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The following is from The Merck Veterinary Manual. See Integumentary System: Tumors Of The Skin And Soft Tissues: Lymphocytic, Histiocytic, and Related Cutaneous Tumors: Tumors with Histiocytic Differentiation
The histiocytoses of Bernese Mountain Dogs are systemic, familial disorders of unknown etiology with two manifestations—a more indolent and generally cutaneous form known as systemic histiocytosis and a more aggressive form in which skin lesions are rare, known as malignant histiocytosis. Malignant histiocytosis has been infrequently identified in other canine breeds. In systemic histiocytosis, males (mean age at onset 4 yr) are affected more often than females. There are multiple cutaneous nodules, papules, and plaques involving the skin (especially of the scrotum), nasal mucosa, and eyelids. The lesions are poorly circumscribed and variably alopecic and may be ulcerated; they develop in waves and slowly regress, only to recur several months later. The clinical disease tends to become more severe with each new wave of eruptions. Although the skin is the primary target organ, lesions may also develop in other organs, including lymph nodes, spleen, and bone marrow. The disease may be episodic in its clinical presentation, but it is progressive and eventually fatal.
Malignant histiocytosis occurs in male Bernese Mountain Dogs (mean age at onset 7 yr) and, less frequently, in other canine breeds. The lungs, lymph nodes, and liver are the most common organs affected, and the disease tends to spare the skin. Grossly, the lesions appear as large, solitary, firm masses that may efface large portions of affected internal organs. It is rapidly progressive and does not wax and wane as does systemic histiocytosis. Few dogs survive more than 6 months.
Various chemotherapeutic regimens have been used to treat both forms. Bovine thymosin fraction 5 may be of benefit in inducing remissions, especially in the systemic form. However, both forms of the disease are ultimately fatal.The Merck Veterinary Manual