Warts on dogs are benign tumors of the skin caused by the papilloma virus; veterinarians refer to dog warts by the more formal terms “papilloma” or “viral papilloma”. With the exception of a small subgroup of papilloma viruses, there are no breeds that are predisposed to developing the papillomas. Most dogs that are affected by viral papillomas are young (under two years of age), but geriatric or immunocompromised dogs can also be affected. This article covers the symptoms, cause, diagnostic tests, treatment, and prevention of viral papillomas.
Warts on dogs, clinical signs
There are a number of different papilloma viruses that cause skin tumors in dogs; the most common is canine papilloma virus-1 (CPV-1) which causes oral papillomas. These tumors have a classic fimbriated appearance, which means they are round, but have a rough surface similar to a sea anemone or a cauliflower. They typically occur on the lips and around the muzzle of a young dog (usually less than two years of age). Rarely, these papillomas can be found on the eyelids or between the toes. They are usually found in groups, so if you find one on your dog, continue looking inside his mouth and around his lips for more papillomas.
Other types of papillomas may be found on the dog. The first are cutaneous inverted papillomas (also known as endophytic warts) and are caused by the CPV-2 papilloma virus as well as a few other papilloma viruses. These papillomas can appear on the belly or groin as single nodules ½ – 1 inch in diameter with a central pore, or they can appear as a scattered lumpy rash. They can also appear on a dog’s toes or foot pads.
Papilloma pigmented plaques are caused by any one of several canine papilloma viruses from the chi subgroup of papilloma viruses and usually affect young pugs and schnauzers. The tumors are dark, scaly, and found on the belly, chest, bottom of the neck, or along the legs, and have been reported to occasionally convert to malignant squamous cell carcinoma.
Digital papillomas are caused by the CPV-2 papilloma virus as well as other papilloma viruses and are found strictly at the junction between the food pads and the adjacent skin at multiple places on all four feet. These papillomas are likely to be painful and could possibly become malignant (cancerous).
Cause of warts on dogs
As mentioned above, papillomas are caused by canine papilloma viruses. Papilloma viruses preferentially invade cells of the mucous membranes and skin cells that are capable of dividing; cells that can divide are found at the bottom layer of the skin. By 2013, nine different canine papilloma viruses had been identified, characterized, and sequenced. Although they are similar to the human papilloma viruses (HPV), the viruses are species-specific, which means your dog can’t get papilloma virus from you, and you can’t get papilloma virus from your dog.
The virus is transmitted by direct contact with the papillomas of an infected dog or with the virus itself in an infected dog’s environment (toys, bedding, food bowls, etc.). Because the actively dividing cells that the virus prefers are deep within the skin layer, the virus must infect a new dog through injured skin; dogs with healthy intact skin will not become infected. The incubation period (time between infection and onset of symptoms) is one to two months.
Once a dog has been infected, the viruses insert their DNA into the dog’s cells and take over control of the cell division process, causing the skin cells to divide abnormally and more often than normal. The virus also prevents the normal process of programmed cell death. Thus once the virus establishes itself, there is a rapid and abnormal growth of skin cells that results in the formation of “warts” or papillomas.
Normally a mature immune system is able to combat the virus before it takes hold, but young dogs who do not have a fully developed immune system are susceptible to the virus. Older dogs with weaker immune systems and those dogs whose immune systems are compromised due to steroid use or chemotherapy, may also be susceptible to the canine papilloma virus.
Diagnosis of warts on dogs
Diagnosis of oral papillomas is usually fairly straightforward since they have a rather unique and characteristic appearance. For the other types of papillomas, or if there is any doubt about the identification of oral papillomas, further diagnostics will be recommended.
One possible diagnostic method is a fine needle aspiration (FNA). With this method, a needle attached to a syringe is inserted into the tumor and a sample of the cells inside the tumor is suctioned into the syringe. The cells are then moved to a microscope slide, stained, and examined under a microscope. While a fairly non-invasive test, the drawback is that the cells needed to make a definitive diagnosis may not be suctioned into the syringe; there’s an element of luck as well as skill in taking the sample.
