Myiasis is a parasitic infestation caused by the larvae of flies from the order Diptera, which feed on the host’s living or necrotic tissue, bodily fluids, or ingested food. These infestations cause a range of clinical issues. Although myiasis presents a global health concern, it is found more frequently in tropical and subtropical regions where the causative flies are abundant. The condition affects both humans and animals, causing significant economic impact on livestock industries worldwide.
Classification of Myiasis
Myiasis is categorized based on the anatomical location of the infestation, which helps guide diagnosis and management. The most common form is cutaneous myiasis, where larvae infest the skin or the tissue just beneath it. This type manifests as furuncular myiasis, where a single larva creates a boil-like lesion, or as wound myiasis, where larvae infest an open injury or sore.
Migratory or creeping myiasis is a different presentation where the larva burrows just under the skin, leaving a characteristic winding, linear track. Beyond the skin, myiasis can affect body openings and internal structures, referred to as cavitary myiasis. This category includes ophthalmomyiasis, which affects the eyes, orbits, and surrounding tissue, and aural myiasis, which is an infestation of the ear canal.
Infestation of the nasal passages, sinuses, and pharynx is known as nasopharyngeal myiasis. These cases can be particularly dangerous due to the potential for larvae to migrate deeper into structures like the brain. Less common forms include gastrointestinal myiasis, resulting from the ingestion of eggs or larvae, and urogenital myiasis, involving the urinary or genital tracts. The specific symptoms and severity of the condition are directly related to the location of the larvae and the particular species of fly involved.
The Fly Life Cycle and Infestation Process
The flies responsible for myiasis undergo complete metamorphosis, including four distinct stages: egg, larva (maggot), pupa, and adult fly. The larva is the pathogenic stage that infests the host and feeds on its tissues. The duration of the life cycle varies significantly depending on the fly species and environmental conditions, but the larval stage typically involves three molting phases, or instars, during which the larva grows rapidly.
Infestation occurs through several distinct mechanisms based on the fly species’ biology. Some flies, known as obligate parasites, must have a living host to complete their life cycle, such as the Dermatobia hominis (botfly). The female botfly often captures a mosquito or other insect, glues her eggs to it, and the eggs hatch and burrow into the host’s skin when the carrier insect takes a blood meal. This mechanism allows for direct penetration into unbroken skin.
Other flies are facultative parasites, meaning they typically lay their eggs on decaying matter but can also infest an open wound or sore. Flies from families like Calliphoridae (blowflies) and Sarcophagidae (flesh flies) are attracted to the scent of blood or discharge from a wound. The eggs hatch quickly, and the larvae begin to feed on the necrotic or living tissue in the wound, leading to wound myiasis.
A third, less frequent route of entry is through accidental ingestion or deposition near an orifice, leading to gastrointestinal or urogenital myiasis. For example, eggs or larvae might be accidentally swallowed with contaminated food or water. The larvae are equipped with features like oral hooks and spines that allow them to anchor themselves within the host’s tissue, making removal difficult.
Identifying and Confirming Myiasis
Diagnosis involves gathering a detailed patient history, especially regarding recent travel to endemic areas, and recognizing characteristic symptoms. Initial signs of cutaneous myiasis include a painful, red, boil-like lesion, often accompanied by itching, swelling, and a sensation of movement beneath the skin. A key finding is the presence of a small opening, called a punctum, in the center of the lesion, which the larva uses for breathing.
Clinical examination is the most straightforward method for identification, as the larvae may be visible in open wounds or protruding from the punctum of a furuncular lesion. For deep or cavitary infestation, imaging techniques such as ultrasound or CT scans may be necessary to determine the extent of the problem.
Confirmation involves the visual identification of the extracted larva, which healthcare providers carefully remove and may preserve for morphological analysis. Identifying the specific species of fly is important because certain larvae, such as those of the New World screwworm, are known to invade healthy tissue and require prompt treatment.
Management and Removal Strategies
Treatment focuses on the safe and complete removal of the larvae and subsequent management of the resulting wound. For furuncular myiasis, a common non-invasive method involves occluding the central breathing pore with a thick substance like petroleum jelly, mineral oil, or even bacon. This suffocation technique forces the larva to migrate toward the surface, where it can then be manually removed with forceps.
Physical extraction must be performed carefully to avoid rupturing the maggot, as retained larval parts can trigger a severe inflammatory reaction in the host. For deeply embedded or difficult-to-remove larvae, a small surgical incision may be required to widen the opening before extraction. Systemic medication such as oral ivermectin may also be administered to kill the larvae or induce their migration, facilitating removal.
After removal, the wound requires thorough cleaning and debridement to remove any dead tissue. Antibiotics are often prescribed to prevent or treat secondary bacterial infections, which are a common complication of the open lesion. Proper wound care is essential to ensure healing and reduce the risk of further complications.