Myiasis is a parasitic condition resulting from the infestation of a living vertebrate animal or human by fly larvae, commonly known as maggots. These larvae, the immature stage of specific fly species, develop inside the host’s body, feeding on tissue, liquid body substances, or ingested food. While myiasis is a concern for livestock industries globally, it also affects humans, particularly in tropical and subtropical regions. The effects vary widely depending on the fly species and the location of development within the host’s body.
Causative Agents and Infestation Mechanisms
Myiasis is caused exclusively by flies belonging to the order Diptera. The most common families responsible are Oestridae (botflies), Calliphoridae (blowflies), and Sarcophagidae (flesh flies). Species like the human botfly (Dermatobia hominis) and the Tumbu fly (Cordylobia anthropophaga) are frequent causes of human infestation.
Infestation occurs through three primary mechanisms. The first is the direct deposition of eggs or larvae onto an open wound, sore, or mucous membrane, where they hatch and begin to feed. Flies like the New World screwworm (Cochliomyia hominivorax) are attracted to open lesions.
Another mechanism involves the fly laying eggs on an intermediary vector, such as a mosquito. The vector then deposits the eggs onto the host’s skin during a blood meal, allowing the larvae to quickly hatch and burrow into the skin. This unique life cycle is characteristic of the human botfly.
The third mechanism is accidental or pseudomyiasis, where eggs or larvae are unintentionally ingested through contaminated food or water, or deposited near orifices. These larvae can survive and cause symptoms like gastrointestinal upset if swallowed.
Other species, like the Tumbu fly, lay eggs on the ground or damp clothing. The larvae hatch and transfer to the host upon contact, burrowing directly into the skin.
Categorization by Location of Infestation
Myiasis is clinically categorized based on the anatomical location of the infestation, which determines the specific symptoms and severity.
Cutaneous myiasis, affecting the skin, is the most frequently observed form and presents in three ways. Furuncular myiasis involves larvae burrowing beneath the skin, creating a boil-like lesion with a central opening (punctum) for breathing. Patients often report itching, a sensation of movement, and sharp, stabbing pain, particularly at night. The larvae develop in this subdermal cavity for several weeks before emerging.
Wound myiasis occurs when flies lay eggs in pre-existing open sores, ulcers, or mucous membranes, often in individuals with poor hygiene. The larvae feed on both dead and living tissue, causing significant tissue destruction and a foul-smelling discharge. In severe cases involving head orifices, this type can lead to bone erosion or fatal cerebral involvement.
Migratory myiasis, also known as creeping myiasis, is characterized by larvae tunneling just beneath the skin’s surface. This movement produces serpentine, red, and itchy tracks that appear to advance as the larva progresses. This presentation is often associated with accidental human parasites, such as larvae from the horse botfly (Gasterophilus intestinalis).
Beyond the skin, myiasis can affect specialized body sites. Ophthalmomyiasis involves the infestation of the eye, eye socket, or surrounding tissue, often caused by the sheep botfly (Oestrus ovis). This condition results in severe inflammation, redness, and a foreign body sensation in the eye.
Enteric or Urogenital myiasis involves the gastrointestinal tract or the urinary and genital organs. This typically results from the accidental ingestion of eggs or their deposition near the orifices. Symptoms for the enteric form include abdominal pain and nausea, while urogenital involvement can cause pain and the discovery of larvae in urine.
Medical Diagnosis and Treatment Protocols
Diagnosis relies primarily on the clinical appearance of the lesion, the patient’s recent travel history, and visual confirmation of the larvae. A healthcare provider identifies the condition by observing the characteristic furuncle with the central breathing pore or by finding the maggots within an open wound. For deep or internal infestation, imaging techniques like ultrasound may be employed to locate the larvae, aiding in removal planning.
The cornerstone of treatment is the physical removal of the larvae. For furuncular myiasis, a common non-surgical method is occlusion, which involves covering the larva’s breathing pore with petroleum jelly or a thick adhesive. This cuts off the air supply, forcing the larva to migrate toward the surface where it can be removed with forceps. If the larva is anchored deeply, a small surgical incision may be necessary to extract it completely and prevent inflammation from retained fragments.
For wound myiasis, treatment involves manual removal of all visible larvae, followed by thorough irrigation and surgical debridement to remove dead tissue. Antiparasitic medications like oral or topical ivermectin may be administered to kill the larvae or encourage migration. Following removal, the wound is cleaned and monitored, and antibiotics are sometimes prescribed to prevent secondary bacterial infections.
Prevention of Myiasis
Preventing myiasis involves a combination of personal hygiene and environmental measures, especially where the condition is common. Travelers to tropical and subtropical regions should take precautions against insect bites, such as wearing long-sleeved clothing and using an EPA-registered insect repellent. Covering all open wounds immediately and changing dressings daily is important, as fluids from sores attract myiasis-causing flies.
Environmental controls are effective in reducing the risk of infestation. Clothes should not be dried on the ground where the Tumbu fly is known to lay its eggs; ironing clothes can kill any eggs or larvae present. Good sanitation, including the proper disposal of refuse and animal carcasses, helps control fly populations around living areas.