Warts in dogs: papillomas, infection and treatment

Overview

Here, a papilloma means a wart-like growth of the surface epithelium, with papillomatosis involving changes on the skin or mucous membranes. Oral warts caused by papillomavirus are particularly common in young, growing dogs. They can appear suddenly and multiply rapidly. They often look like a cauliflower, consisting of finger-like or slender projections.

Growths are often found on the lips, oral mucosa and skin around the lips. Skin warts are common on the paws and around the face and ears. A single wart-like change in an older dog may be something other than a viral wart. A sebaceous gland tumour, for example, can resemble a papilloma.

An ordinary viral papilloma does not itself mean cancer. Skin warts may disappear within about 3 months. However, an individual skin wart can remain for up to 2 years before disappearing. Oral and skin papillomas therefore do not always follow the same course.

A viral papilloma usually takes about 3–6 months to disappear. It involves overgrowth of skin surface cells under the virus's influence. Folding of the thickened epithelium creates the cauliflower-like surface.

Causes

Papillomaviruses spread between dogs through direct contact. Contaminated toys, water bowls and bedding can also transmit infection. A small injury to skin or mucosa can allow the virus into the basal layer of the surface tissue. Viral replication accelerates epithelial cell division, causing a wart to grow.

Not every papillomavirus infection causes a visible lump. When the body's immune response begins to limit viral replication, a papilloma may disappear on its own. Months can pass between infection and a visible wart. You therefore generally cannot identify the exact infection date or the dog responsible from the day the first lump was noticed.

Canine papillomaviruses are species-specific, and dog warts do not spread to humans. Different viruses cause warts in humans, cats and dogs. In the same dog, infection may persist and more papillomas develop if the immune system does not control it.

The same papillomavirus type can cause changes on both the skin and inside the mouth. When a papilloma disappears, it is uncertain whether the virus is eliminated completely or remains in basal skin cells.

Risk factors

Puppies and young dogs are susceptible to papillomas because their immune systems are still developing. Weakened immunity also increases susceptibility. Extensive oral papillomas in an older dog may justify investigating an underlying immune deficiency. An underlying disease can occasionally be lymphoma, a cancer of lymphoid tissue.

Young dogs often have several growths on the skin or mucous membranes. Skin warts are common in Cocker Spaniels and Kerry Blue Terriers, for example. Breed does not determine a lump's diagnosis, however, as wart-like growths can occur in dogs of different ages.

Viral warts often appear between 6 months and 2 years of age. Clusters of infection have also been described in dog day care, where dogs have close contact with one another.

Typical symptoms

A papilloma may be a single lump or part of a dense cluster of growths. Oral warts can appear on the lip, gum or deeper oral mucosa. Common observations include:

• A rough, cauliflower-like surface.
• A pink or white lump.
• Scaling on the surface.
• Rapidly increasing numbers of growths around the mouth.
• Biting a larger oral papilloma while eating, making it bleed.
• Difficulty picking up food, chewing or swallowing because of extensive growths.

Not every papilloma grows outwards: some form a firmer inward-growing lesion. Such a change on a paw can be painful. Flat, dark viral plaques can also occur on the inner limbs and underside of the abdomen. They look different from the usual cauliflower-like oral wart.

A skin papilloma can rarely develop a thick grey keratin layer resembling a small horn. An inward-growing papilloma may look like a thick-rimmed cup with its centre depressed into the skin. Outward or inward growth alone does not determine a papilloma's outlook.

Inverted, or inward-growing, skin papillomas also occur in young adult dogs. Their usual location is the lower abdomen.

Complications and consequences

Biting, scratching or licking a papilloma can cause inflammation and ulceration. A damaged wart in the mouth can develop a bacterial infection. Extensive growths can interfere with eating and swallowing. In a rare, severe case, breathing can also become difficult.

Transformation of a skin papilloma into squamous cell carcinoma, a malignant surface-cell tumour, is extremely rare. However, a persistent or changing lump may need another tissue sample to review the diagnosis.

In rare persistent papillomatosis, new warts continue to appear 6 months after the first changes. The growths can become extensive and cover large areas. An absent immune response specifically against papillomavirus may underlie this, without another obvious immune disease. Papillomas can then recur even after surgical removal.

A small wart that stays unchanged for a long time is distinguished from expanding, persistent papillomatosis. Long duration alone therefore does not mean your dog has a severe immune system disorder.

Diagnosis at the vet

The vet assesses the growth's size, location, shape and texture. The appearance of several typical papillomas may be enough for a provisional diagnosis. However, a sebaceous gland tumour can look almost identical. It may also be impossible to distinguish a benign viral growth from another wart-like epithelial growth by appearance alone.

