Certain parasitic fly species can deposit their eggs or larvae into living hosts. This infestation is known as myiasis, a condition where the fly larvae, or maggots, develop within the body of a mammal. This process can occur on human skin, representing a rare but serious parasitic infection. Understanding the biology of the flies involved and their specific life cycles is necessary to grasp the nature of this affliction.
Defining Myiasis and Specific Culprits
Myiasis is the infestation of living or necrotic tissue in a vertebrate host by the larval stage of dipterous flies. The larvae feed on the host’s tissue, body fluids, or ingested food before maturing. This condition is categorized based on the fly species and the nature of the infestation, with cutaneous myiasis being the most frequently encountered presentation in humans.
Three species are most commonly responsible for cutaneous human myiasis, each associated with distinct geographical regions and patterns of infestation. The human botfly, Dermatobia hominis, is prevalent in Central and South America, and its larvae cause a furuncular, or boil-like, type of lesion. In contrast, the tumbu fly, Cordylobia anthropophaga, is the most frequent cause of furuncular myiasis across sub-Saharan Africa. A third type, known as wound myiasis, is caused by screwworm flies like Cochliomyia hominivorax, which are obligate parasites that feed exclusively on living tissue and are found primarily in the Americas.
These species are obligate parasites, meaning their larvae must develop in a living host to complete their life cycle. The specific species involved dictates the mechanism of entry and the severity of the resulting skin lesion.
The Process of Infestation
The method by which the larvae enter the skin varies significantly between fly species, ranging from an indirect approach to direct penetration. The human botfly employs a unique and indirect strategy involving a phoretic carrier, such as a mosquito or tick, to transport its eggs. The female botfly glues its eggs to the carrier’s abdomen. When the carrier lands on a mammal to feed, the warmth of the host’s skin causes the eggs to hatch, and the tiny larvae then enter the skin through the bite wound or hair follicles.
The tumbu fly uses a more direct method that often involves contaminated items. The female fly lays her eggs in sandy soil soiled with urine or feces, or on damp clothing and bedding left to dry outdoors. The eggs hatch into larvae, which can remain viable in the soil or clothing for up to two weeks while waiting for a host. Upon contact with human skin, the larvae painlessly penetrate the unbroken surface and begin their development.
Screwworm flies typically target existing breaks in the skin. The female deposits hundreds of eggs directly onto the edges of open wounds, lesions, or moist mucous membranes. The eggs hatch quickly, often within 12 to 24 hours, and the larvae immediately burrow into the underlying tissue to feed. Botfly larvae develop within a subdermal cavity for five to ten weeks before they exit the skin and drop to the ground to pupate.
Recognizing Symptoms and Diagnosis
The presence of a developing larva in the skin initially manifests as a small, red, papular lesion, often resembling a common insect bite. As the larva grows beneath the skin, the lesion rapidly enlarges, developing into a firm, painful, boil-like nodule, referred to as a furuncle. A defining feature of furuncular myiasis is the presence of a small central opening, or punctum, through which the larva breathes.
The patient often reports a sensation of movement or stabbing pain, particularly at night when the larva is most active. Serosanguineous fluid, a thin, yellowish-bloody discharge, may drain from the punctum as a result of the larva’s feeding and excretions. Diagnosis is primarily clinical, relying on the characteristic appearance of the lesion, the reported symptoms, and a history of travel to or residence in an endemic area.
For early or atypical lesions where the central breathing pore is not yet visible, medical imaging can confirm the presence of the parasite. High-resolution Doppler ultrasonography is a non-invasive tool used to locate the larva and assess its viability. This technique can identify the larva as an oblong, hypoechoic structure. In more complicated cases, or to assess the depth of tissue damage, computed tomography (CT) or magnetic resonance imaging (MRI) may be used.
Medical Removal and Prevention
The standard medical management for furuncular myiasis focuses on the removal of the larva without causing it to rupture or die within the tissue. A common noninvasive approach is the use of occlusion, designed to deprive the larva of air, forcing it to emerge. Occlusive materials, such as petroleum jelly, liquid paraffin, or heavy oil, are applied over the breathing punctum and left in place for several hours to a full day.
Once the larva is partially emerged, seeking air, it can be grasped and manually extracted using forceps. Caution is necessary because botfly larvae are covered in rows of spines and hooks that anchor them firmly to the subcutaneous tissue. If the larva is difficult to extract, a small surgical incision, often performed with a punch biopsy tool after local anesthesia, can remove the larva intact, minimizing the risk of a foreign-body reaction from retained parts.
Prevention is centered on reducing contact with the flies and their eggs, especially when traveling to or residing in endemic areas. For regions with the tumbu fly, ironing all clothing, including undergarments, after drying them outdoors kills any eggs or newly hatched larvae deposited on the fabric. General prevention includes using insect repellent, wearing protective clothing, and ensuring that open wounds are promptly cleaned and covered to deter screwworm flies.