|Horse botfly (Gasterophilus intestinalis)|
Oestridae are a family of flies variously known as bot flies, warble flies, heel flies, gadflies and similar names. Their larvae are internal parasites of mammals, some species growing in the host's flesh and others within the gut. The human botfly, Dermatobia hominis, is the only species of bot fly known to parasitize humans routinely, though other species of fly do cause myiasis in humans.
A botfly, also written bot fly, bott fly or bot-fly in various combinations, is any fly in the family Oestridae. The life cycles vary greatly according to species, but the larvae of all species are internal parasites of mammals. Largely according to species they also are known variously as warble flies, heel flies, and gadflies. The larvae of some species grow in the flesh of their hosts, while others grow within the hosts' alimentary tracts.
The word "bot" in this sense means a maggot. A warble is a skin lump or callus such as might be caused by an ill-fitting harness, or by the presence of a warble fly maggot under the skin. The human botfly, Dermatobia hominis, is the only species of bot fly whose larvae ordinarily parasitise humans, though flies in some other families episodically cause human myiasis, and sometimes more harmfully.
Family Oestridae 
Of families of flies causing Myiasis, the Oestridae include the highest proportion of species whose larvae live as obligate parasites within the bodies of mammals. There are roughly 150 known species worldwide. Most other species of flies implicated in myiasis are members of related families, such as blowflies and screwworm flies in the Calliphoridae.
Botflies deposit eggs on a host, or sometimes use an intermediate vector such as the common housefly, mosquitoes and even a species of tick (see Dermatobia hominis). The smaller fly is firmly held by the botfly female and rotated to a position where the botfly attaches some 30 eggs to the body under the wings. Larvae from these eggs, stimulated by the warmth and proximity of a large mammal host, drop onto its skin and burrow underneath. Intermediate vectors are often used since a number of animal hosts recognise the approach of a botfly and flee.
Eggs are deposited on animal skin directly, or the larvae hatch and drop from the eggs attached to the intermediate vector: the body heat of the host animal induces hatching upon contact or immediate proximity. Some forms of botfly also occur in the digestive tract after ingestion by licking.
Myiasis can be caused by larvae burrowing into the skin (or tissue lining) of the host animal. Mature larvae drop from the host and complete the pupal stage in soil. They do not kill the host animal, and thus are true parasites.
The equine bot fly presents seasonal difficulties to equestrian caretakers, as it lays eggs on the insides of horse's front legs, on the cannon bone and knees, and sometimes on the throat or nose, depending on the species of bot fly. These eggs, which look like small, yellow drops of paint, must be carefully removed during the laying season (late summer and early fall) to prevent infestation in the horse. When a horse rubs its nose on its legs, the eggs are transferred to the mouth, and from there to the intestines, where the larvae grow and attach themselves to the stomach's lining or they pass into the small intestine and attach there. The attachment of the larvae to the tissue produces a mild irritation which results in erosions and ulcerations at this site. Removal of the eggs (which adhere to the host's hair) is difficult, since the bone and tendons are directly under the skin on the cannon bones: eggs must be removed with a sharp knife (often a razor blade) or rough sand paper, and caught before they reach the ground. The larvae remain attached and develop for 10–12 months before they are passed out in the feces. Occasionally horse owners will report seeing the bot fly larvae in horse manure. These larvae are cylindrical in shape and are reddish orange in color. In 1–2 months adult bot flies will emerge from the developing larvae and the cycle will repeat. Bots can be controlled with several types of dewormers, including dichlorvos, ivermectin and trichlorfon.
In cattle, the lesions caused by these flies can become infected by Mannheimia granulomatis, a bacterium that causes lechiguana, characterized by rapid-growing hard lumps beneath the skin of the animal. Without antibiotics an affected animal will die within 3–11 months.
The human botfly occasionally uses humans as the host to its larvae. The larva, because of its spines, can pose an extremely painful sub-epidermal condition. One removal method is to use the tree sap of the matatorsalo, found in Costa Rica, which is reputed to kill the larva, yet leave its body in the skin. Additionally, one can attempt to seal the breathing hole of the larva with nail polish or vaseline and then, after a day, squeeze out the suffocated, dead larva. Use of adhesive tape can work, but carries additional risk of infection because portions of the larva's breathing tube can be broken off by the tape and make the remainder of the body difficult to remove.
Uses by humans 
In cold climates supporting reindeer or caribou-reliant populations, large quantities of Hypoderma tarandi (warble fly) maggots are available to human populations during the butchery of animals. Some people still relish these maggots as seasonal luxuries containing high levels of proteins, fats and some minerals.
See also 
- Dermatobia hominis, human botfly
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|Wikimedia Commons has media related to: Oestridae|
- Encyclopedia.com article
- "Bug Attack" documentary: includes footage and description of botflies
- A snopes article concerning a botfly infestation
- Human Botfly, Description and Cure
- Cuterebra emasculator, squirrel bot fly
- Dermatobia hominis, human bot fly
- Gasterophilus intestinalis, horse bot fly
- Hypoderma lineatum, common cattle grub