Cutaneous Histiocytoma in Dogs

By Malcolm Weir, DVM, MSc, MPH; Joan Rest, BVSc, PhD, MRCPath, MRCVS

What are histiocyte cells and what do they do?

The histiocyte group of cells is part of the body's immune surveillance system. They take up and process foreign antigens, such as pollens and viral, bacterial, and fungal microorganisms. They then migrate to the local lymph nodes where they present the antigens to other immune system cells (T lymphocytes) to stimulate them into a variety of activities to protect the body. The cells that are involved in cutaneous histiocytoma are called Langerhans cells.

illustration of cutaneous histiocytoma on a dog's faceWhat is a cutaneous histiocytoma?

A cutaneous histiocytoma (not to be confused with histiocytosis) is a common benign (harmless) tumor of Langerhans cells. In their early stages, over the first one to found weeks, they grow rapidly. During this period of rapid growth, they often ulcerate and may become infected. Later, they may regress spontaneously.

What do we know about the cause?

Most dogs that develop these tumors are young, and spontaneous self-cure is common with time. This suggests that they are hyperplasias (overgrowth of cells with regression when the stimulus for the proliferation of the cells is removed) rather than true cancers (where cell proliferation is out of control and does not regress). No infectious agent (such as a virus) has been isolated, but a history of a previous injury that could have allowed entry of such an agent is not unusual. Insects can transmit these agents by biting, carrying the stimulus for histiocytoma from dog to dog.

"Insects can transmit these agents by biting, carrying the stimulus for histiocytoma from dog to dog."

Are cutaneous histiocytomas common?

This is a common tumor. Most affected dogs are less than six years of age, occasionally as young as eight weeks. The tumor can occur in any breed, but some breeds appear to be more susceptible to the tumor, including Boxers and Bull Terriers.

How will this tumor affect my dog?

The most obvious effect of this tumor is the lump. Many will regress spontaneously over a few months. Usually, these tumors are surgically removed because of ulceration, infection, and bleeding. It has been known for a dog to die from secondary infection of an untreated tumor. Occasionally, the local lymph nodes may swell. This may be because the migrating histiocytes have proliferated there or because there is a reaction to secondary infection. It is unusual for more than one tumor to be present on the same dog or for the same tumor to occur later at another site, but both these situations have been found in otherwise normal young dogs.

Less commonly, in older dogs or those with inadequate immune systems, cutaneous histiocytomas become multiple and progress to malignancy (cancer).

How is this tumor diagnosed?

Clinically, this tumor has a typical button-like appearance. Accurate diagnosis relies upon microscopic examination of tissue. Depending on the location, your veterinarian may recommend one or more methods of obtaining a tissue sample for diagnosis. The most common methods include needle aspiration, punch biopsy, and full excision biopsy (removing the growth).

The sample will then be examined by either cytology or histopathology. Cytology is the microscopic examination of aspirated cell samples. This is used for rapid or preliminary assessment. More accurate diagnosis, prediction of behavior (prognosis), and a microscopic assessment of whether the tumor has been fully removed rely on microscopic examination of tissue (histopathology). This is done at a specialized laboratory by a veterinary pathologist. Your veterinarian may submit a small part of the mass or the whole lump. If your veterinarian performed an excision biopsy, the pathologist will also assess whether the cancer has been completely removed.

What treatment is available?

This is one of the rare types of tumor that the body's immune system can eliminate. However, ulceration, itching, secondary infection, and bleeding are often problems that require surgical intervention. Treatment involves the surgical removal of the lump to confirm the diagnosis. In 99% of cases, surgical removal is a permanent cure.

"In 99% of cases, surgical removal is a permanent cure."

How can I care for my dog with a cutaneous histiocytoma?

Preventing your dog from scratching, licking, or biting the tumor will reduce itching, inflammation, ulceration, infection, and bleeding. Any ulcerated area needs to be kept clean. After surgery, you will need to 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, consult your veterinarian.

Are there any risks to my family or other pets?

Although this may be potentially a transmissible tumor between dogs, there are no risks to people or other pets. There are no records of tumors spreading by close contact between animals, and the tumors do not occur in clusters in a household or neighborhood.

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