Malignant Histiocytosis


Malignant histiocytosis is an uncommon disease of dogs that is overrepresented in certain breeds, thereby underlining its heritability. It’s an aggressive, tragic disease that involves the abnormal accumulation of a type of white blood cell called the histiocyte. The histiocyte lives in the body’s connective tissues (not in the blood like other white blood cells do) and works as part of the immune system to consume invading organisms. In the case of malignant histiocytosis, however, this cell’s aggressively cancerous proliferation in multiple sites at the same time typically leads to poor appetite, weight loss, and lethargy. Sadly, death typically ensues within weeks to months of the pet’s diagnosis.

It bears mentioning that malignant histiocytosis is sometimes indistinguishable from another distinct disease entity called histiocytic sarcoma. Histiocytic sarcoma is also a cancerous disease surrounding the uncontrolled proliferation of abnormal histiocytes, but differs from malignant histiocytosis in that it originates first in one site alone then metastasizes to distant sites (instead of to multiple sites at once, as with malignant histiocytosis).

Unfortunately, it is usually impossible to clinically discern the difference between the two diseases due to the difficulty inherent to knowing how long the disease has been active. For that reason, these two diseases are now often conflated by using the term, histiocytic sarcoma complex. For the purposes of this discussion, however, the more common nomenclature, malignant histiocytosis, will be employed throughout.

Another distinct disease entity referred to as a cutaneous histiocytoma often confuses pet owners when researching this disease category. A histiocytoma is a benign skin tumor that often regresses spontaneously and, unlike the other disease entities discussed in this article, carries an excellent prognosis.

Symptoms and Identification

Dogs afflicted by malignant histiocytosis usually show a variety of symptoms. This is because by the time it is detected, the disease has already progressed to the point where it is affecting multiple sites of the body.

Because the proliferating abnormal histiocytes are typically found in the lungs, liver, spleen, bone marrow, and lymph nodes, dogs may be feverish, lethargic, and suffer poor appetites. Weight loss is also common and prominent eyes may be a feature of the disease’s progression as well. Because it often metastasizes to the lungs and can afflict the central nervous system, respiratory symptoms and seizures, respectively, are also possible.

The disease can only be diagnosed definitively by biopsying affected tissues. Ultrasound, X-rays, and sometimes even CT scans are employed by way of identifying the extent of the disease’s masses. These can affect an organ diffusely, leading to organ enlargement, or appear as multiple masses on the affected organ.

Surgery through laparoscopy or exploratory laparotomy (abdominal exploration) is sometimes undertaken to retrieve tissues. Ultrasound-guided fine-needle aspiration, however, is how most samples are typically obtained. Examining the tissues or individual cells through microscopy is ultimately undertaken, usually by a veterinary pathologist.

Affected Breeds

Malignant histiocytosis is perhaps most notoriously recognized in Flat-coated Retrievers and Bernese Mountain Dogs. Though any breed can be afflicted, other predisposed breeds include Rottweilers and Golden Retrievers.

A specific mode of inheritance has not been identified as of yet.


Sadly, there is no effective treatment for malignant histiocytosis at this time. However, trials employing liposomal clodronate (LC), among other chemotherapeutic agents, have recently shown some promise in the treatment of this once-intractable disease.

Palliative care to relieve the symptoms associated with malignant histiocytosis is commonly employed to help bring relief to these patients until they succumb to their disease or are humanely euthanized.

Note: Non-traditional modalities such as acupuncture and traditional Chinese medicine are sometimes elected. Though as yet unproven via controlled clinical trials, many veterinarians believe they can make a difference to their patients’ degree of comfort during this time.

Veterinary Cost

Because it has not been found to be amenable to conventional cancer therapies, the cost of malignant histiocytosis is typically confined to the price of diagnosis and palliative care. Nonetheless, many dogs can accumulate significant expenses in the course of diagnosis and palliation.

Diagnosis of malignant histiocytosis can range anywhere from the cost of an X-rays, bloodwork, ultrasound, fine-needle aspirate, and cytopathological examination (typically between $700 and $1,500) to more than $5,000 depending on the degree to which an owner hopes to pursue a better understanding of prognosis and/or entertain the possibility of definitive treatment, experimental at present though it may be.

The cost of palliative care depends largely on whether owners elect to employ a vigorous hospice protocol that may keep their dogs comfortable for a longer period of time, or whether they elect to euthanize their pets earlier on in the process. Expenses usually range anywhere between $50 to $1,000 a week (or more), depending on which medications and modalities are employed.


There is no known mode of prevention for this tragically aggressive disease.


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Moore PF, Affolter VK, Vernau W. Canine hemophagocytic histiocytic sarcoma: a proliferative disorder of CD11d+ macrophages. Vet Pathol. 2006 Sep;43(5):632-45.

Rosin A. Moore P, Dubizlzig R. Malignant histiocytosis in Bernese Mountain Dogs. J Am Vet Med Assoc 1988: 188:1041-1045. Wellmain ML, Davenport DJ, Morton D. Jacobs RM. Malignant histiocytosis in four dogs. J Am Vet Med Assoc 1985; 187: 919-921.