Botflies are a type of fly belonging to the family Oestridae, known for their distinctive parasitic larval stage. Their larvae develop internally within mammals, with some species growing within the host’s flesh and others within the digestive tract. Understanding their natural distribution and life cycles provides valuable insight into these insects.
Global Habitats and Regions
Botflies are found across various geographical regions, with their presence linked to suitable environmental conditions and host availability. Many botfly species are prevalent in tropical and subtropical climates, particularly across the Americas, extending from southern Mexico through Central America and into South America, including Brazil, Belize, and Argentina. These areas often feature diverse ecosystems such as forests, grasslands, and rural landscapes where their hosts are abundant.
While the human botfly (Dermatobia hominis) is primarily found in these warmer American regions, other botfly species have adapted to temperate zones. For instance, certain Cuterebra species, which parasitize rodents and rabbits, are common across much of the continental United States and southern Canada. The sheep nasal bot fly (Oestrus ovis) is globally distributed, occurring on all six inhabited continents where sheep are raised, including North America, the Mediterranean region, and Africa. Their distribution is influenced by temperature, humidity, and specific host populations.
Hosts They Inhabit
Botflies exhibit host specificity, meaning different species typically parasitize particular animals. For example, many Cuterebra species primarily target rodents and rabbits, forming visible lumps called “warbles” under the skin. These larvae can be found subcutaneously, often in areas like the hip, groin, or neck region.
Domestic animals are also common hosts for various botfly species. Horses can be infested by Gasterophilus species, with larvae developing in their digestive tracts, specifically attaching to the stomach lining or small intestine. Cattle are susceptible to species like Hypoderma lineatum, which cause cattle grub, while sheep and goats are commonly affected by the sheep nasal bot fly (Oestrus ovis), whose larvae reside in the nasal passages and sinuses. Other animals such as deer, cats, and dogs can also serve as hosts.
While many botfly species are host-specific to animals, the human botfly (Dermatobia hominis) is unique as the only species whose larvae routinely parasitizes humans. In humans, the larvae typically burrow into the subcutaneous layer of skin, creating a painful nodule or boil. These infestations are most often seen in travelers returning from endemic areas in Central and South America, where the human botfly is native.
Life Cycle Locations
Botflies occupy various locations throughout their life cycle. Adult botflies are free-flying insects, often found in the vicinity of their potential hosts, focusing primarily on mating and egg-laying during their short adult lifespan. Adult males of some species may gather at high points in the landscape, a behavior called “hilltopping,” to find females.
The location of eggs varies by species. Female human botflies, for instance, capture blood-feeding insects like mosquitoes or ticks and attach their eggs to them; these eggs then hatch and enter a host when the carrier insect takes a blood meal. Other botfly species may lay eggs directly on the host’s fur or skin, or on vegetation and substrates near host burrows or runways. For example, equine botflies lay eggs on the hair of horses’ legs, shoulders, or muzzles, which are then ingested by the horse.
Once hatched, the larvae, or “bots,” burrow into their specific hosts, residing under the skin, in nasal passages, or within the digestive tract, depending on the botfly species. After developing within the host, mature larvae typically exit the host’s body. These larvae then drop to the ground and burrow into the soil to pupate, transforming into adult flies.