Bit & Bridle

Equine Squamous Cell Carcinoma (SCC)

Other Names

  • SCC
  • Squamous Cell Carcinoma in Horses
  • Equine Skin Cancer (Squamous Cell)
  • Penile Squamous Cell Carcinoma (when location-specific)
  • Ocular Squamous Cell Carcinoma (when location-specific)

What is Equine Squamous Cell Carcinoma?

Equine Squamous Cell Carcinoma (SCC) is the most common malignant tumour of horses and the second most common equine tumour overall, after sarcoids. It arises from the squamous epithelial cells — the flat cells that line many of the body’s surfaces, including skin, mucous membranes, and the junction zones between different tissue types. SCC can occur almost anywhere on the horse’s body but has several high-risk anatomical sites that account for the majority of clinical cases.

Unlike some other equine tumours, SCC has the capacity to invade locally into adjacent tissues and, in some cases, to spread (metastasise) to regional lymph nodes and distant sites. Early diagnosis and treatment are therefore critical to achieving a favourable outcome. Advanced, widely invasive SCC carries a poor prognosis and may necessitate euthanasia.

The most common sites for equine SCC include:

  • The eye and periocular tissues (eyelids, nictitating membrane, limbus) — particularly in horses with limited periocular pigmentation
  • The external genitalia — penis, prepuce (sheath), and vulvar region
  • The mucocutaneous junctions — sites where skin meets mucous membranes, including the lips, nostrils, and anus
  • The stomach (non-glandular squamous mucosa) — gastric SCC, though internal and not clinically obvious until advanced
  • The mouth and pharynx

Causes

The aetiology of equine SCC is multifactorial, but several key risk factors have been identified:

  • Ultraviolet (UV) light exposure: A major risk factor, particularly for ocular and periocular SCC. Horses lacking melanin pigmentation around the eyes — particularly appaloosas, paints, and horses with white or pink periocular skin — have significantly higher rates of ocular SCC.
  • Equine Papillomavirus (EcPV): Several equine papillomaviruses, particularly EcPV-2, have been strongly associated with genital SCC in horses. The virus appears to transform squamous cells in the genital region in a manner analogous to HPV-associated SCC in humans.
  • Lack of pigmentation: Depigmented or pink-skinned areas at mucocutaneous junctions are inherently more vulnerable to both UV damage and malignant transformation.
  • Smegma accumulation: In male horses, chronic accumulation of smegma (a mixture of secretions and shed cells) in the sheath is associated with penile and preputial SCC, possibly through chronic irritation and carcinogen exposure.
  • Breed predisposition: Certain breeds — Appaloosas, Belgians, Clydesdales, and draught horses with unpigmented periocular regions — are disproportionately represented in SCC case series.
  • Age: SCC is more common in older horses, typically presenting in animals over 10 years of age.

Symptoms and Signs

Clinical signs depend on the affected site:

Ocular and periocular SCC:

  • Raised, pink, irregular, or proliferative lesions on the eyelids, third eyelid (nictitating membrane), or around the corneal margin (limbus)
  • Excessive lacrimation (watering eye)
  • Blepharospasm (squinting or keeping the eye partially closed)
  • Corneal cloudiness or neovascularisation adjacent to a limbal lesion
  • Bloodshot appearance of the conjunctiva

Genital SCC:

  • Raised, irregular, ulcerated, or cauliflower-like growths on the penis, prepuce, or vulva
  • Discoloured, malodorous, or blood-tinged discharge from the sheath or vulva
  • Difficulty urinating or changes in urinary habits in male horses with extensive preputial/penile involvement
  • Reluctance to extend the penis for urination

Gastric SCC:

  • Weight loss
  • Recurrent colic
  • Dysphagia (if the cardia is involved)
  • Signs may not appear until disease is advanced

Diagnosis

Diagnosis is confirmed by biopsy and histopathology, which is essential to differentiate SCC from other proliferative lesions:

  • Biopsy and histopathology: Small tissue samples are collected from lesions under local anaesthesia and submitted for microscopic examination. This confirms the diagnosis, characterises the tumour (well-differentiated vs. poorly differentiated), and assesses margins where excision has been performed.
  • Clinical examination: Visual and tactile assessment of lesion characteristics — size, surface texture, local invasion, ulceration, and associated lymph node enlargement.
  • Cytology (FNA): Fine needle aspirate from the lesion or regional lymph nodes can provide rapid preliminary information about cell type and metastatic spread.
  • Imaging: Radiography or ultrasonography to assess local invasion into bone or deep tissues; CT available at referral centres for complex periocular or pharyngeal cases.
  • Endoscopy: For suspected gastric SCC — gastroscopy allows visualisation of the stomach lining and targeted biopsy.

