What is cutaneous histiocytosis?
Cutaneous histiocytosis (CH) is a benign (non-cancerous) condition in dogs caused by a proliferation of a type of body cell called histiocytes. These cells are part of the body's immune surveillance system. They function to take up and process antigens (foreign materials) such as pollens and 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 (immunity).
Histiocytes are formed from a type of cell called a dendritic cell. There are many types of dendritic cells; in the case of CH, the dendritic cells are referred to as interstitial dendritic cells (iDCs), which are found in the tissue surrounding blood vessels and in many different types of tissue, including the skin.
CH is part of a large group of diseases that can range from benign to cancerous, all of which have the involvement of the body’s histiocytes as their common factor. In CH, the tissues involved are limited to the skin, subcutaneous (under the skin) tissues, and the regional lymph nodes nearest to the area of inflammation. The histiocytes themselves do NOT exhibit cellular signs of cancer or malignancy in the case of CH.
What are the clinical signs of cutaneous histiocytosis?
CH generally appears as multiple skin nodules, both on and below the skin. These nodules can be found anywhere on the body and can be up to 1.6 inches (4 cm) in diameter. They tend to be painless and not itchy. If they are ulcerated, they may become infected with bacteria. An affected dog may have a history of nodules that seem to spontaneously disappear and appear again at new sites on the body. They may also cause the dog’s nose to enlarge, which is called ‘clown nose’.
"CH can mimic several nodular skin lesions and sometimes requires extensive diagnostic testing."
CH can mimic several nodular skin lesions and sometimes requires extensive diagnostic testing. CH is limited to the skin; no systemic signs (e.g., fever, weight loss, poor appetite, breathing troubles, or gastrointestinal upset) will occur. If systemic signs are present, the dog may be suffering from systemic histiocytosis or one of the other cancerous versions of histiocytic disease.
What causes cutaneous histiocytosis?
CH is an immune dysfunction, possibly due to persistent antigenic stimulation by a variety of antigens. Although some specific infections will cause this reaction, the cause of most cases remains undetermined. It has been suggested that some cases of cutaneous histiocytosis are vector (tick or insect) transmitted infections that provide antigenic stimulation. Often, however, no organism is detectable by routine microbiology.
There is no clear breed predilection for this condition. CH can occur in a dog of any age, and there is no predilection between males and females.
How is cutaneous histiocytosis diagnosed?
Your veterinarian may suspect this condition on physical examination but accurate diagnosis relies upon microscopic examination of tissue. To obtain the appropriate samples, your veterinarian may recommend one or more sampling techniques such as fine needle aspiration, punch biopsy, and/or full excision (removal of a lump or lesion).
Cytology is the microscopic examination of cell samples obtained by needle aspiration. This method is useful for rapid or preliminary screening and may show malignant cells where there is malignancy. Cytology alone, however, will not distinguish reactive histiocytosis from other inflammatory reactions.
"Histopathology will give the most accurate diagnosis and can predict the behavior (prognosis) of the lesions."
Histopathology is the microscopic examination of tissue samples that have been specially prepared and stained. Histopathology will give the most accurate diagnosis and can predict the behavior (prognosis) of the lesions. Histopathology also rules out the possibility of cancer. Your veterinarian will submit the samples to a specialized laboratory for examination and diagnosis by a veterinary pathologist. The piece of tissue may be a small part of the mass (biopsy) or the whole lump.
The information obtained by examining the whole lump will be more informative and also may indicate whether the lesion has been fully removed. However, histopathology of a single lump does not indicate whether there is a systemic disease and to what extent. Combining the information obtained from the biopsy sample along with signs of systemic illness (as outlined earlier) will help to differentiate between cutaneous and systemic histiocytosis.
Other tests your veterinarian may recommend include immunophenotyping of the biopsy sample. This test helps to determine cellular surface antigens on the cells forming the lump or nodule. While this cannot be used to distinguish between cutaneous and systemic histiocytosis, it can be used to rule out other similar-appearing diseases, including various forms of cancer (e.g., lymphoma).
How is cutaneous histiocytosis treated?
For multiple or recurrent CH lesions, immune-modulating drugs are generally the first line of treatment. A variety of drugs have been suggested for use, ranging from a combination of niacinamide (vitamin B3, brand name Nicotinamide®) and tetracycline (antibiotic, brand names Achromycin®, Medicycline®, Sumycin®, Tetracyn®), prednisone (glucocorticoid, brand names Prednis-Tab®, Deltasone®, Rayos®, Pediapred®), cyclosporine (Atopica®), or leflunomide (inhibitor of T-cell activation, brand name Arava®).
"Surgical removal of lumps can be considered for small or solitary lesions, but it does not guarantee new lesions will not develop elsewhere on the body."
Surgical removal of lumps can be considered for small or solitary lesions, but it does not guarantee new lesions will not develop elsewhere on the body.
No single treatment is successful in every case and your veterinarian may need to try several different medication regimes. Medical management generally consists of both an initial phase and a long-term phase (where medication dosages are tapered and the dog is monitored for recurrence of the condition).
Lesions may spontaneously regress, especially during the early stages of the disease, but recurrence is common.
What is the prognosis?
Response to treatment is usually good, though long-term therapy is often needed. The time until the nodules or lesions disappear can be variable, with times ranging from 10-162 days having been reported. Recurrence is always possible and long-term monitoring and periodic rechecks are recommended.