Bot flies are parasitic insects whose larval stage develops inside the skin or tissues of mammals, a condition known as myiasis. The answer to whether these flies exist in the United States is yes, though the species most often encountered by people and pets are different from the notorious human bot fly of Central and South America. These flies complete their life cycle by using a host, which can sometimes include domestic animals or humans. Understanding the specific types found here and their habits is crucial for prevention and safe management.
Identifying Bot Flies in North America
The most common bot fly species in the continental United States belong to the Cuterebra genus, often referred to as rodent or rabbit bot flies. These native insects are widespread, with species like Cuterebra fontinella occurring across most of the continental U.S. and southern Canada, primarily targeting small mammals such as mice, squirrels, and rabbits. When domestic animals like cats and dogs are infested, it is typically an accidental occurrence involving these local Cuterebra species.
The human bot fly, Dermatobia hominis, is non-native to the US mainland, thriving in tropical Central and South America. Cases of myiasis caused by this species in the US are overwhelmingly seen in travelers returning from endemic regions. Although extremely rare, there have been a few reports of localized Dermatobia hominis cases in the US without a history of international travel, suggesting a slight possibility of northward migration or temporary establishment.
The Bot Fly Life Cycle
Infestation begins when the female bot fly, which does not bite, deposits her eggs near the entrance of a host’s burrow or along animal runways. A single female can lay over 2,000 eggs, typically placing them on vegetation or substrate in late summer or early fall. The eggs hatch rapidly when triggered by the body heat of a passing mammal.
The tiny first-stage larvae then crawl onto the host and seek a point of entry, often using a natural body opening like the mouth, nose, or an existing wound. Once inside, the larva migrates through the host’s tissue before settling in a subcutaneous location, usually in the head, neck, or trunk region. Here, it develops into a larger larva, creating a distinctive swelling, known as a warble, which includes a small opening for breathing.
The larva remains within this warble for approximately one month. When fully mature, the larva enlarges the breathing hole and drops out to the ground, where it pupates in the soil. It will emerge several weeks later as an adult fly, completing the life cycle.
Symptoms of Infestation in Hosts
The most recognizable sign of an infestation is the presence of a firm, localized, and well-demarcated swelling, or warble, beneath the skin. This lump typically has a single, small, circular opening, which is the larva’s breathing pore, and may occasionally ooze a thick, purulent discharge. These lesions are most commonly detected in pets during the late summer and early fall months.
In non-natural hosts like dogs and cats, the larva may sometimes take an aberrant migratory path, leading to more serious symptoms. Larvae migrating to the head or nasal passages can cause severe inflammation, resulting in signs like excessive sneezing, respiratory distress, or localized pain. In rare instances, migration into the central nervous system can cause neurological issues, including seizures, tremors, or blindness.
The presence of the warble can also lead to secondary bacterial infection, which often causes more discomfort than the larva itself. Even after the larva exits, a deep abscess or skin infection can develop in the empty cyst, making prompt veterinary attention important for cleaning and aftercare.
Prevention and Safe Removal
Preventing exposure relies on minimizing contact with areas where the eggs are likely to be found, such as dense brush and areas with high rodent and rabbit activity. Keeping domestic pets indoors or closely supervised during the peak season of late summer and early fall significantly reduces the risk of infestation. Regularly checking pets for any unusual lumps or small wounds after outdoor activity allows for early detection.
If a warble is discovered, it is important to seek professional veterinary care for safe removal. Attempting to forcefully squeeze the larva out is highly discouraged because the larva possesses small, backward-pointing spines that anchor it to the host’s tissue. Rupturing the larva while it is still beneath the skin can release foreign proteins into the host’s system, potentially triggering a severe inflammatory reaction or even anaphylactic shock.
Veterinarians typically extract the larva manually using specialized instruments after slightly enlarging the breathing pore or using a technique that encourages the larva to emerge. In some cases, applying an occlusive substance like petroleum jelly over the breathing hole can prompt the larva to surface for air, allowing for easier, safer removal with forceps. Post-removal, the cavity is thoroughly cleaned and often treated with antibiotics to prevent secondary infection.