What is a papilloma of the skin? 
Papillomas are benign, sometimes multiple, tumors caused by viruses. They are commonly known as warts. The tumors often disappear spontaneously because the animal slowly develops immunity to them. Some papillomas may need surgical removal if they become inflamed or infected or fail to regress over time.
Papillomas are uncommon in cats and common in dogs.
What causes papillomas?
After invading the cells of the dog or cat (the host), papillomaviruses insert their genetic information into the host cell's DNA and upset the normal processes of cell division, so the cell divides abnormally and more frequently. The virus activates growth-promoting genes in the DNA (called oncogenes) and simultaneously inactivates suppressor genes that would normally limit cell proliferation and alters the genes that regulate normal, programmed cell death.
There are many types of papillomaviruses, and they occur in all species of animals, including people. Each species has its own viruses and their related tumors. One of the best-known are warts on human feet (plantar wart).
Why did my pet develop a papilloma tumor?
Your dog or cat has been infected with one of the papillomaviruses. All animals and people carry various viruses asymptomatically (without any clinical signs). However, pets with immature immune systems, such as young dogs and puppies, and those who are immunocompromised (have a reduced ability to fight infections), are more prone to developing papilloma warts.
"The virus is transmitted through direct contact with an infected pet or the pet’s environment."
Papillomaviruses are very rent to adverse conditions and can survive for long periods in the environment. The virus is transmitted through direct contact with an infected pet or the pet’s environment (e.g., toys, food and water bowls, bedding, etc.). It then gains access to the body through moist skin, cuts, abrasions, or an insect bite (e.g., flea, mosquito, tick).
What are the clinical signs of papillomas?
Papillomas may appear as multiple oral papillomas in young dogs, solitary cutaneous (skin) papillomas in dogs of any age, venereal (genital) papillomas, eyelid or conjunctival papillomas, and fibropapillomas (in tissues, such as muscle). Various viruses are associated with different sites and in young and old animals. The lesions are usually inflamed polyps (warts), but may be flat, have scaly plaques, or inward-growing hard masses. They may ulcerate (break open) and bleed. Papillomas that grow inward may cause pain, particularly if they are on the feet. In dogs, these tumors are most common on the feet or around and in the oral cavity.
In cats, papillomas are usually flat, plaque-like, and sometimes scaly. There may be one or more lesions, usually on the head, neck, or limbs. There is also a fibropapilloma, or sarcoid, in cats caused by a papillomavirus subtype. These are rare and appear as one or more nodular masses, usually on the head, neck, ventral abdomen, and limbs.
How are papillomas diagnosed?
Most papillomas have a typical appearance, although some more common sebaceous tumors in dogs are very similar. To obtain a definitive diagnosis, your veterinarian may perform a fine needle aspiration (FNA). FNA involves using a small needle with a syringe to suction a sample of cells directly from the tumor. A veterinary pathologist then examines the slide under a microscope.
In some cases, results from FNA may be unclear, so a biopsy may be necessary. A biopsy is the surgical excision of a piece of the tumor. In the case of papillomas, which tend to be small, the entire tumor may be removed. A veterinary pathologist then examines the tumor tissue under a microscope (histopathology).
How do papillomas typically progress?
In healthy animals, papillomas do not spread to other areas of the body and are usually cured by surgery. Rarely, a tumor will regrow after surgery. Additional papillomas may develop if the viral infection persists due to an impaired immune system.
How are these tumors treated?
Some papillomas will regress within one to two months if the animal develops immunity to it. However, some dogs have persistent tumors. The usual treatment is surgical removal. In cats, papillomaviruses are associated with certain cancers (e.g., squamous cell carcinoma), and surgery is usually advised.
In humans, a topical immune-modifying agent that stimulates interferon (a natural that supports the immune system) production has successfully treated papillomavirus lesions. Veterinarians sometimes prescribe it to treat these growths.
The oral antibiotic azithromycin is sometimes used to treat viral papillomas and usually provides good results.
Is there any special care that I should provide to my pet?
It is essential to prevent your pet from rubbing, scratching, licking, or biting the papilloma(s), all of which can cause inflammation, ulceration, infection, and bleeding. Any ulcerated area must be kept clean.
After surgery, you must keep the incision site clean and dry and prevent your pet from rubbing, licking, biting, or scratching at it. Report any loss of sutures or significant swelling or bleeding to your veterinarian. If you require additional advice on post-surgical care, contact your veterinarian.
Since papillomas are caused by a virus, are there any risks to my family?
Although this is an infectious tumor, the viruses are species-specific and not transmissible to humans. The tumors in dogs, cats, and people are not related and cannot be transmitted between species; however, they are transmissible between dogs. To prevent the spread of the virus, keep your dog away from other dogs until the papilloma is gone.
