Signalment and clinical findings
Figure 4A & 4B. The nasal planum (4A) and scrotum (4B) of a 4-year-old intact male German shorthaired pointer with cutaneous
histiocytosis. Note the areas of depigmentation around the nares (4A; arrows). The lesions initially responded to immunosuppressive
therapy. (Photos courtesy of Dr. Karen Farver, DACVD.)
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Cutaneous histiocytosis is a proliferative disorder confined to the skin and subcutis with rare involvement of regional lymph
nodes. The clinical presentation is commonly described as multiple cutaneous nodules, crusts, or areas of depigmentation that
appear to wax and wane on the face, ears, nose, neck, trunk, extremities (including footpads), perineum, and scrotum (Figures 4A & 4B).6,18 Middle-aged to older dogs are commonly affected; no reported breed or sex predilection exists.6
Systemic histiocytosis is a disseminated form of cutaneous histiocytosis that involves the skin (high prevalence at mucocutaneous
junctions), ocular and nasal mucosa, and peripheral lymph nodes.6 Lesions can also develop in the lungs, spleen, liver, or bone marrow.18 The severity of the clinical signs, including weight loss, anorexia, conjunctivitis, and stertor, varies with the extent
of disease.16 Systemic histiocytosis was first reported in young to middle-aged male Bernese mountain dogs, and a polygenic mode of inheritance
has been proved.19 A breed predilection for systemic histiocytosis has also been documented in rottweilers and golden and Labrador retrievers.6,18
Pathogenesis
The cause of both cutaneous and systemic histiocytosis is unknown, but the prevailing hypothesis implicates dysregulation
of the proliferation, activation, and function of dendritic cells and T cells or their direct interactions after antigenic
stimulation.2,6 This theory is supported by the fact that some cases respond to immunosuppressive therapy. However, the specific antigenic
process leading to the immuno dysregulation has not been identified.6
Diagnosis
Cytologically, both cutaneous and systemic histiocytosis resemble granulomatous inflammation. Benign histiocytes possess abundant
cytoplasm, round to indented nuclei, and inconspicuous nucleoli. Variable numbers of inflammatory cells are present, while
multinucleate giant cells and erythrophagocytosis are seen infrequently.19
The histologic features of cutaneous and systemic histiocytosis are identical. Systemic histiocytosis is diagnosed based on
the involvement of organ systems in addition to the skin. Biopsy specimens contain numerous large, pale, round to oval histiocytes
with indented vesicular nuclei.18 Numerous neutrophils and lymphocytes are often present, with eosinophils and plasma cells seen less commonly. The lesions
are usually located perivascularly, with subsequent vascular invasion, thrombosis, and tissue necrosis frequently described.13,18,19
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