Cuterebra, often called the New World Skin Botfly or rodent botfly, is a genus of flies whose larval stage causes cutaneous myiasis in mammals. The resulting condition in humans, known as cuterebriasis, is rare and accidental, as the primary natural hosts are small mammals like rodents and rabbits. Human cases are considered incidental spills from the parasite’s normal lifecycle, usually occurring when people inadvertently cross paths with the fly’s reproductive cycle. The condition is generally treatable.
The Cuterebra Lifecycle and Transmission
The Cuterebra lifecycle progresses through four stages: egg, larva, pupa, and adult fly. The adult botfly is a large, non-biting insect that does not feed, focusing its short lifespan entirely on reproduction. Female flies lay their eggs in clusters, often near the entrances of rodent or rabbit burrows, along animal paths, or on nearby vegetation, typically during the late summer and early autumn months.
The eggs hatch when they sense the body heat of a passing host. The first-stage larvae then adhere to the host’s fur or skin. In natural hosts, these larvae typically enter the body through the mouth or nose during grooming, or through other natural openings.
Human transmission occurs when an individual accidentally contacts contaminated environments, such as brush or soil near a rodent den. The warmth and moisture on human skin can trigger the eggs to hatch, and the larvae will then enter the body. Larvae can penetrate the skin through a hair follicle, a small wet laceration, or via mucosal surfaces like the eyes, nose, or mouth. Once inside, the larva migrates to a subcutaneous location where it begins to develop, forming a characteristic swelling.
Recognizing the Signs of Infestation
The initial sign of a Cuterebra infestation is often a small, red, itchy bump that may be mistaken for a common insect bite. As the larva grows, it creates a larger, firm, and painful lump beneath the skin, known as a furuncle or “warble.” This lesion gradually enlarges over several weeks, typically reaching its full size when the larva has matured to its third stage.
A distinguishing feature of the lesion is the presence of a central pore, called a punctum, which the larva creates for respiration. This pore may occasionally reveal the posterior end of the organism. A serous, bloody, or slightly pus-like discharge may ooze from this opening. Some individuals report feeling a sensation of movement or a subtle, stabbing pain, especially at night, as the larva shifts within its cavity.
While most cases involve the skin, the migrating larva can sometimes settle in less common, more dangerous locations. Infestation of the eye (ocular myiasis) or the pharynx (pharyngeal myiasis) are rare but serious presentations that require immediate medical attention. The presence of fever or significant pain is uncommon with a simple subcutaneous lesion and may indicate a secondary bacterial infection.
Safe and Effective Larva Removal
If a Cuterebra lesion is suspected, seek professional medical care rather than attempting self-removal. Squeezing the swelling or trying to extract the larva at home is discouraged because the larva is covered in backward-pointing spines that anchor it firmly in place. Rupturing the larva during improper removal releases foreign proteins into the host’s tissue, which can trigger a severe inflammatory reaction, including anaphylactic shock.
The preferred medical treatment is surgical excision. Under local anesthesia, the physician makes a small incision to enlarge the punctum or excise the entire lesion, allowing the larva to be removed intact with specialized forceps. Complete removal in one piece is necessary to prevent a foreign body reaction or subsequent infection from retained larval fragments.
A less invasive method involves occlusion, where a substance like petroleum jelly or adhesive tape is applied over the breathing pore. This blocks the larva’s air supply, forcing it to emerge partially or fully in search of oxygen. While sometimes effective, this method is slower and carries a higher risk of incomplete extraction compared to a controlled surgical procedure. Following removal, the resulting cavity must be thoroughly cleaned and flushed with a sterile solution. Prophylactic antibiotics may be prescribed to prevent a secondary bacterial infection, and the patient’s tetanus status is often checked.
Minimizing Exposure and Risk
Prevention of Cuterebra infestation centers on avoiding areas where the adult flies lay their eggs, particularly during the late summer and early autumn when flies are most active. Since the flies target the habitats of small mammals, contact with rodent or rabbit nests and burrows should be avoided, especially in wooded, brushy, or rural environments.
When working or recreating outdoors in high-risk areas, wearing long sleeves and pants can provide a physical barrier against the larvae. Homeowners should ensure that window and door screens are intact to prevent adult flies from entering and laying eggs near human habitation. Regular checks of pets that spend time outdoors are also advised, as they can sometimes bring the larvae into close proximity with humans.