Botflies: Cuterebra in Pets and the Human Botfly Explained
A botfly is a large, bee-like fly whose larvae grow inside living mammals; in the US the one people meet is Cuterebra, the rodent and rabbit bot fly, which turns up as a lump with a breathing hole in cats, dogs, rabbits and squirrels in late summer. The human botfly, Dermatobia hominis, lives only from Mexico to Argentina, and US cases come home with travellers. Botflies do not bite or sting. A larva in a pet is a job for a vet and in a person for a doctor: do not squeeze it.
Botflies have a fearsome reputation built mostly on travel videos. The US reality is narrower: one genus that parasitises pets now and then, two livestock groups that farm medicine has largely brought under control, and a tropical species you can only bring back in your skin.
Which botfly is it?
Botflies are flies in the family Oestridae. The Merck Veterinary Manual and Washington State University list the groups that matter in North America:
| Rodent and rabbit bot fly | Human bot fly | Horse bot flies | Cattle grubs (heel flies) | |
|---|---|---|---|---|
| Genus | Cuterebra | Dermatobia hominis | Gasterophilus | Hypoderma |
| Native to the US? | Yes, throughout North America | No: Mexico to Paraguay and NE Argentina | Yes | Yes |
| Normal hosts | Mice, squirrels, chipmunks, rabbits | Cattle and dogs most often; also people | Horses and other equids | Cattle |
| Accidental hosts | Cats, dogs, ferrets, rarely people | Many mammals | Very rarely people | Occasionally horses, sheep, goats, people |
| Where the larva lives | Under the skin, with a breathing pore | Under the skin, with a breathing pore | Stomach and intestine | Migrates through body, then under skin of back |
| How it gets in | Eggs near burrows hatch on contact; larva enters mouth or nose | Eggs carried by a mosquito or tick | Eggs on leg hairs, licked off | Eggs on leg hairs, larva burrows in |
| Status today | Common in wildlife, occasional in pets | Imported travel cases | Routine target of horse deworming | Nearly clinically nonexistent in North America |
Compiled by The Pest Control Brief from the Merck Veterinary Manual, UF/IFAS EENY-440 and ENY-290, CDC DPDx, the Pennsylvania Game Commission, New Mexico State Guide B-705 and the University of Saskatchewan WCVM.
What is a botfly, and do botflies bite?
No. Merck says adult Cuterebra are large and beelike and do not feed or bite; UF/IFAS says adult Dermatobia do not feed; and horse bot adults lack working mouthparts, according to Saskatchewan’s veterinary college. The adult lives only to mate and lay eggs. UF/IFAS gives the human bot fly adult as 12 to 18 mm long, with a yellow face, metallic blue abdomen and orange legs, and gives adult cattle grubs as averaging 13 to 15 mm.
So what do botfly bites look like? There is no bite. What people photograph is the larva’s warble: a lump under the skin with a small central hole the larva breathes through. Merck describes the Cuterebra lesion in pets as a soft swelling about 1 cm across, often with matted hair over it. For marks that really are bites, see our bite identification guide.
How big are botfly larvae?
Bigger than most people expect. Merck gives Cuterebra first-stage larvae as 1 to 1.5 mm, second-stage as 5 to 15 mm, and third-stage as 3 to 4.5 cm, dark and covered in spines; the Pennsylvania Game Commission gives developed larvae as 20 to 42 mm long and 7 to 10 mm wide. The third stage is the one vets usually find.
Botflies in cats and dogs
Cuterebra lay eggs at burrow entrances, along rodent runways and on nearby stones and plants. Merck says a female deposits 5 to 15 eggs per site and more than 2,000 in her lifetime. Eggs hatch in response to the heat of a passing host, and in a pet the larva usually enters through the mouth or nose during grooming. It then settles under the skin, typically around the head, neck and trunk, and leaves after about 30 days to pupate in the soil.
Merck notes that free-roaming cats are more likely to be affected than indoor cats, and that Yorkshire terriers may be at higher risk. Signs to watch:
- A lump with a hole in it, sometimes oozing; the usual mistake is to call it an abscess.
- A patch of matted fur the cat keeps grooming.
