Cytological description of splendore-hoeppli phenomenon in an actinomycetes mycetoma in the skin of a guinea pig

A 10-month-old male guinea pig was presented with an area of alopecia and pruritus on the back. This progressed over 6 months to an ulcerated, painful skin mass. The aim of this report was to illustrate the unique cytological appearance of the Splendore-Hoeppli (SH) reaction, which has rarely been described in veterinary medicine. The mass was sampled using a fine-needle aspiration technique for cytological examination and bacteriological culture. Following cytological diagnosis and bacteriological results, the mass was surgically excised and histopathological examination was performed. Cytological examination of the mass revealed pyogranulomatous inflammation with filamentous bacteria identified by culture to be Actinomyces spp. The bacteria were often coated with a moderate amount of red to blue staining material, interpreted to represent the SH reaction. Histopathological examination confirmed the presence of the Splendore-Hoeppli phenomenon (SHP). The appearance of SH in histological specimens is well described in veterinary medicine, but the cytological appearance is not well recognized. This report illustrates well the cytological description of the material which should be recognized as a part of the immune response to insult, and not be mistaken as foreign material.