What Pet Owners Need to Know About the Cuterebra

The cuterebra parasite is most common in summer and fall.

Cuterebra, a genus of botfly.
First instar larva of Cuterebra. CDC/ Dr. George Healy

A Cuterebra is an opportunistic parasite found under the skin of small mammals. Most of the time, pet owners encounter these revolting creatures in the larval stage when they notice a new bump on a dog, cat, or rabbit. When you go to examine it, you realize that there's a maggot clearly visible inside your furry friend.

Spotted a Maggot? Don't Remove It 

The important thing to know is that you should not panic and you absolutely should not remove it yourself. Though it is not an emergency situation, this is a time when you need to see the vet. The Cuterebra needs to be removed in one piece in order to prevent infection or an allergic reaction.

The good news is that Cuterebras are rare and, unlike other maggot infestations, they typically only come in very small numbers, most often just a single larva. Veterinarians often mention that they see fewer than 10 cases in their office each year.

The season to watch for them is late summer and early fall, particularly if you live in a rural area with many nesting rabbits and rodents. This is when the Cuterebra fly lays her eggs and she prefers rodent burrows, which are also favorites for curious cats and dogs. If you allow your pet rabbit to play in the grass, keep an eye out for these flies and larvae as well.

How Does Cuterebra Get Inside Animals?

The Cuterebra fly (also known as botflies) is found throughout the Western hemisphere and multiple species are known to live in various parts of the United States. The parasite is the larval stage of the Cuterebra fly, who uses animal hosts to complete its life cycle. The adult files are large, about the size of a bumble bee, and do not feed on or bite animals.

The life cycle of the Cuterebra is important because it's essential to understanding how they invade small animals:

  1. The female Cuterebra will lay about five to 15 eggs in one spot, but she may lay thousands in one breeding cycle. The eggs are deposited around animal burrows and on plants, rocks, and other objects.
  2. The eggs stick onto an animal host as it passes by and they hatch in response to the animal's body heat.
  3. The hatched larvae enter the body through the mouth or nose during grooming or through an open wound in the animal.
  4. The larvae then migrate to specific areas on the body under the skin to make their home (called a warble). Quite often it is in the head or neck area, though it can be anywhere.
  5. The larvae make a small hole in the skin to breathe. This is when the parasite is usually discovered; a noticeable lump in the skin with a small hole. The tip of the larva will often be visible deep in the hole, which is admittedly disgusting and frightens the unsuspecting pet owner.
  6. Roughly 30 days later, the parasite exits the animal host. It pupates on the ground and becomes an adult fly.

What You Need to Do

It is very important that you do not squeeze the skin in hopes of getting the larva out. Also, do not run for the tweezers hoping to pull it out. Either of these can cause the larva to break apart and cause the host animal to have a chronic infection or an anaphylactic reaction.

If you suspect your pet has a Cuterebra parasite, contact your veterinarian as soon as possible. When you get to the vet, they will be able to remove the larva properly (it can be up to one-inch long at maturity). They may also flush the wound and give your pet an antibiotic to prevent a secondary infection.

To prevent another Cuterebra from infesting your pets, there is very little you can do beyond changing their habits. For instance, you might choose to keep dogs on a leash and cats inside or on a leash so they don't go near burrows again. Rabbits in hutches typically don't have problems, but it's good to be aware that their species is a prime target for this fly.

Otherwise, you can keep an eye out for the Cuterebra fly as summer comes to an end. The University of Minnesota has very detailed photos of the adult fly and pupa that you may want to look at. It has a very distinct look and because of its large size, it's hard to miss.