Equine Health Update Issue 1 Volume 16 - Page 25

Howe and Hodges 2006). The incidence of equine MC-SCC in the male external genitalia has been reported to be higher than that associated with the female external genitalia (Valentine 2006). The mean age of horses with periorbital MC-SCC is 13 years (Mosunic et al. 2004; Valentine 2006), while older horses tend to develop genital MC-SCC (mean age 20 years) (Valentine 2006; van den Top et al. 2008b). Aetiology Although risk factors such as chronic ultraviolet radiation exposure and chronic keratosis probably contribute to equine MC-SCC development, a novel equine papillomavirus, termed equine caballus papillomavirus-2 (EcPV-2) has been recently identified as a likely aetiological agent for the development of equine MC-SCC (Scase 2007; Vanderstraeten et al. 2011). EcPV2 DNA was detected in 100% of ocular and genital MC-SCC lesions and genital papillomas, and ~50% each of ocular, penile and vulvovaginal swabs from healthy horses (Vanderstraeten et al. 2011). This suggests that EcPV2 infection is necessary but not sufficient to induce tumoural transformation, which is consistent with bovine papillomavirus infection in equine sarcoids and human papillomavirus (HPV) infection in cervical and anogenital cancer (Carr et al. 2001; Bogaert et al. 2007). Furthermore, identification of EcPV2DNAin healthy equine tissue indicates the existence of latent infection that will not progress to clinical lesions in Fig 1: Periorbital mucocutaneous squamous cell carcinoma in a 28-year-old Paint mare. most horses, which is comparable to that in man where the majority of the population undergoes subclinical HPV infection without progression to clinical disease (Koutsky et al. 1988). Diagnosis Differential diagnoses for equine MC-SCC include equine sarcoid, papilloma (especially of the genitalia Fig 2: Third eyelid mucocutaneous squamous cell carcinoma in a 21-year-old Thoroughbred gelding. Fig 3: Squamous cell carcinoma of nasal mucocutaneous junction in a 26-year-old Appaloosa gelding. Fig 4: Haematoxylin and eosin stained photomicrograph of mucocutaneous squamous cell carcinoma characterised by disorganised nests and trabeculae of squamous epithelial cells proliferating from the surface and invading the underlying dermis. Note the normal stratified squamous epithelium on the right side of the image (100¥ magnification). and third eyelid), mast cell tumour, exuberant granulation tissue, habronemiasis, phycomycosis, cutaneous lymphoma and melanoma. Definitive diagnosis requires submission of affected skin/ mucosa for histopathological examination in the form of a biopsy, although radiography and computed tomography may help differentiate paranasal sinus SCC from sinusitis (Kowalczyk et al. 2011). If possible, a • Volume 16 no 1 • March 2014 • 25