For a definitive diagnosis, a biopsy of the mass is needed. A larger, full-thickness sample of the mass is surgically removed; given the relatively small size of some papillomas, that may mean removing the entire mass as well as a small bit of the surrounding skin. Depending on the location of the mass and the temperament of the dog, the procedure may be done under sedation and local anesthetics, or may necessitate general anesthesia. One or two sutures may be placed to close the surgical site for better healing. The sample is then sent to an outside laboratory where a board-certified veterinary pathologist will examine the sample and provide a full report including a final diagnosis.
Treatment of warts on dogs
Most papillomas in younger dogs will spontaneously disappear within 1-2 months as the developing immune system learns how to deal with the virus. However, older or immunocompromised dogs are more likely to have papillomas that persist for more than a few months, or are more likely to have their papillomas develop into malignant squamous cell carcinomas.
In some cases, oral papillomas may be infected with bacteria from the mouth. If that is the case, oral antibiotics will be prescribed to clear the bacterial infection.
For those dogs whose papillomas persist for more than three months, or whose papillomas interfere with the dog’s normal function (especially eating), or if there are concerns about the possibility of conversion to cancer, surgical removal is recommended. Cryotherapy, the removal of the “warts” by freezing the tissue, is also a possibility in some specialty clinics.
Other treatments that could potentially be used earlier on in the disease process include azithromycin, interferon, and imiquimod. Azithromycin is an antibiotic that seems to have some action against various viruses via an unknown mechanism. In 2008, it was reported that a 10-day course of azithromycin removed all oral papillomas within 15 days. Later clinical trials by other groups yielded mixed results, but since the drug is relatively inexpensive and readily available, it’s reasonable to try azithromycin therapy, particularly in persistent cases.
Interferon is a cell signaling molecule produced by the body that has several important roles in the body’s defense against viruses. Injections of interferon have been used in humans for HPV infections, with mixed results. While there have been a few promising reports of interferon use in dogs, there have not been any controlled experiments demonstrating its effectiveness, nor any studies regarding its safety in dogs. Interferon therapy should be considered experimental.
Imiquimod is a drug that enhances the immune response. It is a topical cream that is used to treat genital warts and skin cancer in humans. Imiquimod has been shown to have some efficacy against dog papillomas. Often when using this drug, the skin is irritated around the papillomas, but this is considered a sign that the drug is working to speed up regression of the tumors.
Another possible treatment is creating a crude vaccine using the dog’s own tumors. Some of the warts are removed and used to create the therapeutic vaccine, which is then administered to the dog. The vaccine is thought to stimulate the dog’s own immune system to remove the rest of the tumors. Unfortunately, malignant tumors have been reported to develop at the vaccine injection site in some dogs.
Prevention of warts on dogs
A preventative vaccine against canine oral papillomas has been developed using recombinant DNA technology. While still considered experimental, the vaccine has been shown to be effective in preventing the development of oral papillomas.
Otherwise, prevention of viral papillomas consists of not allowing your dog to play with dogs that have visible papillomas. You should also prevent your dog from playing with the infected dog’s toys, or using his bowls or bedding. If your dog has wounds or rashes that compromise the integrity of his skin, it’s best to keep him away from places where other dogs congregate, such as dog parks or doggy day care. And if your dog does develop papillomas, keep him away from other dogs until the tumors regress.
Once your dog has recovered from the viral papillomas, he is then immune to reinfection of the same specific virus (but may be susceptible to other papilloma viruses).
Viral papillomas, or “warts”, are benign tumors caused by viruses similar to the human papilloma virus. These tumors generally affect young dogs with immature immune systems, and will usually regress on their own within a month or two as the dog’s immune system learns how to combat the virus. Oral papillomas are easily identifiable, but other papillomas require fine needle aspiration or biopsy for diagnosis. Some of the papillomas are capable of becoming malignant squamous cell carcinoma, particularly in geriatric or immunocompromised dogs. Some non-surgical treatment options are available for persistent or severe infections, but they each have varying degrees of success. Be sure to consult with your veterinarian about any new lumps, bumps, or skin discolorations because it can be difficult to distinguish between a benign tumor from a more worrisome malignant skin tumor.
- Vetinfo.com: Surgical Removal of Dog Warts
- Petmd.com: All about Dog Warts: Types, Causes, and Treatments
- Dog-health-handbook.com: Skin and Coat: Warts
- Vetary.com: Wart Virus in Dogs
- Vetinfo.com: Are Dog Warts Contagious?