A bot fly is a parasitic insect belonging to the family Oestridae. Its life cycle requires the larva, often called a “bot” or “grub,” to develop within the body of a host mammal, feeding on its tissue before emerging. Bot flies are present in the United States, but the established species primarily target livestock and wildlife. These native species rarely infest humans, making human cases uncommon and usually accidental. The main concern for people in the US is a specific non-native fly acquired during international travel.
Endemic Bot Fly Species in the US
Several species of bot flies are endemic and widespread throughout the United States, posing a significant challenge to the agricultural sector. Two of the most notable are the cattle grubs, Hypoderma bovis and Hypoderma lineatum, which are major pests of cattle nationwide. The larvae of these flies migrate extensively through the host’s body, eventually settling beneath the skin of the back where they create characteristic swellings. Another common native species is the sheep nose bot, Oestrus ovis, which infests sheep and goats across the country.
The female Oestrus ovis deposits live larvae directly into the nostrils of its host, leading to development within the animal’s nasal passages and sinuses. The common horse stomach bot fly, Gasterophilus intestinalis, primarily affects horses in North America. While these species can cause economic losses in agriculture, they are highly host-specific. Infestation in humans remains a rare and accidental event in which the larvae usually fail to complete their development.
The Non-Native Human Bot Fly
The species most commonly associated with human infestation is Dermatobia hominis, widely known as the human bot fly. This fly is strictly non-native to the US, meaning it has not established a permanent, breeding population within the country’s borders. Its natural geographic range extends from southern Mexico through Central America and into parts of South America. Consequently, nearly all reported human bot fly cases in the United States are travel-related, acquired by individuals returning from endemic tropical and subtropical regions.
Medical professionals treating these cases rely heavily on a patient’s travel history to accurately diagnose the lesion. The distinction between the endemic animal bots and the human bot fly is important for clinical diagnosis. The presence of Dermatobia hominis in the US is a matter of importation, not establishment. This highlights the link between international travel and the spread of certain parasitic infections.
Understanding the Infestation Process
Infestation by fly larvae is medically termed myiasis, and the life cycle of the bot fly is a specialized form of this parasitic process. The most unusual aspect of the human bot fly’s reproductive cycle is its method of egg delivery, known as phoresia. The adult female captures a blood-sucking insect, typically a mosquito or tick, and glues her eggs onto its body. When the carrier insect lands on a warm-blooded host, the host’s body heat triggers the eggs to hatch instantly.
The tiny first-stage larva quickly penetrates the host’s skin through a bite wound, hair follicle, or abrasion. Once beneath the skin, the larva settles into a subdermal cavity where it feeds and develops over five to ten weeks. The larva maintains a small opening, called a punctum, at the skin’s surface, which serves as a breathing hole. Infestation typically presents as a painful, boil-like lesion, known as a furuncular myiasis, which may exude a clear or bloody discharge. As the larva grows, a patient may report an intermittent, shooting pain or a sensation of movement beneath the skin.
Preventing Infestation
Preventing exposure to endemic bot flies involves managing domestic animals and maintaining good sanitation in agricultural settings. Protecting livestock from species like cattle grubs and sheep nose bots reduces the risk of accidental human exposure. Travelers heading to areas where the human bot fly is endemic, such as Central and South America, should take specific precautions. Using a strong insect repellent containing DEET or picaridin on exposed skin helps deter the carrier insects from landing.
Wearing long sleeves and pants provides a physical barrier against the flies and their vectors. Travelers should also use permethrin-treated bed nets when sleeping in non-screened areas. A unique preventative measure involves ironing all clothing, especially items dried outdoors, as fly eggs may be deposited on fabrics. If an infestation occurs, the standard treatment is the surgical removal of the larva by a medical professional under local anesthesia. Non-surgical methods, such as applying an occlusive substance to suffocate the larva, are sometimes used to encourage the larva to emerge before extraction.