Abstract
A 15 year-old grey Thoroughbred gelding presented for investigation of chronic weight loss and recent onset of respiratory difficulty. Clinical examination confirmed tachypnoea with increased respiratory effort. Thoracic ultrasound examination detected pleural effusion. The dyspnoea was related to the large volume of pleural effusion and, following post-mortem examination, to the presence of a large mediastinal mass. Multiple pigmented masses, likely melanomas, were detected peri-anally. Thoracic radiography, cytological examination of the pleural fluid and a fine needle aspirate of a thoracic mass led to a presumptive diagnosis of malignant melanoma and this was confirmed at post mortem examination. Further metastatic spread to the central nervous system and right guttural pouch was also identified. In conclusion this case manifests the potential malignant behaviour of equine melanomas, and a review of proposed therapies for melanoma treatment highlights the therapeutic options and current areas of research.
Highlights
Melanin is a pigment produced by melanocytes in the basal layer of the epidermis
The horse described in this report had multiple small perianal masses consistent with the benign melanomas reported in approximately 80% of aged grey horses [6]
Equine melanomas were classified based on their growth patterns into three groups: benign melanomas which grow slowly for years without metastasis, malignant transformation of a previously benign melanoma and malignant melanoma from the outset [2]
Summary
Melanin is a pigment produced by melanocytes in the basal layer of the epidermis. It is proposed that, in grey horses, a disturbance in melanin metabolism and transfer associated with progressive greying of the hair, due to increasing age, results in intra-cellular accumulation of pigment. Pigmented peri-anal masses, likely melanomas, had been noted since purchase three years ago. Thoracic ultrasonography revealed a large volume of hypoechoic fluid present in the pleural cavity to the level of the tuber ischii bilaterally. Fine needle aspiration of the mass was performed and the darkly pigmented tissue obtained submitted for cytological examination along with a sample of the pleural fluid. Cytological analysis reported the pleural fluid to be a modified transudate; large cells with abundant foamy cytoplasm containing material consistent with pigment (likely melanophages) were seen on cytospin examination (Figure 2). The fine needle aspirate smears revealed melanocytes with multiple cytological criteria of malignancy including variable pigmentation, multinucleation, moderateto-marked anisocytosis and anisokaryosis, variable nucleus:cytoplasmic ratio and nuclear moulding (Figure 3). Numerous tumour cell emboli were noted in lymphatics within the interlobular septae of the lung This confirmed the diagnosis of malignant melanoma
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