Bot flies, often known as warble flies, are a group of insects notorious for their parasitic larval stage, which develops inside a mammal’s body. This infestation is medically termed myiasis. The adult flies are generally large and hairy, resembling bees, but their main impact comes from their larvae, which feed on the host’s tissue. While the most concerning human species is typically imported, several bot fly species are well-established across the United States.
Presence and Common Species
Bot flies are present across the United States, with approximately 40 different species found in North America. The most common species target livestock, causing significant concern for animal health and agricultural economics. Prevalent groups include the cattle grubs (Hypoderma species) and the horse bots (Gasterophilus species).
The common cattle grub (Hypoderma lineatum) and the northern cattle grub (Hypoderma bovis) are widely distributed, affecting cattle throughout the country. The common horse stomach bot fly (Gasterophilus intestinalis) is the predominant species infesting horses. A third native group, the Cuterebra species, primarily infests rodents and rabbits. The human bot fly (Dermatobia hominis) is not native to the US, being indigenous to Central and South America, and nearly all human cases in the United States are acquired by travelers returning from endemic regions.
The Bot Fly Life Cycle and Infestation
The parasitic stage is the larva, or maggot, as the adult fly often lacks functional mouthparts and lives for only a few days. The infestation process, known as myiasis, is highly specialized and varies between species. Female cattle grubs lay eggs directly on the host’s hair, typically on the legs or lower body. The eggs hatch, and the first-stage larvae penetrate the host’s skin using enzymes.
Horse bot flies, such as Gasterophilus intestinalis, lay eggs on the hairs of the horse’s forelegs. These eggs hatch when stimulated by licking or grooming, and the larvae enter the mouth. They burrow into the tongue or gums before migrating to the stomach. The human bot fly (D. hominis) uses a unique strategy called phoresis, gluing its eggs to a blood-sucking carrier insect like a mosquito. When the carrier feeds on a mammal, the eggs hatch from the warmth of the skin, and the tiny larvae drop onto the host and burrow into the tissue.
Once inside the host, bot fly larvae migrate through the body to specific development sites, which can take several months. Cattle grubs migrate through connective tissue to the esophagus or spinal canal before reaching the subcutaneous tissue of the back. The larva then creates a noticeable swelling in the host’s skin, called a warble, which includes a small breathing hole, or punctum. After developing, the mature larva exits the host through this hole, drops to the ground, and pupates into an adult fly.
Risk Assessment for Humans and Animals
The primary health and economic concern in the US involves livestock, as bot fly species cause substantial impact on cattle and horse industries. Cattle grub infestations cause significant losses due to reduced weight gain, decreased milk production, and hide damage from warble holes. The flies also cause a panic response in cattle, known as “gadding,” which leads to self-injury and reduced grazing time. Horses infested with Gasterophilus larvae can suffer from ulceration of the gastric lining and chronic digestive issues.
The risk for humans in the US is extremely low, and domestic cases are rare, often involving accidental exposure to Cuterebra species. Human myiasis caused by Dermatobia hominis is almost exclusively seen in travelers returning from Central or South America. In humans, the infestation typically presents as a painful, boil-like lesion, or furuncle, which may discharge fluid. While uncomfortable and requiring medical attention, these infestations are rarely fatal.
Managing and Preventing Myiasis
Management of bot fly myiasis in the US focuses primarily on veterinary control to protect agricultural animals. Systemic insecticides and deworming protocols, often utilizing macrocyclic lactones, are highly effective when administered in the fall or early winter. This timing eliminates migrating larvae before they reach sensitive areas like the esophagus or spinal cord, where their death could cause severe inflammatory reactions. If a warble is present with a mature larva, it should not be crushed, and cautious manual extraction is sometimes performed.
For human prevention, the focus is mainly on travel to endemic areas outside the US. Travelers should use high-concentration insect repellent and wear protective clothing to minimize exposure to the flies and their carriers. If an infestation occurs, professional medical help is required for removal. This typically involves a small surgical procedure or a method to force the larva out of the skin opening. Early diagnosis is important, as bot fly lesions can often be misidentified as a common infection.