Finding a lump on your dog is often a cause for concern, but there are many kinds of lumps, bumps, spots and dots that a dog can develop on their skin. One unusual kind is a canine inverted papilloma, or endophytic papilloma. Are these strange doughnut-shaped warts something to be worried about? How do they develop? Is there any treatment?
Canine Papillomaviruses
Canine papillomaviruses (CPV) are a group of viruses that infect dogs and seem to be associated with canine inverted papillomas. CPV mainly infects cells in the deeper layers of the skin (the basal cells of the epithelium), replicating and proliferating inside the cells. Dogs are infected via damage (often microscopic damage) to the skin allowing the virus to enter the cells. As the infected cells migrate to the surface as old skin cells flake off, the virus escapes and enters the environment. Many dogs are infected young, and the majority will never get rid of the virus, remaining infected for life. Many lesions develop on the mouth, feet, ears and face, where microdamage to the skin is most common. However, they can develop anywhere on the body. Though most infections are isolated cases, there are occasional reports of multiple dogs becoming infected together, such as a case of 13 dogs in a kennel developing oral papillomas. There was also a case of a dog that develops papillomas after surgery, with the report suggesting they developed after microdamage from hair being clipped or the reduced immune system following surgical stress.
When the virus hijacks the basal cells to proliferate, it also can cause the cells to replicate. This results in a thicker layer of skin cells, which can present as thickened skin and the development of a papilloma, or wart. Most of these lesions are non-cancerous, but can in rare cases become neoplastic (cancerous). However, many dogs will carry the virus asymptomatically (no symptoms). Sometimes, changes in the dog’s immune-system can cause a normally asymptomatic dog to develop symptoms.
Papillomas
For dogs that develop lesions, one of the stranger appearances is an inverted papilloma. These are thick rough nodules that protrude from the skin but have a sunken ‘cup-shaped’ inner part. They can sometimes be covered by thickened keratin (the same substance that makes up hair and nails). The unique sunken cup-shape is caused by folding of the skin layers inward.
Alternatively, dogs can develop exophytic papillomas. These protrude completely outwards because of the skin layers folding, but otherwise can present very similarly to inverted papillomas. The effects on the dog appear to be the same with inverted and exophytic papillomas, and there is no specific CPV that causes one or the other.
Most of the time, these lesions cause no harm to the dog, though depending where they are they can cause difficulties walking or eating. If caught, the papillomas can break and bleed, and rarely can become secondarily infected.
CPV-Associated Neoplasia
As mentioned above, though the majority of these lesions are non-neoplastic, some CPV can cause cancer. These include squamous cell carcinomas and follicular (hair cell) cancers. It is unclear the mechanism behind the transition from non-neoplastic to neoplastic, but there is evidence in humans that exposure to UV light can change them. There may also be evidence the risk of cancer is higher if a dog is immunodeficient (weak immune system) or is a certain breed (Pugs and Miniature Schnauzers may be at greater risk).
Diagnosing Papillomas
Papillomas can often be presumptively diagnosed based on appearance, particularly if there are multiple lesions and the dog is young. However, definitive diagnosis requires a biopsy, either part or the entire mass, to view cells under the microscope. Special laboratory tests can also be done to detect antigens (viral proteins) in the tissue.
Treatment of Papillomas
In many cases, papillomas are self-limiting – this means that they go away by themselves, usually once the dog’s immune system fights the virus. This can be within 3-6 months but can take years as well. Because of this, unless they bother the dog or are causing harm, treatment is often not recommended.
In cases where the papillomas need to be removed, this can be done so via surgery, cryotherapy or electrosurgery. This usually completely prevents recurrence, though some dogs will regrow the papilloma in the same place if their immune system does not prevent it. A topical cream called Imiquimod can be used to destroy the epithelial cells as well.
There are vaccines for CPV (more on this later) that some vets have used to try and treat papillomas, with the theory being it would stimulate an early immune response. However, there has so far been no reliable evidence that vaccination increases the immune response sufficiently to resolve papillomas.
Preventing Papillomas
As CPV can live for a long time in the environment, is easily transmitted between dogs, and is hard to eliminate without strict hygiene, preventing papillomas is difficult. Remember that even if a dog is exposed to CPV, they may not develop papillomas, and even if they do, they often are self-limiting
There are vaccines that have been trialled to prevent papillomas, but their usefulness is limited. Firstly, they must be given before a dog is exposed to CPV – this generally means it would have to be given early to prevent infection, such as with the core puppy vaccines. Secondly, there has been no evidence that vaccines shrink papillomas, meaning if given after papillomas present, there would be no effect. This means that a vaccine for CPV has not been commercially developed and likely is not a high priority at this stage. Vaccines that are made for human papillomaviruses are more common as they can be given before infection is likely (most human papillomaviruses are spread via sexual contact), and have been shown to reduce the risk of papillomavirus-induced cancers.
Closing Thoughts
If you spot a doughnut-shaped ‘inside-out’ mass on your dog, particularly on the face or paws, it is likely a papilloma, caused by the canine papilloma virus. These warts are usually benign and self-limiting, and your dog rarely needs treatment. Some dogs will have persistent warts, but they rarely become cancerous. There are no effective treatments against CPV, but removal of the papillomas themselves usually is curative. We would always recommend you check with your vet if you spot lumps like these, but they are rarely anything to worry about.
Sources and Further Reading
Lane, Weese & Stull (2017) Canine oral papillomavirus outbreak at a dog daycare facility
Lane & Tübbesing (2007) Multiple cutaneous inverted papillomas in a dog
Discussion