Equine penile squamous cell carcinoma

Equine penile squamous cell carcinoma

Murray Hazlett, Maria Spinato

Animal Health Laboratory, University of Guelph, Guelph, ON.

AHL Newsletter 2019;23(3):15-16.

A search of pathology records since 2007 yielded a total of 67 lesions involving the penis or prepuce that were considered benign (fibropapilloma, squamous papilloma) or malignant neoplasia (squamous cell carcinoma) (Table 1). As expected, the peak number of submissions was in late summer – likely the result of UV exposure on lightly pigmented mucosa, and perhaps increased fly activity spreading virus (Fig. 1). Smegma is also considered to be an irritant that may promote neoplastic transformation. AHL data shows that tumors of the male equine genital tract comprise 6.3% of all tumors reported in this species (78 of 1,240) since 2007; 55 of these were squamous cell carcinomas.

Table 1. Benign and malignant lesions of the equine penis or prepuce, 2007-2019.


Age (average y)

Number of cases

Time of year

(peak submission month)





Squamous cell carcinoma




Other tumors*




*melanoma, giant cell sarcoma

Number of cases of penile SCC (y axis) by month (x axis).

Figure 1. Number of cases of penile SCC (y axis) by month (x axis).

In a recent study, equine papilloma virus 2 DNA was identified in 45% of penile squamous cell carcinomas.1 A subsequent paper found virus in both papillomas and carcinomas and proposes that a transition may occur.2 Figure 2 shows this proposed continuum, starting with a squamous papilloma with probable viral cytopathic effect, and progressing to squamous cell carcinoma.

Differentiating between benign and malignant penile tumor histologically can be challenging for pathologists. Benign squamous papilloma is comprised of hyperplastic or papillated keratinized squamous epithelium overlying sparse fibrous stroma. The absence of stromal invasion by epithelial cells usually differentiates benign papilloma from squamous cell carcinoma. However, an intermediate stage exists: squamous cell carcinoma in situ refers to a tumor that contains sufficient cellular atypia to warrant a diagnosis of malignancy. Overt squamous cell carcinoma is associated with invasion of neoplastic squamous epithelial cells into the subjacent stroma. Metastatic spread to superficial and deep inguinal lymph nodes is reported in a small proportion of cases.   AHL

Equine penile squamous cell carcinoma.

Figure 2. Equine penile squamous cell carcinoma. A. Squamous papilloma showing what likely represents viral cytopathic effect (pale cells at ends of arrows). B. Lesion with large open-faced irregular nuclei = epithelial dysplasia progressing to carcinoma-in-situ. C. Squamous cell carcinoma with islands of invasive epithelial cells (arrows). These islands often keratinize centrally.


1. Knight CG et al, Equine penile squamous cell carcinomas are associated with the presence of equine papillomavirus type 2 DNA sequences. Vet Pathol 2011;48:1190-1194.

2. Lange CE et al, ECPV2 DNA in equine papillomas and in situ and invasive squamous cell carcinomas supports papillomavirus etiology.  Vet Pathol 2013;50:686-692.