A very large portion of of my work deals with parasites, and over the years I've acquired experience with all sorts of invertebrate creatures whose lifestyles and biology sound like something out of a sci-fi horror franchise. Some of these are well known to everyone due to our intimate relationships with canine and feline cohabitants. Others are more obscure. Everyone knows that fleas can cause dermatitis and allergic reactions, but most people don't know the far more fascinating thing about fleas, namely that they are a vital host for one stage of the tapeworm life cycle, and if your dog has tapeworms he has eaten at least one infected flea. Similarly, most people know that mange is caused by skin mites, but hardly anyone knows that not only do all dogs carry these mites from birth, but all humans carry their own species of skin mites. Now there's a happy thought for you.

Working with these little beasties can literally make your skin crawl. There's a story about a vet who deliberately introduced ear mites into his own ears for the purpose of settling, once and for all, the debate on whether ear mites actually cause the host pain. Assuming this story isn't apocryphal, I say bless his scientific little heart, but I think he's absolutely bonkers, because the hairs on the back of my neck get all stiff and crawly every time I look into a microscope and see the little monsters at work. Ear mites are creepy, unsettling creatures. Among the common parasites of dogs and cats, only fly maggots can rival them for sheer ookiness.

Amongst the less common parasites, however, there is one creature that blows every other kind of mite and maggot away. It's called a cuterebra larva, and it's a monster that deserves its own horror franchise. While there are more dangerous parasites (see heartworm), and some that are harder to treat (see giardia), there are very few that are more disgusting than the cuterebra.


The cuterebra is a large kind of fly commonly known as a botfly. There are some 30 species of botflies in North America, and most of them are harmless in their adult forms, which look rather like bees but do not sting. As with many insect species, it's the larvae you have to watch out for.

Female botflies lay their eggs on moist dirt, leafy substrate or other surfaces near the dens or runways of their preferred hosts. Different species of cuterebra target different hosts, mostly rodents and rabbits. The eggs become attached to the host's fur or skin and hatch in response to the host's body heat. The larvae then migrate to openings such as nostrils, mouths, urethras and open wounds, or they can be ingested during the host's self-grooming.

Once ingested, they travel under the skin to their preferred sites, which vary by species and include various spots on the head, neck or trunk. Here's where it gets interesting for dog and cat owners: because these species are abnormal hosts for the cuterebra, the larva that finds itself inside a cat or dog will not travel to its usual development site, but will migrate more or less randomly and may end up inside the host's throat, eyes or brain. Aah, brains!

Once the larva reaches its chosen site, it forms a subcutaneous cyst and begins to develop into its second and third instars. In its third and final larval instar, it is a grub over half an inch long, covered with cuticular spines and a pair of mouth hooks that look absolutely vile, but do little damage to the host.

This third stage is where cuterebra are most commonly noticed, because by the third instar the bot is big enough to create a cyst, otherwise known as a warble4, that can be seen and felt easily. Also, the cyst has a small breathing hole that leaks fluid, which can cause matting of the host's fur. This fluid is very often the first thing a pet owner notices. The breathing hole can usually only be noticed when the surrounding fur is shaved away. Once this is done, the bot itself can sometimes be seen inside the hole, visibly moving around in its little hollow in the host's skin. Right about now, the vet will be excitedly calling all of her staff into the exam room to have a look at the critter, while the host's owner runs for the door trying not to throw up.


So, you've caught a glimpse of the bot's ugly backside, and if you're still conscious you want to grab a tool and pull that bastard out, right? Wrong. The bot can actually get out through that breathing hole (and if you let it mature in there, that's exactly what it will eventually do), but removing it by force is a very bad idea. Extreme care must be taken not to fold, spindle or mutilate the bot, as this will cause it to leak fluids that are almost guaranteed to cause an infection, and pulling it out without rupturing it is almost impossible because of its spines. If infection isn't alarming enough to deter you, there are also reports of hosts going into anaphylactic shock when bots were improperly removed.

What I'm saying here is do not, under any circumstances, no matter how hardcore you are, attempt to do this at home, and I don't care if your name is Ellen Ripley. Make an appointment with your vet. The vet will sedate the host animal and carefully enlarge the opening around the cyst until the bot can be removed without trauma. The cyst should then be flushed and treated with antibiotics. From that point on it can be treated like any other cyst.

Unless there is more than one cuterebra! Unfortunately, this is a very real possibility, and there's no way of knowing until the next cyst gets big enough to notice. It's not unusual to see squirrels and wild rabbits with half a dozen warbles around their throats. However, it's important to note that the grub itself is not producing more cuterebra inside your pet. The larva of the cuterebra cannot reproduce, and the only way to pick up more of them is to pick up more eggs laid by an adult cuterebra fly. I suggest that at this point you clean up your pet's lair (kennel, crate, hutch, what-have-you) thoroughly.


Yes, Virginia, humans can be infected by cuterebra as well. And, like dogs and cats, we are not ideal hosts, so the larvae tend to develop in abnormal parts of our bodies. There has been at least one documented case where a cuterebra larva achieved full development inside a human respiratory system.5

I'd write more, but there's something in my throat.

    Much of this is from personal experience, but here are some more sources and further reading:
  1. Cuterebra Infestation in Small Animals: Introduction:
  2. Cuterebra in Dogs, Cats and Ferrets:
  3. Tree Squirrel Bot Fly:
  4. Warbles:
  5. Tracheopulmonary Myiasis Caused by a Mature Third-Instar Cuterebra Larva: Case Report and Review:

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