Atypical histiocytosis in red squirrels (Sciurus vulgaris)

Four red squirrels were necropsied over three years as part of a broader surveillance study. They presented with cutaneous, subcutaneous and/or internal swellings and nodules that histopathologically consisted of sheets of atypical round cells and multinucleated giant cells. There was moderate anisokaryosis with rare mitoses. Nuclei ranged from oval to indented or C-shaped and some were bizarre, twisted or multilobulated. Many giant cells also had a bizarre morphology, with anisokaryosis within individual cells. Giant cell nuclei were also often multilobulated, ring-shaped or segmented. Affected internal organs varied depending on the squirrel but included lymph node, kidney, intestinal tract and lungs. Representative lesions from each of the four squirrels were negative for acid fast organisms. Formalin fixed tissues from all four squirrels and ethanol fixed tissue from one were PCR negative for Mycobacterium. Immunohistochemically, the mononuclear and multinucleated giant cells were strongly immunoreactive to antibody against vimentin and the vast majority were strongly immunoreactive to antibody against MHCII in all four squirrels. Otherwise, the atypical mononuclear and multinucleated cells were negative for CD3, Pax-5, Mac387, CD18 and E-cadherin. Based on the combination of cellular morphology, arrangement and immunophenotype, a novel form of atypical histiocytosis is considered most likely in these squirrels, although the exact origin and triggering factors remain uncertain.