Equine Health Update Issue 1 Volume 16 - Page 24

Review Article A review of equine mucocutaneous squamous cell carcinoma S. Taylor* and G. Haldorson† Veterinary Clinical Sciences, Purdue University, West Lafayette, Indiana, USA; and †Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington, USA. *Corresponding author email: taylo248@purdue.edu Keywords: horse; squamous; tumour; chemotherapy; photodynamic Summary Squamous cell carcinoma (SCC) accounts for approximately 20% of all equine mucocutaneous (MC) tumours and continues to present a therapeutic challenge to practitioners. Most MC-SCC are locally invasive and slow to metastasise, but metastasis to local lymph nodes is not uncommon. The most common location for MC-SCC is the periorbital region, with the eyelid most commonly affected. Although only 13% of MC-SCC involves the external genitalia, MC-SCC is the most common neoplasm of male genitalia. Equine caballus papillomavirus-2 has recently been linked to MC-SCC and may prove to be necessary for tumour development. Risk factors may include chronic exposure to ultraviolet light and chronic skin irritation. Horses developing genital MC-SCC tend to be older compared to those with periorbital MC-SCC. Histopathology is required for definitive diagnosis of MC-SCC, although horse phenotype and lesion location may suggest MC-SCC. Several treatment modalities have been successful in eliminating or managing MC-SCC, with surgical excision and intratumoural chemotherapy yielding the best results. Other treatment options including cryotherapy, hyperthermia, radiotherapy and photodynamic therapy are often used as adjunctive therapies. Early recognition of tumours and prompt intervention are associated with a positive outcome. Introduction Mucocutaneous squamous cell carcinoma (MC-SCC) is the second most common skin tumour of the horse (Valentine 2006; Theon et al. 2007), as well as the second most common equine tumour overall (Sundberg et al. 1977). Chronic high ultraviolet light challenge is considered a significant aetiological factor in squamous cell transformation and subsequent tumour development (Pazzi et al. 1996; Teifke and Lohr 1996). The nature of MC-SCC varies widely and ranges from slow growing, benign tumours to rapidly growing, highly malignant and invasive tumours. MC-SCC is locally invasive and typically 24 slow to metastasise to local lymph nodes, but the frequency of metastasis for MC-SCC is reported to be as high as 19% (Lavach and Severin 1977; King et al. 1991; van den Top et al. 2008a). Delayed local metastasis without recurrence of primary tumour has been reported following treatment (Elce et al. 2011). Early MC-SCC lesions are often small superficial nodules that may be covered with normal skin. As the epidermal layers are disrupted by the developing tumour, ulceration and malodorous necrosis may be observed. The superficial aspects of these proliferative tumours may become traumatised and infected with opportunistic pathogens. In contrast to slow growing, locally invasive MC tumours, penile forms in younger horses (age <8 years) tend to be aggressive and commonly metastasise to regional lymph nodes. Similarly, gastric SCC is highly malignant with a 68% rate of metastasis and a median survival time of 4 weeks from onset of clinical signs (Taylor et al. 2009). Clinical description The most common location of MC-SCC is the periorbital (ocular and adnexal) region, which constitutes up to 72% of all equine MC-SCC cases (Theon et al. 2007) (Fig 1). The most prevalent periorbital tumour location is the eyelid, followed by the third eyelid, cornea, limbus (corneo-scleral junction), conjunctiva and orbit (Cotchin 1977; Lavach and Severin 1977; Junge et al. 1984; Dugan et al. 1991; King et al. 1991; Mosunic et al. 2004) (Fig 2). Tumour location may also include the genitalia (13%), face and ear pinnae (5%) (Fig 3), perianal region (5%) and extremities (3%) (Theon et al. 2007). An increased prevalence of periorbital MC-SCC is reported in Paint Horses, Quarter Horses, Appaloosas and draught horses (Schwink 1987; King et al. 1991; Mosunic et al. 2004; Valentine 2006), while penile/preputial MC-SCC is most common in Quarter Horses and Appaloosas (Valentine 2006). Although the overall prevalence is low, the most common neoplasm of the male genitalia is MC-SCC (Brinsko 1998; van den Top et al. 2008a), and is thought to result, in part, from chronic keratosis due to chronic irritation (such as that afforded by smegma), although this is controversial (Plaut and Kohn-Speyer 1947; Van • Equine Health Update •