Does my Dog Have Warts?

Viral papilloma on the lower lip/gumline area of a dog. Photo by MarVistaVet

Certain viruses are able to cause the growth of small round skin tumors commonly referred to as warts. Everyone who has ever seen a drawing of a fairy tale witch knows what warts look like, so when the family dog develops small round skin growths, many people assume these, too, are harmless warts.

It is important to realize that viral warts are a specific condition and that a growth on the dog’s skin may or may not be a viral wart. Very few skin growths can be identified visually and many types of skin growths look similar.  Before assuming a growth is "just a wart," it is a good idea to learn about warts (viral papillomas) in case what you are seeing doesn't really fit and needs further evaluation.

Viral papilloma on a dog's body. Copyright VIN 2011

Human warts are round, somewhat flat, and relatively smooth. Viral warts in dogs tend to possess frond-like structures (called fimbriae), creating more of a sea anemone or cauliflower-like appearance, though they can be smooth as well. The classical viral wart patient is a young dog with warts in or around the mouth or eyes. In cases where warts have a typical appearance in a classic patient, diagnosis may be obvious, but in older patients with warts in locations other than the face, other types of growths become more likely. Because many types of growths can appear harmless but behave malignantly, removal and biopsy are often recommended, especially if the appearance of the growth and/or patient does not fit into what is considered typical.

In dogs, we do not call these growths warts; we use the more formal term: viral papilloma. As in people, viral papillomas are caused by a papillomavirus, although dogs and people have different papillomaviruses and cannot transmit their viruses across species lines.

What Do These Papillomas Look Like?

Two viral papilloma lesions on a dog's lip. Photo courtesy of Dr. Jan Bellows

By far, the most common type of viral papilloma in dogs is near the mouth, caused by CPV1 (canine papillomavirus-1). Viral papillomas are classically fimbriated, meaning they are round but often have a rough, almost jagged surface reminiscent of a sea anemone or a cauliflower. They occur usually on the lips and muzzle of a young dog (usually less than 2 years of age). Less commonly, papillomas can occur on the eyelids and even the surface of the eye or between the toes. Usually, they occur in groups rather than as solitary growths so if one growth is noted, check inside the mouth and lips for more.

When the viral papilloma has a classic appearance like the ones shown above, a biopsy is usually not needed to identify the growth. In cases where there is uncertainty, however, a biopsy will settle any questions. The good news is that most oral viral papilloma cases are mild and resolve on their own within 2 months.

How is this Virus Transmitted?

The infection is transmitted through direct contact with the papillomas on an infected dog or with the virus in the pet’s environment (on toys, bedding, food bowls, etc.). The virus requires injured skin to establish infection; healthy skin will not be infected. The incubation period is 1 to 2 months. This virus can only be spread among dogs. It is not contagious to other pet species or to humans, and it appears not to be contagious after the lesion has regressed. Recovered dogs cannot be infected with the same strain of virus but there are several viral strains.

To become infected, the dog generally needs an immature immune system or a suppressed immune system, thus this infection is primarily one of young dogs and puppies.  Dogs taking cyclosporine orally to treat immune-mediated disease may also have an outbreak of papilloma lesions.

Are Viral Papillomas Dangerous?

Not really. They should go away on their own as the dog’s immune system matures and generates a response against the papillomavirus. The process of regression usually takes 1 to 2 months. If lesions are still there after 3 months, treatment is recommended (see below) and a biopsy may be needed to confirm that the growth really is a viral papilloma. Severe cases can actually interfere with chewing and swallowing, but such heavy involvement is unusual.

Sometimes oral papillomas can become infected with bacteria from the mouth. Antibiotics will be needed in such cases to control the pain, swelling, and bad breath.

Treatment

In most cases, treatment is unnecessary; they simply go away on their own. Occasionally an unfortunate dog will have a huge number of papillomas, so many that consuming food becomes a problem. Papillomas can be surgically removed or frozen off cryogenically. Sometimes crushing several growths seems to stimulate the host’s immune system to assist in the tumor regression process. In humans, anti-viral doses of interferon have been used to treat severe cases of warts, and this treatment is also available for severely infected dogs though it is costly and yields inconsistent results.

More recently, a topical medication called imiquimod has been used in both canine and human infections to help boost immune-mediated inflammation and thus facilitate the destruction of the virus by the body. Imiquimod is being prescribed increasingly for dogs with viral papillomas on their skin. Skin irritation is frequently noted adjacent to the growth when imiquimod is in use, but this is generally regarded as a sign that the growth is regressing and the medication is working.

A research group in Turkey in 2008 reported excellent success using a 10-day course of azithromycin. Treatment was able to remove all lesions within 15 days, with no recurrences reported in the subsequent 8 months of the study. Subsequent results have been mixed; however, this therapy is frequently used in more stubborn cases as the medication is reasonably priced and readily available.

Sometimes some of the warts can be removed and made into a vaccine of sorts. The vaccine helps stimulate the patient's immune system so that the virus and tumors can be made to regress naturally. Making the vaccine requires DNA from the patient's virus and can be used as either treatment or prevention. Special biologic companies use their technology to custom-make tumor vaccines in this way. For more information, visit Torigen.com

Other Types of Papillomas

The oral papilloma is by far the most common canine viral papilloma, but it is certainly not the only kind. There are other papillomaviruses besides CPV-1. Here are other types of viral papillomas:

Cutaneous Inverted Papillomas (Also Called Endophytic Warts)
These are caused by the CPV-2 papillomavirus as well as other papillomaviruses. They can appear as single nodules on the belly (1/2- 1 inch across) with a small central pore or they can be disseminated like a lumpy rash. They can also form between the dog's toes.

Papillomavirus Pigmented Plaques
These are caused by several of the "Chi canine papillomaviruses" and usually affect pugs and miniature schnauzers. They are dark, scaly, and of variable size and are usually found on the belly. Unlike other papillomas, these do not regress on their own except in pugs. They are capable of transforming into malignancy.

Digital Papillomas
These are caused by the CPV-2 papillomavirus as well as other papillomaviruses and grow on the foot pads and between the toes. They tend to be painful and have the potential to transform into malignancy.

These non-oral papillomas do not possess the classic fimbriated appearance of the oral papilloma and are likely to require biopsy for identification. As with oral papillomas, though, they can be expected to regress in most cases within a 2-month time frame. If they do not, the same treatments noted above can be applied.