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EPIDERMAL INCLUSION CYST IN MARE – A CASE REPORT

Epidermal inclusion cysts (EIC) are small formations developed out of ectodermal tissue. They are most commonly formed under the surface of the skin inside the nasal diverticulum of horses. A 2-year-old grey Lipizzaner mare was admitted with a swelling in the region of the right nostril. A rhinoscopic examination revealed an ellipsoid cyst, 4 cm in diameter, of soft elastic consistency in the dorsolateral aspect of the nasal diverticulum. A fine needle aspiration (FNA) was performed and a small amount of yellow-green content was aspirated. Cell structures with square, angular to irregularly polygonal pink cells of about 20–30 micrometres in diameter with rarely visible small hyperchromatic nuclei were noted. The cells were consistent with highly keratinised cells and were surrounded by multiple rod-shaped bacteria and a moderate number of erythrocytes. Cytological findings indicated EIC. The cyst was surgically extirpated during standing sedation and an infraorbital nerve block. The cyst was approached from the outside, but the skin of external nares was also aseptically prepared. As a common complication, the diverticulum was penetrated during surgery. The skin and subcutis were closed in the standard manner. A histologic examination revealed a cystic structure that expanded the subepithelial tissue and elevated the overlying epidermis. The cyst was lined by a thick wall of squamous epithelium that underwent gradual keratinisation and contained deeply basophilic keratohyalin granules within the stratum granulosum and melanin pigments throughout the basal layer. The cyst lumen was filled with laminations of keratin. On the basis of histological findings, the diagnosis of EIC was confirmed. Two years after the surgical procedure there were no signs of recurrence. The location of the pathological process in the present case was indicative of EIC ; however, due to the grey colour of the mare, it was paramount to rule out melanoma. This case emphasises the diagnostic value of FNA.

Epidermal cyst, Horse, EIC, FNA

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