Cuterebriasis in Dogs and Cats

By Kate O’Hara, BVetMed
angell.org/generalmedicine
generalmedicine@angell.org
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August 2025

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Cuterebriasis is a parasitic infection caused by the larvae of Cuterebra bot flies.  Cuterebra are natural parasites of rodents and rabbits. Although they do not feed on their hosts, their lifecycle requires an obligatory period within the host tissue for larval development.

Adult Cuterebra bot flies lay their eggs on vegetation or material in or around animal burrows and paths.  The warmth of a nearby host animal causes the eggs to hatch, and the first-instar larva moves onto the passing host animal. The larvae enter the host’s body through natural openings — most commonly the mouth or nostrils. The larvae do not penetrate the skin, but can enter through existing wounds or breaks in the skin.

In the natural rodent or rabbit host, the larvae migrate to the back of the throat and then through the chest and abdomen, ultimately ending up in the tissue under the skin. The larvae will create a cyst in this subcutaneous tissue with an open breathing hole through the skin. They grow rapidly, and about three to six weeks after the initial infection, the larva exits the skin through a breathing hole and then burrows in the soil to pupate and complete its lifecycle.

Although dogs and cats are not natural hosts of bot flies, they can become less common “accidental” hosts if they spend time near rabbit or rodent dens. First-, second, or third-stage instars can be found in cat and dog hosts. Although Cuterebra can fully mature within cat and dog hosts, the larvae are typically unable to mature into adult flies.

In their natural rodent and rabbit hosts, Cuterebra do not usually cause significant disease, except for the skin lesions that they produce. In “accidental” dog and cat hosts, Cuterebra larvae can still migrate to the subcutaneous tissue, but aberrant migrations can also involve the brain, nasal and respiratory passages, and eyes.  Clinical signs in the affected animals will depend on where the larvae have migrated.

The most common problem associated with Cuterebra is subcutaneous infestation. The skin lesions are most often found around the head, neck, and trunk. The encysted larvae will cause a swelling under the skin, characterized by a well-demarcated breathing hole on the surface, which may drain some material.  The first signs an owner may notice are often matted fur over the area and the dog or cat grooming the spot excessively. The Cuterebra lesion may cause varying degrees of discomfort and irritation in the animal, which can be exacerbated by secondary infection.

Although less common, migrations to other areas of the body can cause more significant disease and clinical signs. Larvae migrating into or around the eyes can cause inflammation in these areas, retinal degeneration, and potentially lead to blindness. Inflammation caused by migration through the respiratory tract may lead to symptoms such as sneezing, nasal discharge, or more severe respiratory distress.   Cuterebra larvae have been found in the central nervous systems of dogs and cats and may present as lethargy and depression. Other neurologic manifestations include blindness, abnormal mentation, vertigo, and seizures. Unfortunately, neurologic Cuterebra is typically rapidly progressive and fatal. In a small number of animals, Cuterebra infection can trigger a secondary, life-threatening inflammatory cascade or an immune-mediated disease.

Diagnosis of subcutaneous Cuterebra is fairly straightforward and is based on the identification of the distinctive skin lesion. Often it is possible to see the larva moving below the breathing hole. Diagnosis of systemic manifestations of aberrant larval migrations can be more challenging. Diagnosis is based on a history of the animal’s lifestyle and symptoms, as well as blood work and often more advanced imaging.

Treatment for subcutaneous Cuterebra involves the extraction of the larvae. For larger larvae, this can be done by slightly enlarging the breathing hole and gently grasping the larva with forceps. It is important to carefully extract the Cuterebra, as rupturing the larva can lead to a more significant inflammatory reaction in the host. For less developed larvae, more dissection may be needed to facilitate extraction. After removal of the Cuterebra, the cyst is typically flushed, and the host may also be treated with a course of antibiotics to prevent secondary infection. The residual skin lesion is left to fill in and heal by second intention. Animals treated for subcutaneous Cuterebra typically make a complete recovery.

Removal of aberrant larval migrans in deeper soft tissues, the eyes, or the nervous system may require complete surgical removal; however, this is not always possible. Systemic antiparasitic medication such as ivermectin can be used to treat migrating Cuterebra larvae in some cases. However, the prognosis for animals with neurologic disease is more guarded. At times, this medication may also be used in combination with steroid medication to treat associated inflammation.

Prevention of Cuterebriasis in cats and dogs focuses on avoiding rodent and rabbit dens and controlling populations of those natural hosts. To my knowledge, there are no products labeled for the prevention of Cuterebra in cats and dogs. However, keeping pets on topical parasite preventives (such as fipronil or imidocloprid) may help. Additionally, keeping pets on monthly medications, frequently used for Heartworm prevention (macrocyclic lactones), may also help control migrating larvae.