Myiasis is a parasitic infestation caused by the larvae (maggots) of certain fly species (order Diptera) developing within the living tissue of a human or vertebrate animal host. These flies lay their eggs or deposit larvae directly onto a host. While myiasis is not spread from person to person, it is a global health concern. It is particularly prevalent in tropical and subtropical regions where poor sanitation or close proximity to livestock is common, though cases can occur anywhere, often involving travelers returning from endemic areas.
How Myiasis is Classified
Myiasis is broadly categorized by the biological nature of the fly species and the anatomical location of the infestation. Ecologically, it is classified as either obligatory or facultative. Obligatory myiasis requires a living host for the fly species to complete its larval development cycle. Facultative myiasis involves larvae that usually thrive in decaying organic matter but will opportunistically infest a living host.
Anatomically, classification is based on the site of the body where the larvae are found. The most common presentation is cutaneous myiasis, which affects the skin and subdermal layers. Infestations can also occur in internal or sensitive areas, leading to specific diagnoses such as ophthalmic myiasis (eyes), aural myiasis (ear canal), gastrointestinal myiasis (ingestion of eggs), or urogenital myiasis (urinary and genital tracts).
The Infestation Process
The lifecycle of the myiasis-causing fly dictates the method of human infestation, involving four stages: egg, larva, pupa, and adult. In many species, the adult female fly deposits eggs directly onto an open wound, an existing lesion, or unbroken skin. The eggs hatch rapidly, sometimes within 24 hours, and the emerged larvae quickly burrow into the underlying tissue to begin feeding.
Other species use phoresy, a complex transmission method where the female fly attaches eggs to a blood-sucking insect like a mosquito or tick. When the vector feeds on a human, the host’s skin warmth triggers the eggs to hatch. The tiny larvae drop onto the skin and penetrate the host through the bite site or a hair follicle.
Once inside, the larvae feed on tissue or body fluids, growing over a period lasting from a week to several months. The developing larva maintains a connection to the outside environment through a small breathing hole, or punctum, in the host’s skin. When the larval stage is complete, the mature maggot exits the host and drops to the ground to pupate, completing the cycle.
Identifying Signs and Symptoms
The manifestations of myiasis vary based on the location and the type of fly larva involved. The most recognizable sign of furuncular cutaneous myiasis is a firm, boil-like lesion that slowly enlarges. This nodule often has a small, visible central opening, which is the larva’s breathing pore, from which a serous or bloody discharge may drain.
Patients commonly report a sensation of movement, itching, or intermittent sharp pain within the lesion, often worse at night. Wound myiasis, which affects neglected or open injuries, presents with generalized pain, a foul-smelling discharge, and visible tissue destruction.
For internal forms, symptoms relate to the affected organ system. Ophthalmic myiasis causes eye irritation, pain, redness, and the sensation of a foreign body moving across the eye. Gastrointestinal tract infestation, usually from accidental ingestion, may result in abdominal pain, vomiting, or diarrhea. Urogenital myiasis is characterized by painful urination or the presence of larvae in the urine.
Medical Removal and Treatment
Treatment requires the complete removal of the larvae, taking care to avoid lacerating the parasite, which can cause an inflammatory foreign-body reaction. For furuncular lesions, a common non-surgical method is occlusion. This involves covering the breathing pore with a thick substance like petroleum jelly, nail polish, or mineral oil. This suffocates the larva, forcing it to emerge partially or fully from the skin for extraction with forceps.
Another technique for cutaneous myiasis involves injecting a local anesthetic, such as lidocaine, into the base of the lesion. The resulting fluid pressure can dislodge the larva, pushing it toward the surface for easier removal. Surgical excision is necessary for deeply embedded larvae, those in sensitive areas like the eyes, or when the larva has died beneath the skin.
Pharmacological treatment, particularly with the antiparasitic drug ivermectin, may be used as an alternative or adjunct therapy. Ivermectin can kill the larva or cause it to migrate out, which is useful for difficult-to-reach cavitary or orbital infestations. After removal, the wound requires thorough cleaning with antiseptic dressings. If a secondary bacterial infection is present, antibiotics must be administered.
Practical Prevention Measures
Preventing myiasis requires sanitation and personal protective measures, especially when traveling or residing in endemic areas. Maintaining excellent personal hygiene is important, including regular bathing and proper oral health. All open wounds, cuts, or sores must be cleaned immediately and covered with sterile dressings to eliminate attractive sites for egg-laying flies.
Travelers should wear long-sleeved shirts, pants, and hats to minimize exposed skin and apply insect repellents containing DEET or picaridin. In regions with the Tumbu fly, clothing should not be dried outdoors, as the fly can lay eggs on the fabric. Any clothing dried outside should be thoroughly ironed with heat before being worn, which kills residual eggs.
Ensuring food and water safety helps prevent accidental ingestion of fly eggs. Drinking water should be boiled or filtered, and food must be covered and stored properly to prevent fly access. Fly-control measures, such as installing window screens and using mosquito netting while sleeping, help create a barrier against adult flies.