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Histiocytosis is the most prevalent cancer in Bernese Mountain Dogs and is a common cause of early death. Because it is inherited and there is no known treatment, detecting affected dogs and removing them from the gene pool is a top health priority for the BMD community.

Histiocytosis is a cancer in which histiocytes proliferate rapidly and invade a wide variety of tissues . Histiocytes are a type of white blood cell called macrophages which ordinarily form part of the dog's immune system. Their proper role is to engulf bacteria and other material which should not be present in the body and dispose of it. Histiocytosis is quite rare in other breeds with the exception of Flat Coated Retrievers, but it is the most common cancer in Bernese Mountain Dogs comprising 25% of all cases.

Two types of histiocytosis are discussed in the veterinary literature; malignant and systemic. It is unclear how closely related the two types are. It is possible that they are different forms of the same disease. Malignant histiocytosis is the more aggressive form and usually leads to death in a matter of weeks. It frequently involves the spleen, liver, and lymph nodes. It can also invade other tissues including the lungs and skin. Systemic histiocytosis can have episodes that come and go with varying severity. It invades the skin and peripheral lymph nodes in almost every case but also involves other tissues particularly in later stages of the disease. The appearance of the histiocytes themselves also differs between the two forms. Malignant histiocytes are abnormal. Neither form of histiocytosis should not be confused with benign histiocytomas. Benign histiocytomas are common in dogs. They form skin lumps that do not metastasize and sometimes go away by themselves.

Early symptoms of histiocytosis include depression, lethargy, loss of appetite and weight loss. With systemic histiocytosis skin abnormalities are common, particularly on the face and limbs. If the tumor has metastasized to the lungs there may be trouble breathing. Anemia is also common. X-rays may indicate internal masses and other abnormalities. Histopathology can help differentiate between the malignant and systemic forms. Malignant histiocytosis progresses rapidly and has usually metastasized by the time it is diagnosed. Systemic histiocytosis may under go several remissions or none at all before finally affecting internal organs. In all cases owners should observe their dogs quality of life closely to determine the appropriate time for euthanasia.

There are no successful treatments for histiocytosis at this time. Because the histiocytes metastasize easily and rapidly surgical removal of affected tissues is not effective. Chemotherapy and radiation therapy have been used with some short term success but they are not a cure. Care is generally limited to providing comfort until euthanasia is indicated.

Histiocytosis is known to be inherited in Bernese Mountain dogs. The mode of inheritance is polygenic meaning that many genes are involved. The disease often appears after a dog has been bred so without a conscientious effort by all breeders and owners it cannot be eliminated from the gene pool. Owners should report all cases of histiocytosis to their breeder and also participate in the ongoing research projects which may be found at the Bernergarde health studies page.