Bot flies are a diverse group of insects belonging to the family Oestridae, often recognized by their bee-like appearance in adulthood. Their larvae are internal parasites, developing within the tissues of various mammals. While the adult flies are rarely seen and do not feed, their life cycle revolves around finding a host for their offspring. The presence of bot fly larvae in an animal’s body is known as myiasis.
Global Distribution and Habitats
Bot flies are found across many parts of the world, with different species adapted to various climates and regions. The human bot fly (Dermatobia hominis), for instance, is primarily found in tropical and subtropical areas, extending from southern Mexico through Central America and into parts of South America, reaching as far south as northern Argentina. Other species, such as Cuterebra fontinella, are common across the continental United States, southern Canada, and northeastern Mexico.
These insects thrive in warm, moist environments. Habitats include woodlands, particularly those with abundant animal hosts, and rural areas near livestock. For example, horse bot flies congregate around stables and pastures during mid-summer and fall.
Host Infestation Sites
Bot fly larvae develop in specific locations on their hosts, varying significantly by species. The human bot fly (Dermatobia hominis) larvae infest the subcutaneous tissue, creating a boil-like lesion with a central breathing hole. While humans are incidental hosts, other mammals such as cattle, sheep, goats, pigs, dogs, cats, rabbits, and some birds can be affected.
Cattle are commonly affected by species like Hypoderma lineatum, often called cattle grubs, which cause warbles or swellings under the skin. Horse bot flies (Gasterophilus spp.) primarily infest horses, mules, and donkeys. Their larvae develop in various parts of the host’s digestive tract, including the stomach, rectum, intestines, and throat. The sheep nasal bot fly (Oestrus ovis) specifically targets the nasal passages of sheep and goats, where the larvae develop.
Rodent and lagomorph bot flies (Cuterebra spp.) parasitize rodents and rabbits. Their larvae burrow head-first into the skin, forming a bulging, tumor-like “goose egg” or warble, often in the rear ventral region. Though primarily affecting rodents, these larvae can also infest dogs and cats.
Pathways to Infestation
The methods by which bot fly larvae reach their hosts are diverse and depend on the specific bot fly species. The female human bot fly (Dermatobia hominis) does not directly lay eggs on humans. Instead, she captures blood-feeding insects, such as mosquitoes or ticks, and attaches her eggs to their bodies. When the vector insect lands on a warm-blooded host, the heat from the host’s body causes the bot fly eggs to hatch, and the tiny larvae then burrow into the skin through the bite wound, other abrasions, or hair follicles.
Other bot fly species have different strategies for transferring their larvae. Horse bot flies (Gasterophilus spp.) lay their eggs directly onto the fur of horses, often on the forelegs. The host then ingests the eggs by licking or biting its fur, allowing the larvae to develop internally.
Rodent and rabbit bot flies (Cuterebra spp.) deposit their eggs in or near the burrows and runways of their hosts, or on nearby vegetation. The eggs hatch when a warm body passes by, and the larvae then grab onto the host and crawl into an orifice like the mouth, eyes, or nose, or through a wound, before migrating under the skin.