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EQUINE VETERINARY EDUCATION
Equine vet. Educ. (2020) 32 (1) 8-11 doi: 10.1111/eve.12919
Case Report
Unilateral malignant melanoma of the adrenal gland with metastatic spread to the thorax in a horse
M. H. Hablolvarid Department of Pathology, Razi Vaccine and Serum Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran Corresponding author emails:
hablolvarid@yahoo.com;
h.hablolvarid@rvsri.ac.ir
Keywords: horse; adrenal gland; immunohistochemistry; malignant melanoma; thoracic metastasis
Summary A 7-year-old grey horse that had a history of inappetence, weight loss, weakness, dyspnoea, coughing and intermittent colic was examined post-mortem. A melanoma, confirmed histologically, was found in the right adrenal gland and had eroded into the posterior vena cava. Thoracic metastases were also identified. The skin, including the legs, trunk and perineum was normal with no evidence of melanomas. This was considered to be a primary, malignant melanoma, a novel finding in the adrenal gland of the horse.
Introduction
Melanocytic neoplasms have been reported to account for 4– 19% of the total number of equine cutaneous tumours (Seltenhmmer et al. 2004). Melanomas are common, slow- growing skin tumours, most commonly seen in grey horses (Johnson 1998). Many reports have estimated that as many as 80% of grey horses older than 15 years of age will develop dermal melanomas (Johnson 1998; MacGillivary et al. 2002; Moore et al. 2013; Phillips and Lembcke 2013). Although, the majority of melanomas behave as benign tumours, many of these histologically benign cutaneous melanomas may eventually metastasise and, therefore, should be considered as potentially malignant (MacGillivary et al. 2002; Metcalf et al. 2013). MacGillivary et al. (2002) reported the prevalence of metastatic melanoma to be approximately 14% in 101 horses which were diagnosed with cutaneous melanoma; however, this was a referral population and this prevalence rate is unlikely to be the same in the general horse population. There are several individual case reports that describe malignant melanoma located in different regions, such as the thorax (Milne 1986; Murray et al. 1997), retrobulbar space (Sweeney and Beech 1983), the third and lower eyelids (Rizk 2012; Albanese et al. 2015), foot (Honnas et al. 1990; Floyd 2003), vertebrae (Traver et al. 1977; Schott et al. 1990), peripheral nerves and skeletal muscles (Kirker-Head et al. 1985), and the oesophagus (Caston and Fales-Williams 2010). The purpose of this report is to describe a case of apparent primary melanoma in the adrenal gland with metastatic spread, in a young horse.
Case history
A 7-year-old male horse had been purchased 2 years previously from a local farm. It was normal on clinical examination. Insidious weight loss was noticed 11 months after arrival of the horse. One month later, obvious
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inappetence and weakness were evident, at which time the horse was referred for further examinations and treatment. On presentation, the heart and respiratory rates and rectal temperature were normal. The volume, colour and frequency of urination were normal, but the faeces appeared dry. The horse was treated symptomatically and returned to the herd. However, after a further month, he was presented again for inappetence, weakness, respiratory distress and coughing. On thoracic auscultation, the lung sounds were muffled in the dorso-cranial region, and tachypnoea and tachycardia were evident. The horse was not pyrexic. He was treated again symptomatically with antibiotics and corticosteroids, and his daily exercise programme was halted. Within 4 months after the onset of the clinical disease, despite the above treatments, the horse continued to lose weight and became weaker; the coughing and dyspnoea worsened. In addition, mild to moderate, intermittent colic developed. Terminally, the horse developed severe respiratory distress and he subsequently died.
Post-mortem and histopathology findings
Immediately after death, the horse was submitted for post- mortem examination. Grossly, the skin, including the most common sites of suspected primary melanoma (perineal skin; base of tail; external genitalia; limbs and trunk), was normal. In the abdominal cavity, a large darkly pigmented mass, 8 9 6 9 3 cm, surrounded the anterior pole of the right kidney, in the region of the right adrenal gland (Figs 1 and 2), and a large thrombus, 6 9 5 9 4 cm, extended from the mass into the posterior vena cava (Fig 1). In the thorax bilaterally from the thoracic inlet to the level of the 7th ribs, scattered firm dark red/black masses were present on the parietal pleura (Fig 3). Also, a round, black mass, 3 9 2 9 2 cm, was found adjacent to the thoracic aorta. The remaining organs including lymph nodes, liver, spleen, kidney, lung, eyes, guttural pouch and salivary glands were all grossly normal. Samples of different organs including the adrenal gland,
liver, spleen, kidney, lung, aortic lymph node and pleural metastases were collected for histology and immuno- histochemistry (IHC). Bleached control tissues were also included for assessing the effect of bleaching on melanin tissue antigen. Potassium permanganate was used in the bleaching method. S-100 protein (S-100, clone rabbit anti human1), melanoma marker HMB-45 (Anti-melanosome, clone HMB451), chromogranin A (Chromogranin A, clone DAK-A31) and synaptophysin (Synaptophysin, clone 27G122) were used in IHC staining.
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