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EQUINE VETERINARY EDUCATION / AE / JANUARY 2017


den Top et al. 2008). Both congenital and acquired papillomas have been previously reported (Scott and Miller 2011; White et al. 2004). Papillomas begin as small, approximately 1 mm diameter, raised, smooth, shiny grey to white papules (Van den Top et al. 2010) and can be present over the whole penis, although most appear over the glans, the urethral process and preputial fold (van den Top et al. 2008). Equine genital papillomas have been reported to progress to SCCs but there is also a report of widespread penile papillomatosis associated with EcPV-2 that remained clinically and histologically unchanged over a 2 year period (Knight et al. 2011). Fully developed papillomas are approximately 2–20 mm


in diameter and 5 mm in height, broad-based to pedunculated, grey, pink or white masses with hyperkeratotic frond-like projections (Scott and Miller 2011). Some examples show clear histological evidence of viral infection, namely cytopathic change and the presence of koilocytes, cells undergoing ballooning degeneration, which have eccentrically placed pyknotic nuclei. As a corollary of this observation, equine papillomavirus type 2 DNA has been amplified from equine penile papillomas (Knight et al. 2011; Lange et al. 2013).


Melanocytic tumours Melanomas have been reported in horses aged 2–29 years, with an average of 13 years (Howarth et al. 1991; Gardiner et al. 2008; van den Top et al. 2008). They typically affect all parts of the prepuce and penis other than the glans, with the prepuce listed as the third most common site of occurrence for equine melanocytic neoplasms (Ramos-Vara et al. 2014).


Melanocytic tumours are nodular in appearance (Fig 3) and firm on palpation. They may be solitary or multiple and can be positioned dermally or subdermally over intact or ulcerated skin (Phillips and Lembcke 2013). Several histopathological subtypes of equine cutaneous melanocytic proliferative lesions have been described but


with the exception of the melanocytic naevus, all should be considered to have malignant potential. Diagnosis of the majority of melanocytic tumours is frequently straightforward due to the heavy pigmentation characteristic of this tumour type (Fig 4). However, diagnosis of poorly pigmented or amelanotic examples may be challenging, particularly as these tumours may have a range of gross morphologies, which historically led to amelanotic melanoma in man being dubbed ‘the great masquerader’ (Koch and Lange 2000). These tumours may also exhibit a variable microscopic appearance. Consequently, immunohistochemical staining may be required for increased histopathological diagnostic confidence and, in this regard, it is notable that PNL2 has been suggested to be a sensitive immunohistochemical marker of equine melanocytic neoplasms that is more specific than S100 protein or PGP 9.5, both of which are used in the diagnosis of melanocytic neoplasms in man and other veterinary species (Ramos-Vara et al. 2014).


Other types of equine genital tumours Sarcoids are tumours of fibroblastic origin, usually with an overlying hyperplastic epidermal component. Bovine papillomaviruses (BPV) 1 and 2 have long been implicated in the development of equine sarcoids and BPV nucleic acid has recently been visualised in these tumours using in situ hybridisation, adding to the weight of evidence suggesting a causative association (Gaynor et al. 2016). Although sarcoids may occur at sites all over the body, many different types are described in the preputial and paragenital regions. Fibrosarcomas, a malignant proliferation of fibroblasts,


may also arise in the penile and preputial regions and are usually firm to fleshy infiltrative masses (Scott and Miller 2011). They are invasive and capable of metastasis. van den Top et al. (2008) have reported a fibrosarcoma within the prepuce of a horse. Other tumour types are uncommon findings on the penis and prepuce of horses but reports include lymphomas, lipomas, neurofibromas, adenocarcinomas, basal cell carcinomas and haemangiosarcomas (Van den Top et al. 2010). The case report by De Meyer et al. (2017) describes the rare finding of a preputial fibroma in an 11-year-old gelding. Other than the large size of this mass, there were no


Fig 3: Melanocytic tumour with a nodular macroscopic appearance.


© 2015 EVJ Ltd


Fig 4: Photomicrograph of an equine melanocytic tumour. In this example, the neoplastic cells are heavily pigmented, obscuring nuclear detail. Scale bar indicates 300 microns. Haematoxylin and eosin stain.


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