Certain fly species possess reproductive cycles that involve their larvae developing beneath the skin of a mammalian host, including humans. The question of which insect lays eggs in human skin addresses a verifiable biological event. This biological interaction is a specific parasitic strategy evolved by a few species of flies. Understanding this process requires a detailed look at the insects involved, their unique infestation mechanisms, and the medical condition that results.
The Biological Phenomenon of Myiasis
The medical condition resulting from this type of infestation is called myiasis, which is the parasitic invasion of a live human or other vertebrate animal by fly larvae. These larvae feed on the host’s necrotic or living tissue, body fluids, or ingested food, depending on the species involved. Myiasis is categorized clinically based on the location of the infestation and the behavior of the larvae. The most relevant type is cutaneous myiasis, which affects the dermal layer.
The cutaneous form is classified into furuncular, migratory, and wound myiasis, each caused by different fly species. Furuncular myiasis is the specific presentation where a boil-like lesion forms around a single larva developing in the subcutaneous tissue. This condition is caused by fly species whose larvae enter the skin and remain in a pocket beneath the surface to feed and grow. This behavior distinguishes them from flies that lay eggs in open wounds.
Primary Culprits Identifying the Flies
The insects causing furuncular myiasis in humans belong to the order Diptera, or two-winged flies, and are primarily the Human Botfly and the Tumbu Fly. The Human Botfly (Dermatobia hominis) is indigenous to the Americas, with a geographical distribution stretching from southeastern Mexico down to northern Argentina and Uruguay. This fly employs a unique method to deliver its offspring.
The Tumbu Fly (Cordylobia anthropophaga), also known as the mango or putzi fly, is the most common cause of human cutaneous myiasis across tropical and subtropical regions of sub-Saharan Africa. The larvae of this species are known as the Cayor Worm. While other flies, such as the New World Screwworm (Cochliomyia hominivorax), also infest humans, they typically cause wound myiasis. The Botfly and Tumbu Fly are the main culprits for primary skin invasion.
Infestation Progression and Larval Development
The method of egg delivery is a crucial difference between the two main species. The female Human Botfly captures a blood-sucking insect, such as a mosquito or tick, and glues her eggs to its body in a process known as phoresy. When the carrier insect lands on a warm-blooded host, the heat from the skin stimulates the eggs to hatch. The tiny larvae then burrow into the skin, often through the bite site or a hair follicle. Once beneath the surface, the larva feeds on tissue exudates and develops over five to twelve weeks.
The Tumbu Fly employs a more direct method of infestation. The female fly deposits eggs on dry, sandy soil contaminated with feces or urine, or on damp clothing, such as laundry laid out to dry. The eggs hatch within one to three days, and the larvae can survive for up to 15 days while waiting for a host. Upon contact with a mammalian host, the larvae quickly penetrate the unbroken skin, often taking less than a minute to enter the subcutaneous layer.
In both cases, the developing larva forms a characteristic lesion called a furuncle, which resembles a painful, growing boil. A tell-tale sign is a small central opening, or punctum, which the larva uses to breathe, protruding its posterior spiracles through the pore. The host may experience itching, a sensation of movement, and sometimes sharp, stabbing pain, particularly at night when the larva is most active. After the larval stage is complete (eight to twelve days for the Tumbu Fly and up to ten weeks for the Botfly), the mature larva exits the skin and drops to the ground to pupate.
Medical Management and Removal
Proper medical management of furuncular myiasis requires a trained healthcare professional to ensure the complete removal of the larva. The primary goal is to extract the larva intact to prevent a severe inflammatory reaction if parts of the organism remain in the tissue. This is important because the larva is equipped with hooks and spines that anchor it firmly within the subcutaneous cavity.
One common, non-invasive technique involves occluding the breathing pore with a substance such as petroleum jelly, liquid paraffin, or heavy grease for several hours. This deprives the larva of oxygen, encouraging it to partially emerge from the punctum, making it easier to grasp and remove with forceps. Alternatively, a local anesthetic can be injected into the base of the lesion, which may create enough pressure to push the larva out. If the larva is difficult to extract, a small surgical incision or punch biopsy may be necessary to widen the opening. Following removal, the wound is cleaned and dressed, and antibiotics may be prescribed if there is evidence of a secondary bacterial infection.