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
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