Botflies in Dogs (Cuterebra in Dogs)

What Are Botflies in Dogs?

Cuterebra is a type of botfly whose larvae embed themselves as parasites in a dog’s skin, eyes, upper respiratory tract, or central nervous system (brain/spinal cord). People also call them “warbles” or “wolf worms” when they are in the larval stage.

Botflies can be found in the United States, Canada, and Mexico. 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. The typical hosts for botflies are rabbits and rodents, but dogs can get warbles if they come across the larvae in the grass, especially if they are hunting these animals or are near their burrows.

Symptoms of Warbles in Dogs

Symptoms of warbles, or botflies in dogs, vary depending upon where the Cuterebra infestation occurs on your dog.

Symptoms of Cuterebra in a Dog’s Skin (Cutaneous Cuterebriasis)

• Swelling around the area of skin where the larva is (usually around the face or neck)

• Lethargy

• Lack of appetite

• Vomiting

In very rare cases, a severe systemic inflammatory condition called disseminated intravascular coagulopathy (DIC) develops as a result of warbles in dogs. This causes excessive bleeding or clots that cut off blood supply to vital organs.

Symptoms of Upper Respiratory Tract Infestation

• Sneezing

• Facial/nasal swelling

• Nasal discharge

• Coughing

• Increased breathing rate

• Difficulty breathing

Symptoms of Ophthalmic (Eye) Infestation

• Conjunctivitis (pink eye)

• Eye discharge

• Eyelid swelling

• Inflammation of the inner eye

Symptoms of Central Nervous System (Brain and Spinal Cord) Infestation

• Circling

• Wobbliness (ataxia)

• Head pressing

• Head tilt

• Seizures

• Fever

• Low body temperature

• Paralysis

• Abnormal behavior

• Blindness

Causes of Botflies in Dogs

Cuterebra is caused by an infestation of larvae from the Cuterebra group of botflies. There are 34 species of Cuterebra in North America.

Botflies lay eggs on blades of grass or in nests, where they hatch. 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 or passing host.

The small maggots crawl around until they find an orifice where they can enter (mouth, ear, nose, etc.). Then, they migrate through various internal tissues and, ultimately, make their way into the skin.

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 host and form a pupa in the soil. Eventually, an adult botfly will emerge from the pupa and start the whole cycle again.

Dog Breeds That Are Prone to Cuterebra

Any breed of dog can become infested by Cuterebra. However, one study showed that 80% of affected dogs weighed less than 10 pounds, and 40% of affected dogs were Yorkshire Terriers (likely due to size and popularity of the breed).

How Vets Diagnose Botflies in Dogs

Your veterinarian will look for evidence of the fly larvae in cases of skin or respiratory tract infestation. If your dog has respiratory disease characterized by a sudden onset of symptoms or symptoms only affecting one side, such as nasal discharge and facial swelling, your vet will evaluate them for botfly larvae within the throat, mouth, or nasal passages. This is usually performed under general anesthesia.

In cases affecting the nervous system, CT scans, spinal taps, or MRIs are used to identify evidence of botfly larvae infestation in dogs.

Treatment for Botflies in Dogs

Treatment for skin, respiratory tract, and eye infestations of botflies in dogs includes manual removal of the larvae followed by cleaning of the wound. Your dog may also need antibiotics to treat secondary infection.

Ivermectin, an ingredient used in dog heartworm medications, is used to treat cases affecting a dog’s brain and spinal cord. This can stop symptoms from progressing but may not cure existing symptoms. Your veterinarian might use diphenhydramine (Benadryl) in conjunction with ivermectin to prevent an allergic reaction.

Recovery and Management of Cuterebra in Dogs

After botfly removal, monitor the area for discharge, swelling, or delayed healing. Cases of eye/brain infestation can take weeks to months to completely resolve.

Some dogs with cases of Cuterebra eye infestation do well after removal of the larva and getting anti-inflammatories, but others may develop blindness or glaucoma. Cuterebra skin infestations can result in systemic inflammatory response syndrome (SIRS) or DIC, but this is not common.