Cuterebriasis: Alien Invasion?

Renee DiPietro, CVT
Veterinary Information Specialist 

What is Cuterebriasis?

You are petting or grooming your pet and suddenly you discover a prominent lump (often in the head or neck area) that seems to be a recent occurrence. You part the hair to get a better look and you see a lump or pouch with what looks like an eye looking at you. What is this?? It can be rather alarming at first.

Resist the urge to palpate it vigorously as damage to the creature (Yes, I said creature) inside could be dangerous for your pet.

What you have found on your pet is likely a Cuterebra.

A Cuterebra (also known as a Rodent, or Rabbit Bot) is the larval stage of the bot fly. Approximately 26 species of this insect family inhabit North America. The Cuterebra is parasitic to mammals. It is black/brown/grey in color, may be ridged, and may have spines. It can be seen through the hole in the warble. You may think an alien has attached its young to your pet for gestation. In a way that is what has occurred.

The larvae of the Bot Fly require migration through soft body tissue to complete its life cycle and emerge as an adult Bot Fly. To facilitate this process, the opportunistic adult female Bot Fly (looks like a large bee) lays her eggs where they can contact and adhere to a host animal. The eggs are often laid around the openings to rodent burrows or along other pathways that these animals travel. As the small mammal comes in and out of its home these eggs can stick to the coat. As the infected small mammal grooms itself it can also ingest the eggs passing them into the GI tract.

Body heat from the host animal stimulates the eggs to hatch and the larvae enter the body by penetrating the skin, entering the nose or eyes, or by ingestion. Once penetration has occurred the parasite will migrate and set up in a subcutaneous location. The larvae then creates a home for itself by forming a “warble”.  The warble is a pouch/swelling of skin (approximately 1 cm in size) with a hole on the end that allows the larvae to respire.

Hair matting around the lesion and discharge may be present. Cats may attempt to groom the area.

In most cases the larvae will reside in the warble until it has transformed into a maggot. This process can take 3-7 weeks. Once the larva has matured to the maggot stage it will emerge from the warble and fall off the host animal. Once on the ground is will find a place to overwinter and pupate until it emerges in the spring as an adult bot fly to start its life cycle again by laying eggs around small mammal burrows.

Dogs and cats will often attempt to pursue rodents into holes and for this reason cases of Cuterebriasis are often found as previously mentioned on the face and neck. Infection of ferrets living in outdoor housing is also possible. Cuterebra are most often found on animals during the warmer months of the year.

What to do?

Most of the time the presence of the parasite in it’s warble does not have serious health impacts for the pet. Minor pain, discharge, and secondary dermal infections are possible sequalae.

In some instances though, Cuterebra larvae can migrate inward through the tissues causing more serious health issues.

In rare cases in cats, migration to the brain when the larvae enter through the nose can occur.  This migration can result in Feline Ischemic Encephalopathy. This condition is caused by trauma caused to the brain tissue by larval movement through the tissues. The damage caused by the larvae’s migration can be both mechanical and chemical in nature. A chemical release from the parasite can initiate spasms in the middle cerebral artery.

Symptoms such as blindness, seizures, circling, and aggression can result. A history of breathing issues in the weeks before neurologic symptoms develop can be indicative of the larval migration through the nose. Consistent with the time frame that Cuterebra actively infect pets, this condition is only seen in the summer months in cats and does not infect indoor cats. This severity and duration of the condition is dependent upon the extent of damage caused by the larval migration.

Though the presence of a Cuterebra in a dermal warble is not often expected to cause significant health issues, removal of the parasite is recommended.

This is a procedure that must be done carefully by a veterinarian as crushing or rupturing the larvae can cause the release of substances that can cause a life- threatening anaphylactic response in the host mammal.

This is a minor but delicate surgical procedure. After removal the lesion is generally flushed and at times treated with topical antibiotics.

If you discover a warble on your pet, don’t panic. Simply get your pet to see your veterinarian as soon as is feasible so that appropriate and safe removal of the Cuterebra can be performed.

Please do not ever try to remove Cuterebra at home. This must be done by a veterinarian.

You can reduce the risk of parasitism by the Bot fly by not leaving your pets outside in the summer months for extended periods. Keeping them confined in a yard where they have no access to small mammal burrows can also be helpful.

Enjoy the summer time with your pet, but remember there are always hazards and prevention is the best measure whenever possible.