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: [email protected]
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
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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
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