Background: Equine sarcoid is a rare equine skin tumour seen in any age group of horses, usually in younger horses. Grossly, it appears as multinodular masses which may or may not ulcerate and pink to greyish white coloured. It was suggested that the papilloma virus is a causative organism along with predisposing factors like skin abrasions and wounds. The definitive diagnosis is based on histopathology and classified according to their gross appearance and clinical behavior. To find out the origin and proliferative nature, histochemical and immunohistochemical study was performed. The present communication reflects various patterns of sarcoid on histopathological examination and it was confirmed by histochemistry and immunohistochemistry. 
 
Methods: A four-year-old bay colt was presented to the Madras Veterinary College Teaching Hospital with nodular masses on the buccal cavity. Fine needle aspiration biopsy (FNAB), blood, serum and tumour biopsy materials were collected under local anesthesia. The collected samples were subjected to Leishman-Giemsa staining, haematological and biochemical analysis, histopathology, histochemical and immunohistochemical examination. 
 
Result: Histopathologically, there was mucosal hyperplasia and hyperkeratosis. The neoplastic cells were arranged as storiform or whorl-like pattern. Also, there was perpendicular orientation of fibroblasts towards the basement membrane (picket fence) at the dermo-epidermal junction, which was considered as characteristic feature of sarcoid. Histochemical examination with Picrosirius red revealed strong positivity characterised by deep red coloured mature collagen fibres. Immunohistochemically, the hyperplastic epithelial cells were positive for pan cytokeratin and fibroblast cells were strongly positive for vimentin. Based on the histopathology, histochemistry and immunohistochemistry, the condition was diagnosed as sarcoid which is rare in horses.

Equine Sarcoid-histomorphological, Histochemical and Immunohistochemical Studies in A Thoroughbred Horse