A University of Georgia Extension Service colleague forwarded this query about a mysterious squirrel disease:

"I live in Midtown (a neighborhood in Atlanta), and some of the squirrels that frequent my bird feeder have big and small lumps on their backs, throats and bellies. I've seen both male and female squirrels with these thick, tumor- looking lumps. ... It's disconcerting because there are several like this, and I wonder if they're breeding and spreading this disease, whatever it might be. Secondarily, I worry about my cats which sit in my windows. Anyway, should I be concerned? I actually like squirrels, and I hate to see the little guys looking so pitiful."

Those "tumor-looking lumps" on the squirrels are almost certainly the larvae of Cuterebra emasculator -- a bot fly.

Sorry, no 200 dpi image available Photo courtesy University of Florida < small>http://gnv.ifas.ufl.edu/~fsj/botfly/cutrwrb/cutrwrb1 .htm

Of the array of parasites that squirrels endure, Cuterebra produces the most questions. Usually it's squirrel hunters who notice these lumps and the stout, black maggots that inhabit them.

The hunters often call them "warbles or wolves." They ask what they are and wonder if it is OK to eat the meat of such squirrels. (The meat is OK, and when the squirrel is skinned the parasites come off with the hide.)

The Cuterebra has an interesting life cycle. The adult fly lays its eggs on the squirrel or at a site squirrels frequent. Other Cuterebra species may infest rabbits or other species of rodents.

After the maggot, or larva, hatches, it crawls onto the squirrel and enters the flesh, possibly through the nose. Then it burrows to a place, often on the neck or back, where it grows to a size that can be seen.

The feeding end of the larva penetrates into skin, and the rear end is exposed to the outside. Spiracles in the maggot's rear end are like portholes. They are the exposed ends of the air tubes through which the maggot breathes.

Can these flies infest the cats or humans? It's extremely unlikely. The regular hosts of Cuterebra emasculator are gray squirrels and chipmunks. It's pretty hard for a parasite to change its evolutionary genetic program and parasitize something new.

However, Curtis Sabrosky, an authority on members of the Cuterebra group, did report in his 1986 book that a few human infestations have occurred.

So what should we do about this "problem"? Mostly nothing, in my view. Just watch the squirrels and enjoy knowing more about their life and times.

Soon, as cold weather approaches, the larvae will mature. When their days as a maggot are done, they will withdraw their mouthparts from the squirrel and worm their way out of the little sack the squirrel has grown around them. Then they will drop to the ground to pupate. Next year, the adult flies will emerge to repeat the cycle.

Are these parasites "bad"? No. Like a lot of things in the natural world, they just are.