What is histiocytosis?
Histiocytosis is defined as a proliferation, sometimes cancerous, of the number of histiocytes, these cells of the immune system present in the connective tissue.
There are several types of histiocytosis in dogs:
- cutaneous histiocytosis, a simple proliferative disorder of histiocytes localized to the skin,
- systemic histiocytosis which affects the skin and other organs,
- malignant histiocytosis, a tumoral proliferation of histiocytes. To designate malignant histiocytosis, we also speak of histiocytic sarcoma.
How does histiocytosis manifest in dogs?
Signs of cutaneous and systemic histiocytosis
Cutaneous and systemic histioctyoses are manifested by the formation of plaques or nodules on the skin. These lesions, isolated or multiple, are sometimes alopecic (devoid of hair) and can ulcerate. They often disappear and then reappear. The lesions appear preferentially on the face, neck, scrotum, perineum and at the extremity of the limbs.
In case of systemic histiocytosis, the local lymph nodes can swell and the lesions can affect the eyes, the nasal cavity but also the spleen, liver, lung and bone marrow. The disease evolves chronically and irregularly.
Male Bernese Mountain Dogs are known to be predisposed to systemic histiocytosis but several other breeds such as Rottweiler, Golden Retriever, Belgian Shepherd, Poodle, Border Collie or even Labrador can be affected.Young dogs are more often affected.
Symptoms of malignant histiocytosis
There are two types of malignant histiocytosis:
- histiocytic hemophagocytic sarcoma which is characterized by the rapid appearance of an infiltrating mass, which destroys the underlying tissues, at the level of an extremity and generally close to a joint. It is of particular interest to older dogs.
- Disseminated dendritic histiocytic sarcoma which is mainly found in Bernese Mountain Dogs (80% of cases), Rottweilers and Retrievers (Golden, Labrador, Flat coat). The disseminated form is a highly aggressive form which is manifested by the formation of solid tumor masses in various organs such as the spleen, liver, lymph nodes, lungs and bone marrow. In affected dogs, the disease results in symptoms such as lethargy, weight loss, hyperthermia, respiratory signs (cough, dyspnea), lameness when bone is involved and seizures when of brain damage.Skin lesions, on the other hand, are infrequent.
How are histiocytosis diagnosed?
Your veterinarian may suspect the condition when examining your animal, in view of the clinical signs it presents, but the diagnosis of certainty is based on the histological examination of its tissues. To obtain samples of injured tissue, your veterinarian may recommend a biopsy or complete excision of the mass. The sample taken will then be prepared for examination by a specialized veterinary laboratory.
What treatments are available?
Systemic histiocytosis responds to immunosuppressive drugs in 50% of cases. These may be glucocorticoids used in high doses or other immune system modulators, such as cyclosporine or luflenomide. These treatments allow a definitive cure in some animals but others show systematic recurrences when treatment is stopped.
Malignant histiocytosis is treated by surgical excision or even amputation for highly infiltrating tumours. Chemotherapy does not seem to be effective for this condition and the prognosis for dogs with it is very poor.