Cuterebra: A Creepier, Crawlier Side of Veterinary Medicine

Bambi, a 7 year female spayed Papillon dog, presented to Oswego Animal Hospital for evaluation of a swelling on her side that the owners noticed the day before. The owners were especially concerned because when they examined the swelling at home, they thought they saw something moving inside of it.

On presentation, Bambi’s vitals were stable and she had a red, raised, painful swelling 2 cm in diameter located on the left lateral aspect of her thorax. The swelling was draining blood tinged fluid through a small 4 mm draining tract present in the middle of the swelling.

The fur surrounding the swelling was shaved and a local anesthetic was injected into the skin to desensitize the area and allow thorough assessment of the swelling without causing pain to Bambi. Indeed, upon close examination there was something moving beneath the skin that could be visualized through the small opening in the swelling. This opening was enlarged slightly by a surgical incision and forceps were used to remove a 1 cm, cream colored larva from beneath the skin. The larva, though small, had created a large amount of damage in the tissue, leaving behind a hollowed out cavity that extended 2 cm. The cavity was flushed with sterile saline to remove the waste material the larva had left behind.

This case sounds like something out of a horror film- the question becomes where and how did Bambi get this bug? Unfortunately, cases like Bambi’s are not uncommon in the veterinary world. The culprit is a parasite called a Cuterebra, also known as the rabbit bot fly. Cuterebra most commonly affect wild rodents and rabbits. The female bot fly deposits her eggs near high traffic areas for rodents and rabbits including their nests, burrows, or along their normal traveling routes. The eggs then hatch in response to heat from passing animals releasing larvae. Once on a host, the larvae then migrate to the subcutaneous tissue where they develop and make a breathing pore to communicate with the air.

In Bambi’s case, the larva was fortunately not fully mature and remained small. However, mature larvae reach ~ 3 cm in size and are dark black in appearance and heavily armored with spines. Once the larvae grow to maturity in the animal host for approximately 30 days, they then exit through the breathing hole and pupate in the soil prior to hatching into the adult fly.

Despite Bambi being a mostly indoor dog, she was likely exposed to Cuterebra eggs outdoors near a rodent or rabbit den in the owner’s backyard. Even though the bot fly’s target hosts are rabbits and rodents, if a companion animal, such as a cat, dog or ferret, passes through these areas were the female bot fly has deposited her eggs, the eggs will hatch and the Cuterebra larvae can opportunistically affect our pets.

It is very important if you suspect your pet has Cuterebra that you do not attempt removal of the larvae by yourself. Professional veterinary removal is necessary to avoid traumatizing the larvae before removal. Ruptured or damaged larvae prior to or during removal can lead to secondary infection and in some cases an anaphylactic reaction, which can be life threatening. The best action to take if you suspect a Cuterebra is to have a veterinarian evaluate the wound and remove any larvae if present.

Following removal of Bambi’s Cuterebra larvae, she was sent home on anti-inflammatory pain medication and a 2 week course of antibiotics to prevent infection of the damaged tissue. Within several weeks the area had completely healed and Bambi today has made a full recovery without complications.