Are Bot Flies in the US? Types, Risks, and Prevention

Bot flies are indeed present in the United States, with several species found across various regions. While some species primarily affect wildlife and livestock, others can occasionally infest humans or pets. Understanding their types, life cycles, and host interactions is important for prevention and management.

Bot Fly Presence and Types in the US

The United States is home to several species of bot flies, with the most common being those from the Cuterebra and Hypoderma genera. Cuterebra species, often referred to as rodent or rabbit bot flies, are prevalent across most of the continental U.S., excluding Alaska. These adult flies are relatively large, measuring about 15 to 17mm, with a bee-like appearance.

Female Cuterebra flies do not lay their eggs directly on a host, but instead deposit them on vegetation, wood chips, or near the entrances of rodent and rabbit burrows. The eggs hatch into larvae when they detect a sudden increase in temperature, such as the body heat of a passing animal. The tiny larvae then attach to the animal’s fur and typically enter the host’s body through natural openings like the nose, mouth, or even through skin wounds.

Hypoderma species are commonly known as cattle grubs or warble flies. Two species, Hypoderma bovis and Hypoderma lineatum, primarily affect cattle. These flies are found in North America between latitudes 25° and 60°. Female Hypoderma flies lay their eggs on the hair of cattle, particularly on the legs and lower body regions. The eggs hatch within a few days, and the larvae then penetrate the host’s skin to begin their migratory journey through the tissues.

Impact of Bot Fly Infestations on Hosts

When bot fly larvae infest a host, they cause myiasis, the invasion of living tissue by fly larvae. In humans, infestations often result from accidental contact with larvae, commonly from Cuterebra species or from the human bot fly (Dermatobia hominis), which is native to Central and South America but can affect travelers returning to the U.S. The initial sign is typically a small, firm nodule in the skin that gradually enlarges. This nodule, often called a “warble” or furuncular lesion, usually has a central pore through which the larva breathes.

Infested individuals may experience itching, pain, and a sensation of movement beneath the skin as the larva grows. The wound often discharges a yellowish or bloody fluid because the larva needs to keep the opening clear for respiration and waste excretion. While human bot fly myiasis is rarely fatal, the presence of the larva can be painful and uncomfortable, and secondary bacterial infections can occur around the wound. In rare instances, larvae can migrate to less common sites such as the eyes, ears, or nose, leading to more severe complications.

In animals, such as livestock and pets, bot fly infestations also manifest as subcutaneous swellings or warbles. For cattle, Hypoderma larvae cause visible lesions on the back, which can lead to hide damage and reduced productivity. These larvae migrate through the animal’s body, and their presence can cause inflammation and tissue damage. Horses can be affected by Gasterophilus species, where larvae primarily infest the gastrointestinal tract, leading to irritation, ulcers, and potentially affecting digestion or causing colic in severe cases. Pets like dogs and cats, particularly those that hunt rodents or rabbits, can accidentally become hosts to Cuterebra larvae, resulting in lumps, discharge, and discomfort, especially around the head and neck.

Prevention and Management of Bot Flies

Preventing bot fly infestations involves strategies tailored to both human and animal exposure. For humans, especially when traveling to areas where the human bot fly (Dermatobia hominis) is endemic, protective measures include wearing long-sleeved clothing and pants, using insect repellents containing DEET or picaridin on exposed skin, and sleeping under permethrin-treated bed nets. Ironing clothes that have been line-dried in tropical areas can also help, as bot flies may lay eggs on damp laundry. Avoiding direct contact with vegetation in areas known for Cuterebra activity can reduce the risk of accidental infestation.

For animals, preventing bot fly infestations often focuses on environmental management and direct protective measures. Regular inspection and grooming of horses can help remove bot fly eggs from their coats, particularly from legs and manes. In livestock, controlling adult fly populations and applying larvicides can be part of an integrated management plan.

Systemic insecticides, such as macrocyclic lactones (e.g., ivermectin), are effective against migrating larvae in cattle. For pets, limiting access to areas where rodents and rabbits are abundant, especially during peak bot fly season (late summer and early fall), can reduce exposure. Topical repellents approved for animals may also offer some protection.

If an infestation is suspected or confirmed, professional medical or veterinary care is important. For humans, bot fly larvae are typically removed surgically by a healthcare provider to ensure complete extraction and prevent complications like secondary infections or inflammation. Attempting to squeeze or puncture the larva can cause it to rupture, leading to severe reactions.

In some cases, occlusive methods like applying petroleum jelly over the breathing hole can encourage the larva to emerge due to oxygen deprivation. For animals, veterinarians usually perform surgical removal of the larvae, followed by proper wound care. Systemic antiparasitic medications, such as ivermectin, are often used to treat animal infestations, especially for internal larval migrations.

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