Botfly

Botflies belong to the fly family Oestridae and go by several common names, including warble flies, heel flies, and gadflies. All botfly species have larvae that live as internal parasites inside mammals, though the location varies: some develop within the host's flesh, while others grow inside the digestive tract. The species *Dermatobia hominis* is the only one that regularly uses humans as hosts, but other types of flies can also cause myiasis in people.

The word "bot" refers to a maggot, and a "warble" is a skin lump or callus, often caused by a botfly larva living under the skin. The family Oestridae now includes what were once separate families—Oestridae, Cuterebridae, Gasterophilidae, and Hypodermatidae—as subfamilies. This family belongs to the superfamily Oestroidea, alongside blow-flies and others. Among all fly families that cause myiasis, Oestridae has the highest proportion of species whose larvae are obligate internal parasites of mammals. About 150 species are known worldwide.

Botflies lay eggs directly on a host or use an intermediate carrier, such as a housefly, mosquito, or tick. After mating, a female botfly grabs the carrier insect by its wings and attaches 15 to 30 eggs to its abdomen. She repeats this process to distribute the 100 to 400 eggs she produces during her short adult life of 8 to 9 days. When a large mammal gets close, the warmth triggers the larvae to drop onto the skin and burrow in. Many animals flee when they see a botfly approaching, so the flies often rely on these intermediate vectors. Some botfly larvae enter the host through ingestion, hatching in the digestive tract after the animal licks eggs off its fur.

The larvae cause myiasis by burrowing into the host's skin or tissue lining. Once mature, they drop off and complete the pupal stage in the soil. They do not kill the host, making them true parasites. In horses, botflies lay eggs on the front legs (especially the cannon bones and knees) and sometimes on the throat or nose. These eggs look like small yellow drops of paint and must be carefully removed during late summer and early fall to prevent infestation. When a horse rubs its nose on its legs, the eggs transfer to the mouth and then to the intestines, where larvae attach to the stomach lining or small intestine. This attachment causes mild irritation, leading to erosions and ulcers. Removing the eggs is tricky because the skin over the cannon bones is thin; a sharp knife or rough sandpaper is used, and the eggs must be caught before they fall. The larvae stay attached for 10 to 12 months before passing out in the feces, where they appear as cylindrical, reddish-orange maggots. Adult botflies emerge one to two months later. Dewormers like dichlorvos, ivermectin, and trichlorfon can control them.

In cattle, the lesions from botfly larvae can become infected with *Mannheimia granulomatis*, causing lechiguana—rapid-growing, hard lumps under the skin. Without antibiotics, the animal dies within 3 to 11 months. The mouse botfly, *Cuterebra fontinella*, parasitizes small mammals across North America, while *Dermatobia hominis* occasionally uses humans.

In cold regions where people rely on reindeer or caribou, large numbers of *Hypoderma tarandi* maggots are available during butchery. In an episode of the TV series *Beyond Survival*, survival expert Les Stroud and two Inuit guides ate one larva each after skinning a caribou; Stroud said it tasted like milk and noted that the Inuit historically ate them.