EQUINE VETERINARY EDUCATION Equine vet. Educ. (2019) 31 (8) 403-409 doi: 10.1111/eve.12815


Case Report Primary corneal malignant melanoma in a horse R. A. Strauss†


, R. A. Allbaugh†, J. Haynes‡ and G. Ben-Shlomo†*


†Department of Veterinary Clinical Sciences; and ‡Department of Veterinary Pathology, Iowa State University College of Veterinary Medicine, Ames, USA. *Corresponding author email: gil@iastate.edu


Keywords: horse; malignant melanoma; superficial keratectomy; strontium-90 beta irradiation; mitomycin C


Summary A 13-year-old Missouri Fox Trotter gelding of chestnut colour was referred for a 6-week history of blepharospasm and epiphora of the left eye. Due to the presence of irregular corneal masses, superficial keratectomy was performed along with adjunctive strontium-90 beta irradiation and subsequent topical mitomycin C chemotherapy. A diagnosis of poorly melanised malignant melanoma was made based on histopathological examination and immunohistochemistry. There has been no recurrence of the neoplasm over 10 months of follow-up. To the authors’ knowledge, this is the first documented case of a primary corneal melanocytic neoplasm in a horse.


Introduction


Melanocytic tumours, which can be malignant or benign, are neoplasms originating from melanocytes. These tumours have been described in many domestic species, including horses. The most frequently diagnosed equine melanocytic tumours are those affecting the skin in older grey horses (Valentine 1995; Giuliano 2010; Phillips and Lembcke 2013). In contrast, equine ocular melanocytic neoplasms are rare, but have been described in adnexal, orbital, epibulbar and intraocular tissues (Ramadan 1975; Murphy and Young 1979; Moore et al. 2000; McMullen et al. 2008). In horses, neoplastic extension into the cornea has been described from conjunctival malignant melanoma (Moore et al. 2000) and epibulbar malignant melanoma (McMullen et al. 2008). To the authors’ knowledge, there are no confirmed equine cases of primary corneal melanocytic tumours. One case report in 1975 described a primary ocular melanoma in a horse with possible corneal origin; however, due to the advanced state of the tumour infiltrating most of the globe, the origin could not be confirmed (Ramadan 1975). Primary melanocytic neoplasms of the cornea are rare in


all species, but have been reported in human patients (Naseri et al. 2001; Romaniuk et al. 2002; Uc


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et al. 2012) and one dog (Bauer et al. 2015), and can be either pigmented or amelanotic with variable clinical appearance. These tumours are often treated with surgical excision by superficial keratectomy; adjunctive treatment, such as topical chemotherapy, can be administered to help decrease recurrence (Naseri et al. 2001; Uc


et al. 2012). Metastasis is rare, but recurrence and local invasion requiring enucleation is reported (Uc


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et al. 2012). The equine cornea can be affected with other


neoplasms that have the potential for local invasion and metastasis, with the most common being squamous cell


carcinoma (SCC) (Lavach and Severin 1977). Corneal SCC typically appears as raised, white-pink, fleshy lesions or as nonraised lesions infiltrating the corneal stroma (Clode 2011). Definitive diagnosis is made via histopathological evaluation of biopsy specimens; characteristic features of SCC include sheets, cords and whorls of malignantly transformed epithelial cells, intercellular bridges, individual cell keratinisation and keratin pearl formation (Grahn et al. 2013). Other corneal tumours that have been reported in horses include vascular tumours, mast cell tumours and lymphosarcoma (Clode 2011). Surgical excision of corneal tumours with an adjunctive therapeutic modality tends to be the most effective treatment to eliminate the tumour and prevent recurrence (King et al. 1991; Clode 2011). Adjunctive therapies that have been reported for equine ocular neoplasia include cryosurgery, topical chemotherapy, radiofrequency hyperthermia, carbon dioxide laser ablation and beta irradiation (English et al.1990; King et al. 1991; Schoster 1992; Rayner and Van Zyl 2006; Plummer et al. 2007; Malalana et al. 2010; Clode et al. 2012). In this report, we describe a case of a primary corneal


malignant melanoma in a horse that was diagnosed based on histopathology and immunohistochemistry, and was treated with surgical excision, adjunctive strontium-90 beta irradiation, and subsequent mitomycin C topical chemotherapy.


Case presentation


History and initial examination A 13-year-old Missouri Fox Trotter gelding of chestnut colour was referred to the Iowa State University Lloyd Veterinary Medical Center (ISU-LVMC) for a 6-week history of blepharospasm and epiphora of the left eye (OS). The horse was initially treated by the referring veterinarian for suspected corneal scar tissue with a topical steroid, and a few weeks later was treated for a superficial corneal ulcer with a topical antibiotic. The corneal ulcer healed with treatment, but blepharospasm and epiphora persisted. A complete eye examination including rebound


tonometry (TonoVet)1, slit-lamp biomicroscopy and indirect ophthalmoscopy was performed. On initial examination, the horse had mild blepharospasm and epiphora OS. Intraocular pressures were 19 and 26 mmHg in the right eye (OD) and OS, respectively. No fluorescein stain uptake was noted on either eye. Ophthalmic examination OD was unremarkable. Slit-lamp biomicroscopy OS revealed two adjacent irregular pale pink corneal masses rising minimally from the dorsal corneal surface (Fig 1). The masses were approximately 4 mm and 5 mm in diameter. Superficial corneal


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