Myiasis, the parasitic infestation of a host’s living tissue by fly larvae, is a condition that can affect many mammals, including humans. Humans can contract botflies, though it is a rare occurrence outside of specific geographic regions. This infestation is caused by the larvae of flies belonging to the Oestridae family. Humans typically encounter this parasite when traveling to tropical and subtropical areas where the species is endemic, where the infection is generally localized to the skin.
The Botfly Species That Infect Humans
The primary species responsible for cutaneous myiasis in humans is the human botfly, Dermatobia hominis. This fly is adapted to parasitize a wide range of mammals, including humans and livestock, and resembles a large bumblebee with a blue-gray thorax and a metallic blue abdomen. The geographic range of D. hominis is confined to the Americas, extending from southern Mexico through Central America and into parts of South America. While other botfly species, such as the Cordylobia anthropophaga (tumbu fly) found in Africa, can also cause human infestation, D. hominis is the main concern for travelers to the Neotropics. The adult fly does not feed and dedicates its short lifespan entirely to reproduction.
How Larvae Enter the Human Host
The process by which the human botfly larva enters the skin is indirect, involving a strategy known as phoresis. The adult female botfly does not lay her eggs directly on a human host. Instead, she captures a blood-feeding arthropod, often a mosquito or tick, and glues 10 to 50 eggs onto its abdomen. The vector later lands on a human host to take a blood meal, and the warmth of the skin triggers the eggs to hatch rapidly. The newly emerged, tiny larva then uses the puncture wound created by the vector or enters the skin through a hair follicle or pore. Once beneath the skin, the larva uses its mouth hooks and rings of small spines to anchor itself firmly into the subcutaneous tissue.
Recognizing the Signs of Infestation
Once the larva is established beneath the skin, the condition is known as furuncular myiasis due to its resemblance to a boil. Initially, the site presents as a small, itchy nodule that gradually enlarges over several weeks. The most telling sign is a raised, reddened lesion, typically one to three centimeters in size, with a small central opening called a punctum. This punctum is maintained by the larva as a breathing hole to allow its posterior spiracles to access the air. Patients frequently report sharp pain or a tingling sensation as the larva moves within the skin cavity. The larva positions itself head-down, feeding on tissue exudates and growing larger as it develops over five to ten weeks, after which the fully grown larva will exit the skin and drop to the ground to pupate.
Medical Procedures for Safe Removal
Seeking medical attention for botfly removal is important, as improper self-removal attempts, such as squeezing the lesion, can cause the larva to rupture. A ruptured larva can lead to a severe inflammatory reaction, secondary bacterial infection, or even an anaphylactic response. The primary goal of medical intervention is to extract the entire larva intact with minimal trauma to the surrounding tissue.
One common non-surgical technique involves occluding the breathing hole with a substance like petroleum jelly or thick oil. This lack of oxygen suffocates the larva, forcing it to partially emerge from the punctum, allowing a healthcare professional to grasp and remove it with sterile forceps. Because the larva is anchored by rows of spines, the occlusion method may sometimes be ineffective on its own.
If the larva cannot be removed through occlusion, minor surgical excision under local anesthesia may be performed. This involves making a small incision to enlarge the punctum and carefully remove the larva and any surrounding necrotic tissue. Following removal, the wound is thoroughly cleaned, and a course of antibiotic ointment is typically prescribed to prevent secondary bacterial infection.