Cuterebra, Bot Fly Larvae in the Skin

Here it is again, cuterebra (cute-a-ree-bra) season. This is the time of year when my personal favorite clinical entity rears it’s not so cute little head on dogs and cats and rabbits around town. The appeal of this little beastie to me is that it is quick and easy to diagnose, quick and easy to fix, and you get a suitably gross souvenir for your efforts. I am talking about a type of fly larva that makes a little nest for itself inside the skin of mammals while it is waiting to pupate and become an adult fly.

Usually the pet comes in with the complaint of some sort of sore or bump. On close inspection the bump, which is usually about ½ to 1 inch in diameter, it looks like a volcano with a perfectly round hole at the top and a little discharge coming out of the mouth. On still closer inspection the rear end of the larva can be seen moving up and down inside the hole. At this point I usually give the patient a local block at the mouth of the opening and enlarge the hole with a small incision with a scalpel blade. When the hole has been enlarged enough the larva can either be pulled out or squeezed out. It is important that the opening be enlarged before trying to remove the larva or the larva tends to squish inside out and its skin, or cuticle, remains in the hole. That foreign material causes lots of irritation and possible allergic reactions if left in there, and it is hard to fish it out once the larva is eviscerated. I find that the still-squirming larva is best showcased in a small glass tube, where it can be admired to varying degrees by the proud owners of the pets.

Adult cuterebra flies look a little like bees. Their preferred hosts are wild rabbits and squirrels, so they lay their eggs around the openings to rabbit burrows or in places where squirrels like to go. When a furry creature brushes past them the eggs stick to the coat and they are either ingested during grooming and migrate from the gut to the skin after hatching, or they hatch on the hair and burrow directly into the skin. The developing larvae live quite happily and quietly, causing very little discomfort or other problems on their host, until they start getting big enough to stimulate some tissue irritation. The discharge from the tissue irritation usually causes a pet to be brought to the vet, where the wiggly, white souvenir is soon procured. Occasionally a cuterebra gets lost during its migration to the skin and can cause problems in the organs it goes wandering through. Lost cuterebra are a very rare occurrence and can be diabolically hard to diagnose.

Now before you think smugly to yourself that there is no way your immaculately clean, pampered, indoor only pet would ever have such a disgusting problem, I will tell you that it seems to me (in a very unscientific personal survey) that many of the patients I see with cuterebra are tiny, immaculately clean, fuzzy little white dogs that never go outside. My crackpot theory to explain this phenomenon is that adult female flies get trapped inside the house and, even though it is not their preferred site, they have to lay their eggs somewhere. That makes the house pets the only suitable hosts available for the eggs, and thus the tiny, fuzzy little white dog with a volcano shaped worm house on its skin. Just remember that the idea of a larva in the skin may be revolting, but it doesn’t usually do any real damage or cause illness, and it will bring a smile to your veterinarian when you come in.