Fine-needle sampling collects cells from the lump onto a slide using a small needle. A pathologist examines the cells under a microscope. The sample can often be taken at the practice without general anaesthesia. Sedation may be necessary if your dog cannot remain safely still.

If the cell sample is inconclusive, a tissue sample, or biopsy, is taken. A small papilloma may be removed completely at the same time. Histopathology, or tissue examination, shows the growth's cellular structure. A viral origin can be assessed from the virus or the changes it causes in cells.

PCR testing can look for papillomavirus genetic material within the growth. A positive result alone does not prove the virus caused the lump, as infections without signs occur. Tissue samples can be checked for characteristic viral cell changes. Clear halos around cell nuclei, for example, can support a papillomavirus diagnosis.

A sebaceous adenoma is a benign growth associated with a sebaceous gland. It is seen particularly in older dogs. Its surface can also become crusted and inflamed, like a wart-like skin lesion.

A biopsy removes tissue from the growth, whereas fine-needle sampling collects individual cells. Dogs are sedated for skin biopsy collection. The small skin wound left afterwards may be closed with a suture.

Treatment

The vet prescribes any treatment and medicines. Confirmed small papillomas can often be monitored if they do not interfere with eating or swallowing. An irritated, inflamed or persistent papilloma may need removal. Removal is also considered for an awkwardly located or repeatedly bleeding growth.

Growths can be removed surgically or using laser, electrosurgery or radiosurgery. Removed tissue can be sent for testing to confirm the type of lump. New growths may still follow a virus-associated papilloma if immunity is weakened. Treatment planning then also considers any underlying illness.

Keep a surgical wound clean and dry according to veterinary instructions. Prevent licking, biting and scratching during healing. A suture coming out, significant swelling or bleeding is a reason to contact the vet treating your dog. An ulcerated papilloma that has not been removed is also kept clean.

Papillomas can also be treated by freezing, called cryotherapy. Assessments of medical treatments' benefits conflict. Papillomas have reportedly disappeared within 2–3 weeks with azithromycin. However, its ability to speed up spontaneous wart regression has not been confirmed. The usual spontaneous resolution of warts makes treatment response difficult to interpret.

Vaccine treatment for severe papillomatosis may increase antibodies, but has not been shown to initiate the cell-mediated immune response needed for growths to regress.

Removing a benign, non-viral wart-like epithelial growth can also resolve the local problem completely.

Removing one papilloma can sometimes trigger regression of other growths too.

Prevention

Keep a dog with viral warts apart from other dogs until the papillomas have disappeared. Avoiding shared toys and food or water bowls reduces contact with potentially contaminated objects. The viruses can survive in the environment for a long time. However, transmission may already have occurred before the first warts were noticed because of the long incubation period.

Avoid damaging the skin around a wart, as small surface injuries make it easier for the virus to enter the epithelium. Do not squeeze or cut off a papilloma at home. Preventing your dog from scratching also helps avoid inflammation and bleeding.

Important for dog owners

Photograph a new lump and record its location. Take later photographs from the same angle and at the same scale so growth can be compared. Note whether new changes appear and whether oral growths affect eating. Tell the vet about medicines used and previous skin tumours too.

If your dog attends day care or group activities, agree with your vet how to manage infection risk and return to the group. Do not label a new or growing lump a wart based on a photograph alone. Ask whether the diagnosis has been confirmed by a sample or the change is being monitored on a provisional assessment. Keep any pathology report for later comparison.

Frequently asked questions

Can dog warts spread to humans or cats?

Canine papillomaviruses do not infect humans, and papillomas in dogs and cats involve different viruses. Infection can, however, pass between dogs. People at home therefore do not need isolation because of the warts, but exposure of other dogs must be considered.

How long do oral warts in dogs usually last?

Many oral papillomas in young dogs disappear on their own within 4–8 weeks. However, papillomas in an individual dog can last considerably longer. Skin warts usually disappear in about 3 months, but some remain for up to 2 years.

Is a wart in an old dog always a papilloma?

No. A single wart-like lump in an older dog is not necessarily caused by a virus. A sebaceous gland tumour, for example, can resemble a papilloma. Have every new or changing lump examined by your vet and sampled when necessary.

Does every papilloma need removal?

No. Small, harmless oral papillomas can be monitored because they often disappear without a procedure. Removal may be needed if a growth affects eating, becomes inflamed or repeatedly bleeds. An uncertain diagnosis may also require a tissue sample.

Can I use a human wart remover?

Do not treat your dog's lump with a human wart remover or try to remove it yourself. First, the type of growth must be established. Lumps resembling papillomas may be sebaceous gland tumours, for example, which need a different assessment. The vet chooses a suitable removal method if needed.