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Breed & Health Resources

Cuterebra Infestation

By Jacqueline Brister, CVM

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Cuterebra infestation is a parasitic infection in dogs and cats caused by Cuterebra fly larvae (i.e., immature worms or maggots). Cuterebra larvae are known by many names, including rodent bot fly worms, wolf worms, warbles, and wools. Typically, Cuterebra larvae infect rodents, especially squirrels and rabbits.

How Pets Become Infected

Adult Cuterebra flies lay eggs near a rabbit burrow or rodent den during the summer. Larvae hatch from the eggs when exposed to warmth from nearby animals. This can include unexpected hosts such as dogs or cats that sniff around or rest near burrows, nests, and dens. Once hatched, larvae attach to the animal’s coat and enter the pet through a body opening.

Each larva migrates and moves through the pet’s body to subcutaneous tissues (i.e., areas just beneath the skin). After 3-4 weeks, the area in which a bot fly larva rests becomes swollen and a hole develops in the skin to allow the larva to breathe. A larva will continue to grow under the skin and will eventually leave the pet through the breathing hole to finish developing into an adult bot fly.

Symptoms of Cuterebra Infestation

Swollen, mass-like area with an opening similar to a puncture wound (i.e., the breathing hole) is a common finding. Sometimes the swelling is red, painful, or itchy. If migration of the larva moves beyond subcutaneous tissues (i.e., not just under the skin), other symptoms may be seen. Migration to the eyes or nose can occur which can lead to swelling of one of the eyes or over the bridge of the nose. Sneezing, coughing, or increased tear production may be seen depending on where the bot fly larva has migrated.

Movement of the larva through the tissues can occasionally cause inflammatory reactions that can make the pet very sick. Laying around, not wanting to eat, trouble breathing, and/or inability or unwillingness to respond to the pet owner could occur in these cases.

Treatment for Cuterebra Infection

In most cases, veterinarians can remove the Cuterebra larva through its breathing hole. Sedation may be necessary to keep the pet comfortable and to enlarge the opening to remove the worm. Removal should not be attempted outside of a veterinary setting because injury to the worm can lead to anaphylactic reactions (i.e., severe allergic reactions), infections, and recurring swellings.

Bacterial infections in or around the larva’s breathing hole are common, so sometimes antibiotics are needed in addition to removing the worm. Anti-parasitic medications may be necessary if additional larva are suspected but cannot be removed. This type of medication may be required for weeks to months because Cuterebra larvae are difficult to kill.

How to Prevent Cuterebra Infestations

Little can be done to prevent rodent bot fly adults from laying eggs. Keep pets away from rabbit and rodent homes when possible, especially during summer months. If you suspect your pet has a Cuterebra infestation, call your veterinarian as soon as possible.

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