Histiocytes are a type of white blood cell, and they’re responsible for fighting off foreign invaders. Their function as part of the immune system of mammals is key in staying healthy, but as in most cases, too much of a good thing can be bad.
Malignant histiocytosis is the result of the uncontrolled proliferation of histiocytes. This proliferation is rapid and aggressive, and it occurs in several sites simultaneously. The bone marrow, spleen, kidney, liver, central nervous system, and lymph nodes can all be affected.
This disease is sometimes called disseminated histiocytic sarcoma, and in fact, this might be the more accurate term. Histiocytic sarcoma is the proliferation of histiocytes that occurs in localized areas. For example, lesions may occur in the skin, spleen, or liver, but generally they occur in only one place initially. Histiocytic sarcoma can eventually spread to involve many organs, at which time it becomes known as disseminated histiocytic sarcoma. It is very difficult to differentiate between this disease and malignant histiocytosis, as clinical signs and lesions appear very similar.
Unfortunately for the affected dog, differentiating between the two diseases isn’t that important, as a diagnosis of disseminated histiocytic sarcoma or malignant histiocytosis is very bad news, indeed. Both diseases are aggressive and rapidly progress to death, often in a matter of weeks.
We see this disease most often in Bernese Mountain Dogs, for whom there is a familial association. Because it most often shows up after dogs have already been bred, it can continue on in blood lines more easily than congenital problems. Golden retrievers, Flat Coated Retrievers, and Rottweilers are also more prone to histiocytic diseases as well. It is important to note, however, that the diseases are not limited to these breeds—they can occur in any breed, including mixed breeds.
Clinical signs will depend on which organ system is affected, but generalized signs like weight loss, lethargy, and decreased appetite are common. Coughing and respiratory distress are common when the lungs are involved, as are seizures if the central nervous system is involved. Enlarged lymph nodes (lymphadenopathy), an enlarged spleen, and/or an enlarged liver can also be detected by your veterinarian if these organs are involved.
Diagnosis will be based on clinical signs as well as the results of diagnostic tests. Fine needle aspirates of affected organs will show abnormal histiocytes, and biopsy of affected organs or skin lesions can confirm this awful diagnosis.
If histiocytic sarcomas are caught early, it is possible that the surgical removal of lesions can prolong survival. For example, skin lesions can be excised or if joints of the limbs are affected, the limb can be amputated. Unfortunately, however, for disseminated disease or malignant histiocytosis, there is just nothing that can currently be done to save these dogs. While chemotherapy may provide a brief response, eventually all patients will succumb or be humanely euthanized due to a poor quality of life.