Histiocytic syndromes are a complex group of disorders which overlap. They result from an accumulation of histiocytes - cells which arise from the bone marrow and circulate throughout the body as part of the normal defense system. Cutaneous histiocytosis is a benign form of histiocytosis in which there are small skin lesions around the head and neck which may come and go, and may or may not respond to treatment. Dogs with systemic histiocytosis also have head and neck lesions, with lymph nodes affected as well. In malignant histiocytosis, skin masses are uncommon; instead, internal organs are affected (eg. spleen, liver, lungs, bone marrow).Unfortunately, malignant and systemic histiocytosis are characterized by rapid and invasive spread, with poor response to treatment.
The disorder has a polygenic mode of inheritance in the Bernese mountain dog. The disorder is also more common in the other breeds listed below.
In both systemic and malignant histiocytosis, signs of illness include loss of appetite, weight loss, and lethargy. In systemic histiocytosis, there are skin lesions (crusting, firm nodules, ulcers) but these are uncommon in malignant histiocytosis. Both affect middle-aged to older dogs, while cutaneous histiocytosis occurs in younger dogs. There may be periods of remission with systemic histiocytosis, but ultimately the histiocytes infiltrate other organ systems especially the lungs, liver, spleen, bone marrow, and lymph nodes. Malignant histiocytosis is a rapidly progressive and invasive disorder, which commonly metastasizes to the lungs.
The clinical signs of this disorder vary depending on where metastasis of abnormal cells occurs. Tumours in the lungs are common, causing respiratory signs such as coughing and shortness of breath. Your veterinarian will likely suspect a tumour based on clinical signs, radiology, and blood tests; he or she will submit tissue samples to a pathologist for histopathologic examination to determine the type of tumour.
Malignant histiocytosis spreads rapidly and metastasis is generally present at the time of diagnosis, so that surgical treatment is ineffective. Dogs with systemic histiocytosis may experience periods of remission, or the disease may progress rapidly. Various types of chemotherapy have been tried for both types of histiocytosis, but with little success. Your veterinarian will work with you to keep your dog as comfortable as possible, until the quality of life deteriorates to the point where euthanasia is the best treatment option.
CLINICAL: most common in Bernese mountain dogs; see anorexia, weight loss, lethargy, lymphadenopathy, lameness if joint or bony involvement, plus signs associated with metastasis (commonly respiratory, also gastrointestinal or nervous); with systemic histiocytosis, you will also see multiple cutaneous plaques, ulcers, nodules and papules, which can be anywhere but are seen primarily on face and limbs.
RADIOLOGY: commonly see thoracic abnormalities such as single or multiple masses, hilar lymphadenopathy, pleural effusion; may see hepato or splenomegaly.
LABORATORY: hematology may be normal, or there may be regenerative or non-regenerative anemia; may see elevated liver enzymes; fine needle aspirates of accessible masses or lymph nodes may show fairly normal cells with systemic histiocytosis (cf. malignant histiocytosis).
Because this invariably fatal disorder generally does not develop until the dog is middle-aged or older, it can be hard to identify parents that carry the trait. It is very important that the veterinarian and/or owner inform the breeder when this disorder has been diagnosed, so that he or she can modify the breeding programme accordingly, to limit the spread of the harmful gene(s) in the Bernese mountain dog population.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
Sargan DR. Histiocytosis. In IDID - Inherited diseases in dogs:web-based information for canine inherited disease genetics.
Selting K. Histiocytic diseases. In: Côté E, ed. Clinical Veterinary Advisor Dogs and Cats. Missouri: Mosby Elsevier, 2007:523-525.