myiasis in individuals returning from the Americas. The Tumbu fly, Cordylobia anthropophaga, is the counterpart found throughout Sub-Saharan Africa, sometimes referred to as the mango fly. Both species cause a similar lesion characterized by a boil-like swelling that houses the developing larva.
A third, less common but more aggressive group includes the screwworm flies, such as Cochliomyia hominivorax. Unlike the botfly and Tumbu fly larvae, screwworms actively invade and feed on healthy tissue, often creating large, deep lesions. The New World screwworm is found in the Western Hemisphere, while the Old World screwworm, Chrysomya bezziana, is found across Africa, India, and Southeast Asia.
Mechanisms of Entry
The way these flies introduce their offspring to a human host varies significantly between species. The human botfly employs a complex method known as phoretic transfer. The adult female captures a blood-feeding arthropod, often a mosquito or a tick, and glues her eggs onto its body.
When the carrier lands on a host, the warmth of the skin causes the eggs to hatch. The newly emerged larvae drop from the carrier and burrow directly into the skin through a bite wound or hair follicle. The larva then remains in a subdermal cavity for a period of five to ten weeks to mature.
In contrast, the Tumbu fly uses a more direct approach. The female fly typically lays her eggs on soil contaminated with animal waste or on damp clothing left to dry outdoors. The larvae hatch within two to three days and can survive without a host for up to 15 days. When a person contacts the contaminated soil or puts on the infested clothing, the larvae attach to the skin and burrow in rapidly.
Recognizing Symptoms and Diagnosis
The characteristic symptom of furuncular myiasis is the development of a painful, boil-like lesion. This lesion may be accompanied by itching and a stabbing pain, often reported to worsen at night. Some patients report a distinct sensation of movement within the swelling as the larva grows.
A defining feature that aids in diagnosis is the presence of a small central opening, called a punctum. This opening serves as the breathing pore for the developing larva, which requires oxygen to survive. A serosanguinous discharge, a thin, yellowish or bloody fluid, often drains from this punctum as the larva excretes waste.
Diagnosis is primarily clinical, relying on the lesion’s appearance and a thorough patient history that includes recent travel to endemic areas. Submerging the lesion under water may confirm the diagnosis, as a live larva will cause bubbling while attempting to breathe. Medical imaging, such as ultrasound, can also visualize the larva beneath the skin, confirming the presence of the parasite.
Medical Treatment and Removal Procedures
Once myiasis is confirmed, the main goal of treatment is the complete and intact removal of the larva. Non-surgical techniques are often attempted first to avoid damaging the larva, which could lead to a severe localized inflammatory reaction if fragments are left behind.
A common method involves occlusion, where an air-blocking substance is applied over the breathing pore. Occlusive agents like petroleum jelly, mineral oil, or nail polish are used to cut off the larva’s oxygen supply. The asphyxiated larva is then forced to migrate toward the surface of the skin where it can be easily grasped and removed with forceps. This process typically takes up to 24 hours.
If the occlusion method is unsuccessful, a medical professional will perform a surgical extraction under local anesthesia. This procedure involves making a small incision to enlarge the punctum or using a punch excision to gain direct access to the larva. Care must be taken during removal, as the larvae are anchored by rows of spines and hooks that resist simple pulling. A local anesthetic may also be injected into the base of the lesion, attempting to create hydraulic pressure to push the larva out of the pore. Following removal, the wound must be thoroughly cleaned to prevent secondary bacterial infection. Oral or topical medications, such as ivermectin, may also be administered to kill the larvae or facilitate their migration.
Strategies for Prevention
Preventing myiasis requires targeted measures, especially when traveling to regions where these flies are common. Using strong insect repellents containing DEET is a fundamental step to deter adult flies and the arthropods that serve as carriers for botfly eggs. Wearing long-sleeved shirts and pants also provides a physical barrier against initial contact with the skin.
A primary measure against the Tumbu fly involves the treatment of laundry. Because the female Tumbu fly commonly lays eggs on damp clothing left to dry outside, all items must be ironed thoroughly before they are worn. The heat from the iron is sufficient to kill the eggs and newly hatched larvae embedded in the fabric. Travelers should avoid leaving clothing to dry on the ground or in shaded areas. Where possible, clothes should be dried indoors or using a machine dryer, and sleeping directly on the bare ground in endemic areas should be avoided.