20-year-old quarter horse (Equus ferus caballus) gelding.This 20-year-old gelding horse had been part of a small herd that was used for antibody production against various antigens. It suddenly presented with muddy-red fluid discharge from the prepuce and physical examination revealed a large, proliferative mass within the sheath that was bleeding and friable. The animal was anesthetized for a more thorough exploration of the mass. The mass was determined to be deep within the prepuce, palpable up to, and involving, the body wall. Due to the location and infiltrative nature of the mass, resection was not possible and the animal was euthanized. A full necropsy was not performed; only the penis and prepuce was removed for post mortem examination.

This animal was part of a research project conducted under an IACUC approved protocol in compliance with the Animal Welfare Act, PHS Policy, and other federal statutes and regulations relating to animals and experiments involving animals. The facility where this research was conducted is accredited by the Association for Assessment and Accreditation of Laboratory Animal Care, International and adheres to principles stated in the Guide for the Care and Use of Laboratory Animals, National Research Council, 2011.

Gross Description:  

Extending from the base of the glans penis, involving the surrounding prepuce, and extending deep to the body wall is a 13x13x12 cm friable, proliferative, ulcerated mass. 

Histopathologic Description:

Prepuce: Multifocally, effacing and replacing normal tissue architecture, affecting approximately 70% of the tissue section, extending to cut borders, is an unencapsulated, infiltrative, well-demarcated, moderately cellular neoplasm composed of polygonal cells arranged in nests and packets, often surrounding variably sized areas composed of keratin, admixed with degenerate neutrophils, necrotic cellular debris, sloughed, often necrotic, neoplastic cells, and hemorrhage, on a moderately dense desmoplastic stroma. Neoplastic cells have distinct cell borders, moderate to abundant amounts of brightly eosinophilic cytoplasm, a round to oval nucleus with finely stippled chromatin, and up to 4 variably distinct nucleoli. The mitotic rate is brisk, with up to 5 mitotic figures in some high-powered fields. Anisocytosis and anisokaryosis are marked. Neoplastic cells at the periphery of nests and packets occasionally surround concentric lamellations of brightly eosinophilic material (keratin pearls), and often exhibit dyskeratosis and single cell necrosis. Multifocally, neoplastic cells are found within expanded lumens of lymphatics. There are multifocal aggregates of lymphocytes and plasma cells that infiltrate the neoplasm and surrounding fibrovascular connective tissue. The surrounding connective tissue is edematous with dilated lymphatics.

Morphologic Diagnosis:  

Prepuce: Squamous cell carcinoma.


Squamous cell carcinoma

Contributor Comment:  

Squamous cell carcinoma (SCC) is the most common genital malignant tumor in horses and is considered a classic neoplastic lesion in the study of veterinary pathology.(1,5) SCCs are epithelial neoplasms that arise from keratinocytes, and often develop at mucocutaneous junctions, such as the eyelids, perineum, anus, and external genitalia in stallions and geldings.(2,3,4,9) Gross lesions are often proliferative and may exhibit ulceration, hemorrhage, necrosis, and infection.(2,3) Histologically, neoplastic cells are usually well-differentiated, keratinization is almost always present, and the neoplasm is frequently infiltrated by neutrophils and/or eosinophils.(3) Historically, potential promoters such as ultraviolet light, chronic inflammation, and smegma accumulation have been associated with development of the neoplasm, although the role of these elements remains unclear.(5)

There are numerous recent reports that reveal increasing evidence to support a potential causal relationship between equine papillomavirus type 2 (EcPV2) and equine penile squamous cell carcinoma (SCC).(1,4,5,8) Studies have suggested that equine penile papillomas, in situ carcinomas, and invasive carcinomas belong to a continuum of papilloma-induced disease, which is supported by detection of EcPV2 DNA in horses with characteristic SCC lesions, and in various lesions of the penis in a proportional number of control cases.(6) When comparing the levels of EcPV2 viral load in equine SCC lesions, penile non-SCC, or precursor disease lesions, and tissues without observable lesions from SCC-prone sites on clinically normal horses, EcPV2 DNA was present significantly more often, and in higher copy numbers, in the equine penile SCC lesions than the others. The presence of EcPV2 DNA has also been demonstrated in anal lesions, a lymph node, and contact metastases.(1) Additionally, in one report, viral mRNA was detected in all examined EcPV2 DNA positive lesions, while only 2.6% of specimens from healthy horses had detectable mRNA.(7) This finding provides evidence of intralesional viral transcriptional activity which further supports an active role of the virus in equine SCC disease.(9)

Note: Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the U.S. Army.

JPC Diagnosis:  

Prepuce: Squamous cell carcinoma.

