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, thus they are true parasites.
The equine botflies present seasonal difficulties to equestrian caretakers, as they lay eggs on the insides of horses’ front legs, on the cannon bone and knees and sometimes on the throat or nose, depending on the species. 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 lining or the small intestine. The attachment of the larvae to the tissue produces a mild irritation which results in erosions and ulcerations at the 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 sandpaper 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 report seeing botfly larvae in horse manure. These larvae are cylindrical in shape and are reddish orange in color. In one to two months, adult botflies emerge from the developing larvae and the cycle repeats itself. Botflies can be controlled with several types of dewormers, including dichlorvos, ivermectin and trichlorfon.