- In cats, rarely, larvae wander into the brain or airway. Merck lists seizures, blindness and odd behaviour, usually between July and September, and says a bout of violent sneezing weeks or months earlier is a key clue.
How to get rid of botfly larvae in a pet
| Do | Do not |
|---|---|
| Book a vet visit as soon as you see a lump with a hole | Squeeze the lump |
| Let the vet widen the pore and lift the larva out whole | Try to pull it out at home; a broken larva leaves remnants |
| Follow the vet’s wound care; healing can take weeks | Put insecticide, spot-on or household spray into the hole |
| Go straight back if the site swells again | Assume a closed scab means it is all gone |
Merck explains why squeezing is the worst option: it can rupture the larva and cause a chronic foreign body reaction and infection, and there are anecdotal reports of rupture causing anaphylaxis. The larva should come out in one piece; recurrent abscesses at the site suggest pieces were left behind. Healing can take weeks to months. Keep household pesticides away from the wound too; our guide to pest control that is safe around pets and children covers why products must match the animal on the label.
When pets catch them
Merck says Cuterebra infestation of outdoor animals is most common in late summer and early autumn. Our iNaturalist data shows the adult flies on the same calendar.
The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026, found 1,427 US records of Cuterebra. Their share of everything recorded rises from June, peaks in August at 2.3 times its yearly average, and falls away after October. Prevention follows from that: in July to October, keep cats indoors and keep dogs from nosing into rabbit nests and rodent burrows.
Where do botflies live? Are botflies in the US?
Yes, but not the one in the viral videos. Cuterebra species live throughout North America, and the Pennsylvania Game Commission says they have turned up in deer, foxes, mink, cattle, pigs, mules, cats, dogs and humans. Horse bots and cattle grubs are native too. The human bot fly is not: UF/IFAS gives its range as Mexico to Paraguay and northeast Argentina.
Squirrel hunters see Cuterebra most. The Pennsylvania Game Commission says warbles are unsightly but can be cut out, the meat is safe to eat if properly cooked, and there is no need to discard the animal. Squirrels and chipmunks around the house are the natural hosts; see our guide to chipmunks, groundhogs and ground squirrels.
Why are there botflies in my house?
Botflies cannot breed indoors. Every larva needs a living mammal, then soil to pupate in, so a botfly problem starts outside, usually with a pet roaming near burrows. A large, dark, spiny grub found indoors in late summer may be a mature larva that has left a pet to pupate; take it, and the pet, to the vet. Pale maggots in a bin are a different insect; see maggots in the trash can.
The human botfly, and botflies in people
Dermatobia’s trick is borrowed transport. UF/IFAS explains that the female catches a blood-feeding insect, usually a mosquito or tick, and glues her eggs to it. When that carrier bites a person or animal, the eggs hatch from the warmth and the larvae enter through the bite or a hair follicle. CDC says larvae feed in a cavity under the skin for 5 to 10 weeks; UF/IFAS gives 5 to 12.
US cases are travel cases. UF/IFAS cites seven confirmed cases in the US documented between 1999 and its 2008 publication, all in people recently back from Central or South America, and 33 cases imported into Japan from 1974 to 2005. For travellers, UF/IFAS advises insect repellent and protective clothing; our guide to what attracts mosquitoes covers bite avoidance.
Native Cuterebra infect people only rarely. A 1989 review in the Journal of the American Academy of Dermatology reported 17 new human cases and reviewed 54 more, finding that most occurred in children in the northeastern US or Pacific Northwest, usually as a single boil-like lump, with most lesions appearing in late August, September and October.
Do botfly larvae hurt?
Often, yes. UF/IFAS describes a raised lesion that becomes hard and sometimes painful, often seeping pus, and says patients may feel the larva move. CDC describes swellings on the body or scalp that may discharge and be painful. In pets, Merck says pain varies and usually comes from secondary infection.
How to get rid of botfly larvae in skin
See a doctor, and mention any recent travel. UF/IFAS says the most conventional removal is a simple surgical procedure under local anaesthetic, with healing in one to two weeks and little scarring. UF/IFAS also notes that backward-projecting spines anchor the larva in the wound, which is one reason not to try squeezing it out yourself.
