Botfly larvae are the parasitic stage of flies belonging to the family Oestridae. The infestation of a vertebrate animal, including humans, by the larval stage of a fly is a condition known as myiasis. Botflies are classified as obligate parasites because their larvae must feed on the host’s tissues to complete their growth. Infestations are most commonly observed in tropical and subtropical regions worldwide, with different species dominating in the Americas and Africa.
The Botfly Larvae Life Cycle
The most medically relevant species, the Human Botfly (Dermatobia hominis), employs a unique strategy for infestation. The adult female fly does not directly approach a host. Instead, she captures a blood-feeding arthropod, such as a mosquito or tick, and glues between 10 and 50 eggs to the vector’s abdomen, a process called phoretic transmission.
When the vector lands on a host, the host’s body heat triggers the eggs to hatch. The first instar larva then drops onto the skin and quickly burrows into the tissue, often using the bite wound or a hair follicle as an entry point. Once settled in the subcutaneous layer, the larva forms a warble, where it develops over several weeks.
Inside the host, the larva matures through three instars, feeding on the host’s tissue exudates. The parasitic stage typically lasts five to ten weeks, during which the larva grows significantly, reaching up to 25 millimeters in length. When fully mature, the third-instar larva exits the host’s skin through the original entry hole and drops to the ground. It then burrows into the soil to begin the pupal stage before emerging as a non-feeding, short-lived adult fly.
Types of Botflies and Host Specificity
The Oestridae family is diverse, with species specializing in different host animals and infestation sites. The Human Botfly (Dermatobia hominis) is endemic to the Americas, ranging from Mexico to northern Argentina. It is the primary cause of myiasis in travelers returning from Central and South America, and its larvae typically infest the subdermal layer of mammals, including cattle, dogs, and humans.
Another common species causing similar lesions in humans is the Tumbu Fly (Cordylobia anthropophaga), prevalent across sub-Saharan Africa. Unlike the Human Botfly, the female Tumbu Fly lays eggs directly on dry soil or on clothing, and the larvae burrow into the skin upon contact. Other botflies primarily target livestock, such as the Gasterophilus species, which develop in the stomach and intestines of horses.
Cattle botflies (Hypoderma species) migrate extensively through the host’s body before settling under the skin of cattle. The sheep botfly (Oestrus ovis) is a cavity-dwelling species whose larvae develop in the nasal passages and sinuses of sheep and goats. These species rarely infest humans but illustrate the specialized nature of botfly parasitism.
Signs, Symptoms, and Host Impact
Human Botfly infestation results in furuncular myiasis, characterized by a painful, boil-like lesion on the skin. The lesion is a raised, firm, reddish nodule that often resembles a large insect bite or a bacterial abscess. A defining feature is a small, central opening called a punctum, which the larva uses as a breathing hole.
Through this punctum, the larva extends its posterior spiracles to breathe. The host may observe a clear or serosanguineous discharge, and the presence of the larva often causes intermittent, sharp, or stinging pain, thought to be caused by its movement or feeding. Some individuals also report a creeping sensation or nocturnal itching.
As the larva grows, its movement becomes more noticeable, and the pain intensifies. Although the infestation is usually self-limiting, rupture of the larva during forceful removal can lead to complications. The release of foreign proteins from the larva’s body can trigger a severe localized inflammatory reaction or a secondary bacterial infection.
Extraction and Prevention
When a botfly larva infestation is confirmed, professional medical removal is recommended to prevent complications. The larvae possess backward-pointing spines that anchor them firmly within the host’s tissue, making manual extraction difficult and often causing rupture. Surgical removal, involving a small incision to enlarge the punctum and allow for intact extraction, is a common and definitive technique.
Non-surgical methods, which aim to suffocate the larva and force it to emerge, are also employed. These involve covering the punctum with an occlusive substance like petroleum jelly, mineral oil, or thick bacon fat for several hours. By depriving the larva of oxygen, this method encourages it to partially emerge, allowing a healthcare provider to grasp and remove it intact with forceps.
Prevention strategies are relevant for travelers visiting endemic areas in Central and South America. Since the Human Botfly’s eggs are often deposited on clothing via a vector insect, ironing all clothes, especially seams, can kill the eggs.
Prevention Methods
- Using insect repellents containing DEET.
- Wearing long-sleeved clothing.
- Avoiding hanging laundry outdoors.