There are several distinct types of cutaneous (skin) and subcutaneous (beneath the skin) cysts that can be seen in domestic dogs. Typically, cysts are identifiable visually and by physically feeling them on palpation. Dermoid cysts can be distinguished from follicular and infundibular cysts in several ways. A procedure called a fine-needle aspirate can be performed to obtain a tiny sample of the internal cyst material. This involves inserting a sterile needle into the center of the cyst and extracting a sample through an attached syringe. The sample is then expressed onto a glass slide and examined under a microscope.
A more definitive diagnosis can be made by performing an excisional biopsy. This procedure involves removing the entire cyst and a bit of surrounding tissue surgically and submitting it to a pathology laboratory for microscopic evaluation through a process called histopathology. The histopathology results will determine the specific type of cyst and the source of tissue that it originated from. They also will enable the veterinarian to rule out skin cancer as a potential cause of the mass. Dermoid cysts are the only cysts that, when cut open, actually contain hair follicles and shafts on the inside.
If a dog’s owner is concerned enough about a skin mass that she consults with a veterinarian, it probably is best to have the mass removed surgically, if for no other reason than cosmetic appearance.
Owners should never squeeze any cyst or other lump on or under their dog’s skin in an attempt to evacuate its contents (“pop it”). This can greatly increase the risk of secondary infection and inflammation. It also can cause the dog to suffer unnecessary discomfort and pain. Moreover, the dog’s immune system may react to the ruptured cyst as if it were a foreign body, such as a sticker or thorn.