Horse bots and cattle grubs
New Mexico State Guide B-705 says the common horse bot glues eggs to hairs mostly on the forelegs; the horse licks them, larvae enter the mouth, and they then feed attached to the stomach lining for about 10 months. Light burdens do little harm, but heavy ones can cause poor condition and colic. Saskatchewan’s veterinary college says treatment usually follows the first autumn frost, when the adults are dead; New Mexico State gives mid-October as the earliest date there. Ask your vet about timing and follow the dewormer label.
Cattle grubs earned the name heel flies because, UF/IFAS explains, cattle kick and stampede to escape egg-laying females. Merck says widespread use of macrocyclic lactone treatments has cut Hypoderma to an almost clinically nonexistent level in North America, though serology shows cattle are still exposed.
Is a botfly dangerous?
For people in the US, rarely. Cuterebra infections in humans are uncommon, and the Pennsylvania Game Commission states these bots are of no public health significance. The real risks are an untreated infection around a larva, a ruptured larva in a pet, or a cat whose larva has migrated into the brain. CDC notes death from human bot fly is rare, though larvae have entered the brain in children. For the other big biting flies of summer, which do bite, see horse flies and deer flies, and for adult fly lifespans see how long flies live.
Frequently asked questions
Are botfly larvae dangerous?
Usually not life-threatening, but they need proper removal. In pets, Merck warns that squeezing can rupture the larva, causing chronic inflammation and infection, with anecdotal reports of anaphylaxis; larvae that migrate into a cat’s brain can cause seizures or blindness. In people, US Cuterebra cases are rare, and the human bot fly is a travel infection. See a vet or doctor rather than removing it yourself.
What is a botfly?
A botfly is a fly in the family Oestridae whose larvae develop inside living mammals. Adults are large, hairy and bee-like and do not feed or bite. In the US the common one is Cuterebra, which parasitises rodents and rabbits and occasionally cats and dogs. Horse bots and cattle grubs are related. The human botfly lives in Central and South America.
How to get rid of botfly larvae?
Do not squeeze. For a pet, a vet widens the breathing pore and removes the larva whole, then cleans the wound, which Merck says can take weeks to heal. For a person, a doctor usually removes it with a simple procedure under local anaesthetic, according to UF/IFAS. Household insecticides have no place in the wound.
Do botfly larvae hurt?
They can. UF/IFAS describes the lump in people as raised, hard and sometimes painful, often seeping pus, and some patients feel the larva moving. CDC also describes painful swellings. In pets, Merck says pain is variable and is mostly linked to secondary infection, so a cat may only show matted fur and a lump with a small hole.
What do botfly bites look like?
Botflies do not bite, so there is no bite mark. What you see is the warble: a swelling under the skin with a small central breathing hole, sometimes oozing fluid. In pets Merck describes it as about 1 cm across, often under matted fur. In people a 1989 review describes a single boil-like lump, which UF/IFAS says often seeps pus.
Sources
- Merck Veterinary Manual, Cuterebra Infestation in Small Animals, K. A. Moriello, full review February 2025.
- Merck Veterinary Manual, Hypoderma spp (cattle grubs), R. Panadero-Fontán, September 2022.
- UF/IFAS Featured Creatures EENY-440, Human Bot Fly, Dermatobia hominis, C. R. Connelly and S. K. Hill, 2008, reviewed December 2024.
- UF/IFAS Extension ENY-290, Cattle Grub Management, P. E. Kaufman and E. N. I. Weeks, 2013.
- CDC DPDx, Myiasis.
- Baird J. K., Baird C. R. and Sabrosky C. W., North American cuterebrid myiasis: report of seventeen new infections of human beings and review of the disease, Journal of the American Academy of Dermatology 21(4), 1989.
- Pennsylvania Game Commission, Bots and Warbles.
- Washington State University PestSense, Bot Flies (Warbles).
- New Mexico State University Cooperative Extension Guide B-705, Horse Bots.
- University of Saskatchewan Western College of Veterinary Medicine, Learn About Parasites: Gasterophilus.
- The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026.
