EQUINE VETERINARY EDUCATION / AE / JANUARY 2017
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tumour accessibility. Acknowledging these factors is critical when formulating decisions regarding treatment protocol and surgical approach. For such cases, histopathology of the tumour is frequently only performed after treatment-based surgery has been undertaken if gross appearance of the mass is consistent with common tumour types. Whilst the concern of metastasis is important to both veterinarian and owner, the possibility of local tumour recurrence is also a critical question. Histopathological assessment of surgeon cut edges/margins of surgically removed masses may be informative if histopathological analysis is undertaken post surgery. Consideration of all of these factors may in many cases lead to an individual diagnostic approach to equine genital masses.
Squamous cell carcinomas (SCC) Squamous cell carcinoma is the most common neoplasm of equine external genitalia, with Brinsko (1998) and van den Top et al. (2008) reporting incidences of 49–82.5%. Male genital SCC predominantly affects older horses and most studies document an average age range of 17.4–19.8 years (Strafuss 1976; Howarth et al. 1991; Mair et al. 2000; van den Top et al. 2008). The veterinary literature does not conclusively support any specific breed predilection for genital SCC but ponies have been frequently highly represented (Howarth et al. 1991; Mair et al. 2000; van den Top et al. 2008). It has also been proposed that breeds with nonpigmented genitalia may be at higher risk for development of SCCs (Schumacher 2006). Papillomaviruses in man have long been implicated in the aetiology of SCC development and recent work has identified a number of variants of Equus caballus papillomavirus 2 (EcPV2) within the tissue of equine penile papillomas, penile intraepithelial neoplasia (PIN) and SCCs (Scase et al. 2010; Bogaert et al. 2012; Lange et al. 2013; Newkirk et al. 2014; Van den Top et al. 2015; Zhu et al. 2015). Histological evaluation of equine penile tumours frequently shows papillomas undergoing a spectrum of changes as part of a continuum of transition to SCC (Van den Top et al. 2011). Genital SCCs in horses can have a number of different
gross appearances depending on the stage of disease. Early lesions include depigmented plaques (Fig 1), irregularities of the penile or preputial surface and nonhealing erosions, with or without accompanying granulation tissue (Van den Top et al. 2011). More advanced lesions can appear as solid masses and may have a typical cauliflower-like appearance or contain necrotic areas. Owners often notice SCCs incidentally during micturition but associated clinical signs can include dysuria, preputial oedema, or sanguineous/purulent discharge secondary to infection or tissue necrosis. Other reported abnormalities are wide-based stance, frequent protrusion of the penis, excoriation of the genital integument and changes in gait (Van den Top et al. 2011). Squamous cell carcinomas are malignant tumours but tend to be slow to metastasise, although it should be noted that pulmonary or skeletal metastases may occur in advanced cases (Cramer et al. 2011; Nelson et al. 2015). Histologically, squamous cell carcinomas, including those
of the penis and prepuce, tend to have a very characteristic appearance, with clusters of neoplastic cells exhibiting varying degrees of keratinisation, prominent nuclei often with conspicuous nucleoli and frequently prominent
Fig 1: Squamous cell carcinoma. Arrow indicates the raised plaque-like appearance of the lesion.
Fig 2: Photomicrograph of an equine squamous cell carcinoma. Arrow indicates a focus of keratinisation within a cluster of neoplastic cells. Scale bar indicates 100 microns. Haematoxylin and eosin stain.
mitotic figures (Cramer et al. 2011) (Fig 2). Equine SCCs are frequently infiltrated by CD3+ T lymphocytes, CD79+ B lymphocytes, IgG+ plasma cells and macrophages (Perez et al. 1999).
Papillomas Papillomas (warts) tend to occur on the nose, distal limbs and external genitalia. They are the most common tumours in young horses, age 1–3 years (Scott and Miller 2011). However, papillomas on the external genitalia of male horses tend to affect older horses and the published mean age range is 16.2–18 years (Howarth et al. 1991; Gardiner et al. 2008; van
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