A cuterebra is a type of botfly native to the Americas whose larvae burrow under the skin of animals, most commonly rabbits, rodents, dogs, and cats. The adult fly is large and beelike, roughly 30 mm long, and doesn’t bite or feed at all. It’s the larval stage that causes problems: thick, grub-like maggots that grow up to an inch long inside a pocket of swollen tissue just beneath the skin. If you’re reading this, you probably found a strange lump with a small hole on your pet, and that’s the classic sign.
How Animals Get Infected
Cuterebra flies don’t attack animals directly. Instead, the female lays her eggs near the entrance of rodent burrows, rabbit dens, or along paths where small mammals travel. Those eggs sit and wait. When a warm-blooded animal passes close enough, body heat triggers the eggs to hatch almost instantly, within about a week of being laid. The tiny, moist larva sticks to the animal’s fur and works its way into the body through a natural opening: the mouth, nose, or occasionally an open wound. Cats and dogs typically pick them up while sniffing around rodent holes or brushing past vegetation in areas where wild animals live.
Once inside, the larva migrates through tissue until it settles just beneath the skin, usually around the head, neck, or chest. There it creates a small breathing hole called a “warble pore” and begins to grow, feeding on fluid and tissue for several weeks before dropping out to pupate in the soil and eventually emerge as an adult fly.
What a Cuterebra Warble Looks Like
The most obvious sign is a firm, round lump under the skin with a tiny opening at the center. You might notice a dark spot or slight movement inside the hole. The surrounding skin is usually swollen and may ooze a small amount of fluid. In cats, these lumps commonly appear around the neck, face, or under the front legs. Dogs tend to show them in similar areas. The lump grows noticeably over days as the larva develops.
If you part the fur and look closely at the hole, you may be able to see the larva’s breathing apparatus, two small dark spiracles, just inside the opening. The larva itself is cream-colored to brown and covered in rows of small spines. Fully grown, it can reach 22 to 27 mm long, roughly the size of a large grape.
When Larvae Migrate to Dangerous Places
In most cases, a cuterebra larva stays under the skin and causes only a localized infection. But occasionally, especially in cats, the larva takes a wrong turn during its initial migration and ends up somewhere far more dangerous. Because larvae commonly enter through the nose, they can travel along the nasal passages toward the brain. A study of ten cats with cuterebra larvae in the nervous system found that the most common signs were progressive depression, blindness, and behavioral changes. The larvae were most often recovered near the olfactory structures and optic nerves, consistent with entry from the nasal cavity.
Researchers believe the damage comes from two sources: a toxic substance released by the parasite that spreads through the fluid surrounding the brain and spinal cord, and direct damage to blood vessels that can cause brain infarction (a type of stroke). This condition, sometimes called feline ischemic encephalopathy, is rare but serious.
Cuterebra larvae can also affect the eyes. Documented cases in cats include swelling around the eye socket, corneal ulcers, bulging of the eye, and in some instances a visible larva protruding from the tissue around the eye. Cats with eye or orbital involvement sometimes develop fever, lethargy, loss of appetite, and difficulty breathing or swallowing.
Why You Should Not Remove It Yourself
It’s tempting to squeeze a cuterebra warble the way you’d pop an abscess, but this is one of the worst things you can do. If the larva ruptures during removal, its body contents can trigger a severe allergic reaction in the host animal, potentially including anaphylaxis. Leftover larval fragments left in the wound also create a high risk of secondary infection and prolonged inflammation.
A veterinarian will typically enlarge the breathing hole slightly, then use forceps to extract the larva intact and in one piece. The cavity is flushed and the animal may be given antibiotics to prevent or treat any bacterial infection that developed while the larva was present. When the larva is removed whole, most animals recover quickly and the wound heals within a couple of weeks.
Peak Season and Geography
Cuterebra are found throughout North and Central America, with 72 recognized species in the genus. Infestations follow a clear seasonal pattern tied to the fly’s reproductive cycle. In studies of small mammals in Kansas, the infection window ran from June through December, with the highest rates in July and August. A smaller second peak appeared in October and November, suggesting the flies can reproduce in two distinct waves at temperate latitudes.
Outdoor cats and dogs that roam near wooded areas, fields, or anywhere rabbits and rodents are active face the highest risk during summer and early fall. Cats that hunt or explore rodent burrows are especially vulnerable. Indoor-only pets are almost never affected.
Can Cuterebra Infect Humans?
According to the CDC, cuterebra species primarily parasitize rodents and rabbits, and human infection is rare but has been reported. When it does occur, it typically presents as a single warble under the skin, similar to what you’d see in an animal. Cuterebra is distinct from the human botfly (Dermatobia hominis), which is found in Central and South America and more commonly infects people. Both belong to the same broader family but target different hosts.