02/17/2026
THE LUMP THAT BREATHES.
You are watching your bird feeder or walking the dog in the February woods when you spot a squirrel or a cottontail rabbit. It has a large, hairless lump on its shoulder or neck.
You assume it is a tumor, an abscess, or a sign of terminal cancer.
Look closer.
If you watch the center of the lump, you might see it move. You might even see it open.
That isn't a tumor. It is a passenger.
While the New World Screwworm is the tropical nightmare we fear (and thankfully eradicated from the U.S.), the North American Botfly (Cuterebra spp.) is the native reality currently living beneath the skin of our local wildlife.
The Myth of the "Death Sentence"
We recoil at the idea of a parasite eating living flesh. We assume the host is dying.
The Biological Reality: Unlike the Screwworm, which feeds indiscriminately and kills its host, the Botfly is a "Prudent Parasite."
It has no interest in killing the rabbit. If the host dies, the larvae die.
Therefore, the Botfly essentially "manages" its own home. It feeds only on tissue fluid and white blood cells, avoiding vital organs. It even secretes natural antibiotic compounds to keep the wound from getting infected, ensuring its landlord stays healthy enough to support it through the winter.
The Scientific Reality: The Warble
What you are seeing is scientifically known as a Warble.
The Breathing Pore: The larva is currently living in a subcutaneous pocket. To survive without suffocating, it maintains a small, open hole (pore) through the host's skin. It backs up to this hole to breathe through its posterior spiracles.
The "Wolf" Worm: The larva inside is huge—often an inch long, dark, and covered in spines. It anchors itself into the flesh so it cannot be easily dislodged by the host’s scratching.
What is Happening Right Now (February)
The Maturation Phase: Botfly eggs were laid last summer, but the larvae develop slowly over winter inside the host. Right now, in February, these larvae are reaching their final "instar" (growth stage). They are at their largest size, making the lumps highly visible on squirrels and rabbits this month.
The Drop: Within the next few weeks (as the ground thaws), these larvae will back out of the hole, drop to the ground, and burrow into the soil to pupate. They will emerge as large, bee-like flies in late spring.
Why This Matters Ecologically
While disturbing to look at, Cuterebra play a role in regulating wildlife populations, though rarely by killing adults. They tax the energy of the host, which can make them more vulnerable to predators like foxes and hawks—a subtle thumb on the scale of the food web.
Practical Action: The "Hands-Off" Rule
If you see a lump on a wild animal or your outdoor pet:
Do Not Squeeze (Wildlife): Never try to squeeze the lump on a wild animal. Rupturing the larva can cause anaphylactic shock in the host, killing it instantly. Nature has a timeline; the larva will leave on its own, and the wound usually heals rapidly afterward.
Check Your Pets: Outdoor cats and dogs can pick up Botflies (usually by investigating rabbit burrows). If you find a breathing lump on your pet, do not remove it yourself. The larva has backward-facing spines that anchor it. Pulling it can leave parts behind, causing severe infection. A vet can remove it safely and cleanly.
Know the Difference: In the U.S., this is almost certainly a Botfly (one lump), not Screwworm (a gaping, festering wound with hundreds of maggots). Screwworm is an emergency reportable disease; Botfly is a gross but natural occurrence.
The Verdict
It looks like a tragedy, but it is actually a truce.
The parasite takes a meal; the host keeps its life.
It is a lump that breathes—and soon, it will leave.
Scientific References & Evidence
Parasitology: Slansky, F. (2007). "Insect/Mammal Associations: Effects of Cuterebrid Bot Fly Parasites on Their Hosts." (Details the "prudent parasite" strategy and lack of mortality in adult hosts).
Life Cycle: Catts, E. P. (1982). "Biology of New World Bot Flies." (Documents the overwintering timeline and spring emergence of Cuterebra).
Veterinary Medicine: Bowman, D. D. (2009). Georgis' Parasitology for Veterinarians. (Protocols for extraction and risks of anaphylaxis from crushing larvae).
