Histiocytosis refers to an abnormally large amount of histiocytes, which are the white blood cells in the immune system that come from the bone marrow. There are two kinds of histiocytes, which are macrophages, which remove debris and other foreign materials from the blood, and dendritic cells that tell the lymphocytes what molecules are good and which are bad. Both CCH and CH produce benign tumors that are restricted to the skin and may come and go at will. SH is a more serious form that involves the organs as well as the skin and lymph nodes although it is not malignant. HS and MH are both malignant tumors that are life threatening. Removing the tumor before it spreads is the best form of treatment because once it spreads into the vital organs, it is almost always fatal.

There are several different types of histiocytosis, which are reactive histiocytosis (includes cutaneous and systemic), canine cutaneous histiocytoma, and histiocytic sarcoma complex (includes histiocytic sarcoma and malignant histiocytosis). Cutaneous reactive histiocytosis (CH) and canine cutaneous histiocytoma (CCH) are benign skin tumors that come and go but do not cause any internal damage. Systemic reactive histiocytosis (SH) causes skin tumors that progress to the vital organs and bone marrow and can be unresponsive to treatment. Histiocytic sarcoma (HS) is a malignant tumor that can spread quickly without treatment. Malignant histiocytosis (MH) is an aggressive disease with several tumors that progresses quickly and is usually fatal.

There are several types of histiocytosis and each have their own set of side effects. Some of these are:

The cause of most histiocytoses are unknown, but they are most often seen in:

The veterinarian will perform a physical examination to check vitals and examine your dog from head to tail for signs of lesions. Bring your dog’s medical records if possible, and let the veterinarian know if your pet is on any medications. Fine needle aspiration of the tumors or lesions is usually taken for microscopic examination. Some diagnostic tests may reveal low hemoglobin and iron reserves (anemia), low number of platelets, and increased liver enzymes. Radiography (x-rays), CT scans, MRI, and ultrasound can give the veterinarian an idea of how far the disease has spread.

Your dog’s treatment plan is dependent on which type of histiocytosis is found. No treatment is usually needed for CCH, but the veterinarian will want you to keep checking it and schedule follow up appointments for 3-6 months. The treatment for CH and SH are medications and observation. The treatment for HS is surgery, medications, and hospitalization. MH is fatal within days and euthanasia is usually recommended.

Medication may include aggressive steroid therapy, azathioprine, tetracycline, or cyclosporine. The veterinarian will want you to continue to observe the tumor growth and watch for side effects from the medication.

Surgery to remove the tumor (amputation, if necessary) is necessary right away to stop the cancer from spreading. Chemotherapy and radiation are sometimes used but once the cancer reaches the vital organs it is usually too late, since it has then progressed into MH, which is untreatable and fatal.

Prognosis for CCH is excellent since it usually resolves on its own. CH and SH are good, but since steroids affect the immune system, you must continue to watch for infections. Even a mild infection can be fatal with a weakened immune system. HS prognosis is fair as long as you have caught it before it spread to the internal organs. However, infections need to be watched for with this type of histiocytosis as well,  because steroids are used. The fate of MH is grave and your veterinarian may suggest euthanasia.