Conference Comment:  

Conference discussion included a review of neoplastic features of malignancy observed within this neoplasm, including marked anisocytosis and anisokaryosis, invasion of lymphatics, elevated mitotic rate, infiltrative pattern, and desmoplasia. As noted by the contributor, additional features included dyskeratosis, areas of necrosis, infiltration of a mixed population of inflammatory cells as well as few areas of mineralization. Although the specific anatomic location could not be determined histologically in this case, participants briefly discussed common locations for SCC in horses, including the urogenital area, ocular/periocular regions, and the stratified squamous portion of the stomach. Locations for squamous cell carcinoma in other species include the nares, pinnae and lips of white cats, the vulva of sheep and goats and the nonglandular stomach of rodents and pigs.

In addition to horses, squamous cell carcinomas of the eyelid epithelium occur in other species, most commonly cattle, cats, and dogs. Squamous cell carcinomas develop through a stepwise process, often consisting of precancerous stages and the formation of a papilloma. Tumor development begins with initiation, which is an irreversible genetic change typically caused by solar radiation. The next step is promotion, which includes the growth of genetically altered or initiated cells within a favorable environment. Promotion is followed by progression, which results in increasing malignancy of the developing tumor and involves both genetic and epigenetic changes to tumor cells.(7) Ultraviolet radiation induced squamous cell carcinoma of the eyelid occurs most commonly in Hereford cattle which have nonpigmented eyelids.(7) The ultraviolet radiation induces DNA damage and subsequent mutation; of the three wavelengths of ultraviolet light, UVB is thought to be most associated with development of cutaneous neoplasms. The energy in UV light absorbed is by DNA and results in covalent crosslinking of pyrimidine bases and the formation of pyrimidine dimers, preventing proper base pairing. It is postulated that nucleotide excision repair mechanisms are overwhelmed with continued UV exposure, which may result in the propagation of cells with genomic mutations.(6)

As mentioned by the contributor, recent evidence supports an association between equine penile squamous cell carcinoma and equine papillomavirus 2 (EcPV2). The expression of two proteins, E6 and E7, has been demonstrated in the human papillomavirus associated with cervical cancer, which interact with proteins associated with cell cycle regulation.(10) Zhu et al. demonstrated that a subset of equine penile squamous cell carcinomas contained the E6/E7 nucleic acid of EcPV2, and a majority of neoplastic cells contained virus, providing additional evidence for the role of EcPV2 in penile and preputial SCCs in horses. Additionally, in that study the E6/E7 oncogenes of EcPV2 were present in metastatic SCCs. There was also evidence of solar damage in cases of penile and preputial SCC not associated with EcPV2.(10)


1. Bogaert L, Willemsen A, Vanderstraeten E, et al. EcPV2 DNA in equine genital squamous cell carcinomas and normal genital mucosa. Vet Microbiol. 2012;158:33-41.

2. Brinsko SP, Blanchard TL, Varner DD. Male Reproductive Disorders. In: Smith BP, ed., Large Animal Internal Medicine, 4th ed. St. Louis, MO:Mosby Elsevier; 2009;1475-1476. 

3. Foster RA, Ladds PW. Neoplasms of the penis and prepuce. In: Maxie MG, ed., Jubb, Kennedy and Palmers Pathology of Domestic Animals, Vol 3. 5th ed. Philadelphia, PA:WB Saunders Co; 2007:617-619. 

4. Knight C, Dunowska M, Munday JS, Peters-Kennedy J, Rosa BV. Comparison of the Levels of Equus caballus papillomavirus type 2 (EcPV-2) DNA in equine squamous cell carcinomas and non-cancerous tissues using quantitative PCR. Vet Microbiol. 2013;166:257-262.

5. Knight CG, Munday JS, Peters J, Dunowska M. Equine penile squamous cell carcinomas are associated with the presence of equine papillomavirus type 2 DNA Sequences.  Vet Pathol.  2011;48:1190-4.

6. Kumar V, Abbas AK, Aster JC. Pathologic basis of disease. 9th ed. Philadelphia, PA: Elsevier Saunders, 2015: 324-325.

7. Kusewitt DF. Neoplasia and Tumor Biology. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO: Mosby Elsevier; 2012:298-299, 315.

8. Lange CE, Tobler K, Lehner A, et al. EcPV2 DNA in equine papillomas and in situ and invasive squamous cell carcinomas supports papillomavirus etiology. Vet Pathol. 2012;50:686-692.

9. Sykora S, Samek L, Schonthaler K, et al. EcPV-2 is transcriptionally active in equine SCC but only rarely detectable in swabs and semen from healthy horses.  Vet Microbiol. 2012;158:194-198.

10. Zhu KW, Affolter VK, Gaynor AM, Dela Cruz FN, Pesavento PA. Equine genital squamous cell carcinoma: In situ hybridization identifies a distinct subset containing Equus caballus papillomavirus 2. Vet Pathol. Online first May 12, 2015:1-6.

Click the slide to view.

1-1. Prepuce

1-2. Prepuce

1-3. Prepuce

1-4. Prepuce

Back | VP Home | Contact Us |