Has your dog or cat suddenly developed a nickel to quarter sized lump along the trunk (side)? Is your pet irritating the site further or excessively grooming the site? If the cyst contains a circular hole in it, you may be dealing with a parasite called Cuterebra or the warble worm.
A Cuterebra adult is actually a non-biting fly that resembles a bee. This fly is a prolific egg layer that deposits its eggs typically near rabbit or rodent burrows or trails frequented by these critters. Although Cuterebra typically infect rodents, pets may become exposed by brushing against eggs while outside near the burrow of a rodent. These eggs will hatch once they are exposed to the warm body temperatures of the host. The newly hatched larva then penetrate the mucus membranes of the mouth, nasal passages or an external wound.
The larva migrates under the skin typically on the head, neck or trunk finding a comfortable location where it continues to grow, developing a thick capsule or cyst around it. The smaller circular area is actually a breathing hole were the tip of the larva may occasionally be seen. There is often a discharge dispensed from the hole due to the severe inflammatory reaction caused by the larva. These lesions are most commonly seen in the summer and early fall. After a month on the pet host, the larva will emerge from the breathing hole and drop to the ground where it undergoes further development (pupate) before becoming an adult fly.
Treatment for cuterebriasis involves removal of the larva, flushing of the wound with antiseptics and the administration of systemic antibiotics. Removal of the larva is best conducted by a veterinarian. If the larva is not removed; intact serious allergic reactions, anaphylaxis and infection my result. With large Cuterebra cysts, the larva may remain elusive within the cyst and occasionally may be multiple in number.
Kahn, Cynthia Editor. The Merck Veterinary Manual. 9th Edition. Merck and Co. Inc. 2005. P. 710.