Botfly Invasion: Why Parasites Pop Out of Animals’ Skin

It’s fall, and that means squirrels are hurriedly securing food for the winter—and so is another creature, the parasitic botfly.

As forest animals scurry about transferring nuts from one hiding place to another, they’re also unknowingly carrying this parasite, which tunnels through squirrel flesh, causing swollen protrusions like the one pictured below.

A botfly is removed from an infected mouse. Photograph by Jill Wilson Bull

These inflamed tubules are called warbles, and about a quarter of U.S. squirrels have them, according to research conducted in southeastern Ohio. Mice, dogs, cats, livestock, and even humans can grow warbles. The open sores are caused by botflies, which in the U.S. lay their eggs in late summer through to the first frost.

Once inside the animal, the larvae eat their way out of the host: It’s like a much tamer version of the scene in Alien where the extraterrestrial bursts from a man’s body. (Watch a video of a botfly being pulled from a girl’s scalp.)

“[The botflies] essentially create this lesion because they need some easy way to get to the outside,” said Roger Applegate, an environmental biologist with the Tennessee Wildlife Resources Agency.

Basically what happens is squirrels come into contact with a botfly egg, which attaches itself to their fur, Applegate added. Once the egg hatches the larvae “enter the squirrel through its nose or mouth and migrate to a site under the skin.” That’s when they start to grow, and in the process create a cocoon of skin, which scientists have dubbed warbles.

“The sores look like a mosquito bite with an opening that’s large enough to see the worm,” Applegate said. “They’re feeding on your flesh and just getting bigger and bigger, working their way toward being mature.”

Though the wounds may look fatal, botflies usually don’t kill their hosts.

Beware the Botfly

Hairy insects that belong to the Oestridae family, botflies are found in different types worldwide.

Some of these buggers even live in horse intestines, stealing oxygen and nourishment from the animal before they make their way outward. Depending on the type of botfly, it can take weeks or even months for the wriggling parasites to finish growing and leave the body.

For botflies that invade squirrels and other North American critters, the larvae will pop out of the warble and fall to the ground, where it metamorphoses into a pupa—the stage right before adulthood. Once the bot fly reaches sexual maturity, it’s off in search of a new host to house its wormy offspring.

With winter fast approaching, the squirrel’s coat will start to thicken, and those botfly warbles will begin to heal. In colder climes, squirrels are generally safe from further infection—at least for a few months.

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Meet the Author
Mollie Bloudoff-Indelicato is a science journalist who loves em dashes, ’80s music and parasites. She has a master’s degree from the Columbia University Graduate School of Journalism with concentrations in science journalism, photography, and radio reporting. Contact her at, and follow her on Twitter at @mbloudoff.