Primary intraparotid peripheral nerve sheath tumour with characteristics of benign schwannoma in a horse.

Schwannomas arising in the parotid gland are rare in man and are not documented in domestic animals. This report describes the clinical, morphological and immunohistochemical findings of a benign schwannoma in the parotid gland of a 12-year-old warmblood gelding. A slow-growing mass was surgically excised from the parotid gland and did not recur within the following 6 months. The tumour was well circumscribed and was composed of densely packed spindle cells partly arranged in an Antoni A pattern and intermixed with hypocellular areas resembling the Antoni B pattern. Tumour cells expressed vimentin, S100 protein, glial fibrillary acidic protein and variably neuron-specific enolase. Less than 5% of the neoplastic cells were immunolabelled with the proliferation marker Ki67. Although malignant melanomas frequently involve the parotid gland in horses, schwannomas should be included in the differential diagnosis of parotid masses.