Botflies are a type of parasitic fly, known for their unique life cycle that often involves their larvae developing within the skin of mammals, including humans. This article aims to clarify how these insects manage to infest human skin.
The Botfly Life Cycle
Botflies, belonging to the family Oestridae, undergo a complete metamorphosis, through egg, larval, pupal, and adult stages. The adult botfly, often resembling a large bee, has a short lifespan, typically lasting only a few days, and does not feed. Its primary purpose during this adult phase is reproduction.
The female botfly lays her eggs, which then develop into larvae. This larval stage is parasitic and infests a host. After maturing, the larvae exit the host, pupate in the soil, and eventually emerge as adult flies.
Transfer to the Host
The primary mechanism by which human botfly larvae, specifically Dermatobia hominis, transfer to a human host’s skin is through a process called phoresy. The female botfly captures another arthropod, frequently a mosquito, and glues 10 to 50 eggs to its abdomen. When this carrier insect lands on a warm-blooded animal, including a human, to take a blood meal, the host’s body heat stimulates the botfly eggs to hatch.
The newly hatched larvae then quickly penetrate the host’s skin. This entry can occur through the bite wound created by the carrier insect, through a hair follicle, or even through intact skin. Once inside, the larva develops, burrowing into the subcutaneous layer.
Development Within the Skin
After penetrating the skin, the botfly larva creates a raised, boil-like lesion known as a “warble” or furuncle. The larva positions itself head-down within this cavity, with its posterior end, which contains its breathing tubes (spiracles), oriented towards a small opening in the skin for breathing. It feeds on tissue fluids and exudates from the host, not blood.
Over a period of approximately 6 to 10 weeks, the larva grows through several developmental stages. During this time, a person might feel movement under the skin or experience intermittent stabbing pains. Once fully mature, the larva, which can reach about 1 inch in length, exits the host through the opening it maintained, drops to the ground, and pupates.
Different Botfly Species and Their Methods
While Dermatobia hominis is the botfly species most commonly associated with human infestation, other botfly species employ different methods of host transfer. For example, the sheep botfly (Oestrus ovis) does not use an intermediate carrier. Instead, the female directly deposits live larvae into the nostrils of sheep and goats, and occasionally humans, while in flight. These larvae then migrate into the nasal passages.
Another genus, Cuterebra, often referred to as rodent or rabbit botflies, lays its eggs near the entrances of rodent and rabbit burrows or along their pathways. The eggs hatch in response to the warmth and carbon dioxide emitted by a nearby host, and the larvae then enter the host through natural openings like the nose or mouth, or through skin abrasions. These varied strategies highlight the diverse adaptations within the botfly family.