Botflies (Maggots) in Dogs

Cuterebrosis in Dogs

Flies of the genus Cuterebra are found in the Americas, where they are obligatory parasites of rodents and rabbits. Called botflies, they proliferate by laying eggs on blades of grass, or in nests, where they hatch, releasing maggots that crawl onto the skin of the passing host. The small maggots enter a body orifice, migrate through various internal tissues, and ultimately make their way to the skin, where they establish themselves within a warble (a small lump in the skin). The mature maggots, which may be an inch long, then drop out of the rodent or rabbit host and pupate in the soil.

Dogs become infected with a botfly larva when they come into contact with a blade of grass that has a maggot on it. The dog's movement against the blade of grass stimulates the maggot to crawl onto the dog. The maggot then crawls around on the dog until it finds an orifice in which to enter.

In the northern U.S. the disease is seasonal, with most cases occurring in late summer and early fall when the adult flies are active. Seasonality is less determined in areas with warmer temperatures, where flies are active through longer periods of the year.

The condition or disease described in this medical article can affect both dogs and cats. If you would like to learn more about how this disease affects cats, please visit this page in the PetMD health library.

Symptoms and Types

Cuterebra infection may be presented by warbles below the surface of the skin, or the dog may show signs associated with the larvae migrating within their tissues. Symptoms may include respiratory signs, neurological signs, opthalmic (eye) lesions, or the aforementioned maggots under the skin.

Respiratory symptoms:

Cough
Fever
Shortness of breath

Neurological symptoms:

Dizziness
Circling
Paralysis
Blindness
Lying down

Opthalmic symptoms:

Lesions (caused by the larvae in the eyeball)

Skin symptoms:

Lump in the skin containing the maggot, also called a warble; there will be a raised opening in the lump so that maggot may breathe

Causes

The most likely places for your dog to acquire this parasite are in environments where the botfly flourishes: grassy areas where there are adequate populations of rodents and rabbits. Even dogs without access to the outdoors, such as newborn puppies, can be infected from larvae brought home on the mother's fur.

Diagnosis

Your veterinarian will want to consider the following conditions before positive diagnosis of a cuterebra infection is made. Respiratory symptoms will be evaluated for allergies, and for other possible parasites, like lungworms, or other migrating worms that use the respiratory tract as a passage. Conditions that might produce similar neurological symptoms, but are of graver consequence, will need to be ruled out before treatment is given for a cuterebra infection. These conditions include rabies, distemper, and heart worms. If your pet has lesions on the eye, there may be a more serious parasitic larval infestation, one that can lead to permanent blindness, that needs to be ruled out.

The clearest indication of a cuterebra infection is, of course, a warble under the skin, in which case your veterinarian will be able to quickly determine whether it is the botfly.

Treatment

If the maggot is at the end of its migratory stage and has settled into a spot on the body, such as under the skin, eyes, or nose, your veterinarian will be able to remove it safely. Manifestations of lung migration may be alleviated by corticosteroids. If the parasite has led to irreversible neurological damage the prognosis will be poor and euthanasia may be the only option.

Your veterinarian will probably prescribe a broad-spectrum anti-parasite medication, which should kill maggots still in the migrating stage. A corticosteroid treatment will be given before administering the medication. The anti-parasite medication can be administered either to alleviate the signs caused by maggots suspected of migrating in the lungs, or to kill larvae in other tissues, including the central nervous system.

Prevention

There does not seem to be any prolonged immunity to infestation; a dog can develop skin lesions several years in a row. Application of monthly heartworm preventives, flea development control products, or topical flea and tick treatments may either prevent the maggots from developing in the dog, or may kill the maggots before they have time to gain access to an orifice for entry.