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EQUINE VETERINARY EDUCATION
Equine vet. Educ. (2022) 34 (11) 570-572 doi: 10.1111/eve.13610
Clinical Commentary Corneal neoplasia in horses
F. Malalana Institute of Infection, Veterinary and Ecological Sciences, University of Liverpool, Neston, UK Corresponding author email:
f.malalana@
liverpool.ac.uk
Keywords: horse; corneal neoplasia
Summary Primary corneal neoplasia in horses is rare. In most cases, tumours extend into the cornea from the adjacent limbus, sclera and/or conjunctiva with only a handful of cases described originating on the cornea. The most common tumour reported affecting the cornea is squamous cell carcinoma with other types of neoplasia described as sporadic case reports. This short clinical commentary accompanies and intends to complement a report of primary corneal lymphoma in a horse (Equine Vet Educ, 2022, https://
doi.org/10.1111/eve.13601).
Corneal neoplasia in horses
Primary corneal neoplasia is rarely encountered in equine practice. In the current issue of Equine Veterinary Education, Opgenorth and colleagues (2022) report a case of primary corneal lymphoma. Multiple cases of multicentric lymphoma affecting the eye have been described in horses (Martabano et al., 2019), but primary ocular lymphoma is rare. A previous case of corneal lymphoma has been reported in the literature (Vallone et al., 2016). Interestingly, in both of these cases of corneal primary lymphoma, there was a suspicion of malignant transformation following treatment of presumed immune-mediated keratitis with immunomodulating medications such as tacrolimus and cyclosporine. In contrast to the case described by Opgenorth and colleagues, where immunohistochemistry suggested a diagnosis of T-cell lymphoma, Vallone et al. (2016) documented B-cell lymphoma. Surgical intervention, in the case presented by Vallone et al. (2016) followed by adjunctive therapy with mitomycin-C and electronic surface brachytherapy also, appeared curative in both cases. Squamous cell carcinoma is the most common tumour
affecting the cornea in horses. Most of these arise from the conjunctiva or sclera, making up around 34%–37% of all ocular squamous cell carcinomas (Dugan et al., 1991a). Corneal squamous cell carcinomas typically appear as raised, pink-white masses with an irregular surface affecting mainly the corneal epithelium (Montgomery, 2014) (Fig 1). A second, less frequent form has also been described affecting the corneal stroma. These corneal stromal invasive squamous cell carcinomas appear clinically as smooth lesions, which grow directly into the corneal stroma and which may be confused with immune-mediated keratitis (Kafarnic et al., 2009). Diagnosis mostly relies on the characteristic clinical appearance. Confirmation is easily achieved by histopathological examination after excisional surgery. Ultraviolet (UV) radiation exposure is a well-documented
risk factor for the development of squamous cell carcinoma in many species, with the risk rising with increasing exposure
© 2022 EVJ Ltd.
Fig 1: Squamous cell carcinoma invading the cornea in a 16- year-old Haflinger mare.
to solar radiation and higher altitude (Dugan et al., 1991). The condition appears more frequently in breeds with the least periocular pigmentation such as Appaloosa and American Paint breeds, although horses with a chestnut coat colour and some heavy horse breeds, such as Clydesdale, with darkly pigmented periocular regions also show an increase in incidence (Dugan et al., 1991a; Lassaline, 2021). In addition, data suggest that Haflinger horses are also overrepresented. Recent genetic studies in Haflinger horses have identified an association between a mutation in a gene associated with repairing UV-irradiated DNA damage and limbal squamous cell carcinoma (Bellone et al., 2017). Sex has also been associated with a risk of SCC, with castrated males five times more likely to develop ocular/adnexal squamous cell carcinomas than stallions and two times more likely than females (Dugan et al., 1991). The management of ocular squamous cell carcinoma
remains challenging. Local infiltration is common, and metastasis rates vary between 6% and 18% (Kafarnic et al., 2009). This author routinely scopes the guttural pouches of horses presented for the management of ocular and periocular squamous cell carcinoma for the assessment of the regional lymphatic nodes prior to any treatment. However, in several studies, metastasis to distant sites was identified several months following the treatment of the primary ocular lesion, with no evidence of recurrence at the original site, confirming the importance of long-term monitoring of these cases (Elce et al., 2011; Mair et al., 2015). A large number of treatment options have been described, most commonly surgical excision via lamellar keratectomy (Ollivier et al., 2006) combined with adjunctive therapy
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