Papillomas are benign (non-cancerous) tumors that originate from the squamous (resembling the shape of scale of a fish) epithelial cells. Several types of papilloma have been described.

In puppies, papillomas may appear as tufts of cauliflower-like tissue growing on the lips and in the mouth. These papillomas are viral in nature and usually disappear in 2 to 3 months as the puppy's immune system matures. Occasionally, in severe cases difficulty swallowing and breathing may develop which will require surgical removal and electrocautery (a procedure involving burning a tissue with electrical current using a specially designed apparatus).

Warts, or singular papillomas are found on the skin of older dogs. If necessary, the diagnosis can be confirmed with cytology or surgical biopsy. If they are small and look like mushrooms on a stalk or just fingerlike, they are typically nothing to worry about. However, if they grow in size and ulcerate, they should be removed and biopsied to make sure they are cancerous, such as mast cell tumors. If they are black and are growing on the eyelids or lips, they should be removed and biopsied to rule out the possibility of malignant melanoma.

There are several variants of canine papillomavirus:

1. Canine Oral Papillomavirus (COPV)

Canine Oral Papillomavirus (COPV) causes Canine Oral Papillosis, the most common presentation of papilloma viruses. Canine viral papillomas are common tumors occurring in puppies and young dogs. These multiple tumors are contagious and the incubation period is about 30 days. Areas primarily affected are mouth (from lips to esophagus), nostrils, and on the eyelid and adjacent haired skin. The lesions begin as small, flat, greyish papules, and can grow rapidly into larger cauliflower-like masses. When the mouth is severely affected, chewing and swallowing is difficult. Canine viral papillomatosis can rarely transform to squamous cell carcinoma. Canine oral papillomavirus (COPV) typically regresses without treatment.

2. Canine Papillomavirus Variants

1. Canine exophytic cutaneous papillomatosis
2. Cutaneous inverted papilloma
3. Pigmented epidermal plaques

Exophytic papillomas (papillomas growing outward) are similar to common warts in humans. Most exophytic papillomas on haired skin are caused by a PV that is different from canine oral papillomavirus. Multiple or numerous lesions may have a broad base or positioned on a stalk. These are referred to as cutaneous papillomatosis. Lesions are typically less than 1 cm in diameter. Overgrowth of skin cells may be severe enough to form a cutaneous horn. Exophytic viral papillomas occur mainly on the face, ears and limbs, but usually do not involve the pads. Lesions can also be found on other parts of the body. Exophytic papillomas can occur at any age, but most often seen in dogs less than 2 years of age. Many but all exophytic papillomas regress without treatment within weeks or months. Transformation of exophytic papillomas into malignant tumors is extremely rare.

Cutaneous inverted papilloma is quite rare in dogs. Inverted papillomas usually appear as multiple, raised, firm masses that are less than 2 cm in diameter. Most of them occur on the abdomen, and, unlike exophytic papillomas, they do not regress spontaneously.

Inverted viral papillomas (papillomas growing inward) occur primarily in dogs less than 3 years of age. Whippets, Irish Setters, Beagles, Bernese Mountain Dogs, Cocker spaniels, Great Danes, and Kerry Blue terriers are reported to be predisposed to develop viral papillomas.

Pigmented epidermal plaques (PEP)

Pigmented epidermal plaques (PEP) have been reported as a condition in the Pug, Miniature Schnauzer, Chinese Shar-pei, and in individual cases of an English Setter and an English Pointer. Affected dogs have multiple cutaneous discolored spots in the skin (macules), pimples (papules), and plaques of heavily pigmented skin. The dark pigmentation in the lesions results from an abnormal distribution of melanin (dark skin pigment) within the outermost skin layer (epidermis).

Squamous cell carcinoma (SCC) may develop by malignant transformation from multiple plaques. This risk of malignant transformation emphasizes the need to recognize PEP and to control plaque progression. Antiviral and antineoplastic therapy, such as low-dose oral IFN-alpha, may be an effective component of the therapeutic management of both PEP and papillomavirus-associated cutaneous neoplasia.

SCC originates from squamous epithelial cells. SCC is common in dogs and cats and usually affects old animals. In cats, like in people, there is a strong correlation between the development of SCC and the exposure to ultraviolet light. Squamous cell carcinoma is typically seen around 10 years of age in large breed dogs, but any dog at any age can develop an oral tumor. Treatments options for dogs with squamous cell carcinoma include surgery, radiation, chemotherapy, and any combination thereof.