Various types of bot flies are present in Wisconsin. These insects, known for their parasitic larval stages, primarily affect animal populations within the state. While certain species are highly host-specific, accidental infestations in humans can occur, particularly with species commonly found in rodents and rabbits.
Bot Fly Presence in Wisconsin
Wisconsin is home to several bot fly species, each with preferred animal hosts. Deer nasal bot flies (Cephenemyia species) are common, with larvae residing in the nasal passages and pharyngeal cavity of deer. These larvae generally cause little harm to individual deer and do not infect humans.
Rodent and rabbit bot flies (Cuterebra species) are also prevalent in Wisconsin. These flies primarily parasitize small mammals such as squirrels, chipmunks, mice, and rabbits. Female Cuterebra flies lay their eggs near small mammal burrows, and larvae enter the host through natural openings or wounds. While not typically associated with humans, accidental human infestation can occasionally occur, especially in rural areas where these animal hosts are abundant.
Cattle grubs, also known as warble flies (Hypoderma lineatum and Hypoderma bovis), are found in North America and affect cattle. Adult flies lay eggs on cattle hair, and hatched larvae burrow into the skin, migrating through the animal’s body before developing into cysts under the skin on the back. The “human bot fly” (Dermatobia hominis) is largely confined to tropical regions of Central and South America, meaning human cases in Wisconsin are typically related to travel to those endemic areas.
Identifying Bot Fly Activity
In animals, a common indicator of infestation is visible lumps or swellings, referred to as “warbles,” beneath the skin. These warbles often feature a small, central opening that allows the bot fly larva to breathe. For deer, behaviors like excessive snorting or head shaking can suggest the presence of nasal bots.
When accidental human infestation occurs, typically with Cuterebra species native to Wisconsin, the signs can resemble a boil. An individual might observe a painful, firm nodule on the skin, which may itch or discharge a yellowish or bloody fluid. Some people report a distinct sensation of movement under the skin as the larva develops.
The larva creates a breathing hole in the skin, and its body is covered with tiny spines that help it anchor itself. Attempting to self-extract a bot fly larva is not recommended.
Minimizing Exposure
For humans, wearing protective clothing when spending time in areas with dense vegetation or high insect activity can create a physical barrier. Using insect repellents on exposed skin can also deter flies. Avoid direct contact with wild animals, especially small mammals, as these are common hosts for Cuterebra species.
For animals, regular grooming and physical checks can help in the early detection of any suspicious lumps or skin lesions that might indicate a bot fly infestation. Avoiding areas with dense rodent populations can also reduce exposure for pets. In livestock management, veterinary care and the use of appropriate parasiticides can effectively prevent and control cattle grub infestations. If an infestation is suspected in a human or an animal, consulting a medical professional or veterinarian is the recommended course of action for proper diagnosis and safe removal.