Assessment of regional lymph nodes (submandibular, inguinal, pre-scrotal) for enlargement is essential to evaluate the extent of disease spread at presentation.

Treatment

Multiple treatment modalities are available, and combination approaches are often the most effective:

  • Surgical excision: Wide surgical removal of the tumour with adequate margins is the gold standard for localised SCC. For periocular SCC, this may involve removal of the nictitating membrane, partial eyelid resection, or, in advanced cases, enucleation (removal of the eye). For penile SCC, partial or total penile amputation (phallectomy) may be required.
  • Cryotherapy: Application of liquid nitrogen to freeze and destroy tumour tissue. Effective for small, superficial lesions. Can be used alone or following debulking surgery.
  • Hyperthermia: Radio-frequency heat treatment applied to the tumour — an effective treatment for periocular SCC, particularly in combination with surgical cytoreduction.
  • Photodynamic therapy (PDT): A photosensitising agent is applied to the tumour and activated by specific wavelengths of light. Increasingly available at specialist equine referral centres.
  • Intra-lesional chemotherapy: Cisplatin in oily emulsion or slow-release beads injected directly into the tumour. Effective for small to medium-sized lesions and can be used post-surgically to treat incompletely excised margins.
  • Radiation therapy: Available at specialist equine oncology centres; effective for periocular SCC. Brachytherapy (implanted radiation sources) or external beam radiation may be used.
  • Piroxicam: An NSAID that has anti-tumour properties against SCC via COX-2 inhibition. Sometimes used as an adjunct to local treatment.

Treatment decisions should always be made by a specialist with experience in equine oncology, taking into account tumour size, location, degree of local invasion, and the horse’s overall health.

Prevention

Preventive measures focus on reducing UV exposure and managing known risk factors:

  • Apply UV-protective fly masks with solid or UV-filtering lenses for horses with unpigmented periocular skin, particularly in bright climates
  • Stable horses with high ocular SCC risk during peak UV hours (10am–3pm) in sunny weather
  • Apply equine-safe sunscreen to depigmented periocular skin and muzzle areas
  • Clean the sheath of male horses regularly to remove smegma accumulation
  • Perform regular visual examination of the genitalia, eyes, and mucocutaneous junctions, particularly in horses over 10 years of age
  • Castrate male horses with preputial or penile SCC to reduce the hormonal environment that may promote tumour growth

Prognosis

Prognosis depends heavily on the tumour location, size, degree of local invasion, and whether metastasis has occurred. Early, small, well-circumscribed lesions treated with appropriate modalities carry a good to excellent prognosis — many horses achieve complete remission. Large, deeply invasive, or metastatic tumours carry a guarded to poor prognosis, and treatment may be palliative rather than curative. Regular monitoring post-treatment is essential as local recurrence is common, particularly with incomplete surgical margins. Gastric SCC and SCC involving the pharynx or oesophagus typically carry the worst prognosis due to late detection and anatomical constraints on treatment.

When to Call the Vet

Contact your vet promptly if you notice:

  • Any new or changing growth on your horse’s eyelids, conjunctiva, or around the eye
  • A watering, squinting, or bloodshot eye that does not resolve within a few days
  • Unusual growths, ulceration, or discharge from the sheath, penis, or vulvar area
  • Any proliferative skin lesion at a mucocutaneous junction that fails to heal or is growing
  • Unexplained weight loss combined with colic in an older horse — consider gastric SCC

Frequently Asked Questions

Is equine SCC the same as human skin cancer?

Equine SCC and human cutaneous SCC share the same cell of origin (squamous epithelial cells), similar histological appearance, and some common risk factors including UV exposure. However, they are distinct diseases in different species. The role of equine papillomavirus in genital SCC in horses is analogous to HPV’s role in human genital SCC, reflecting a shared mechanism across species. The disease is not transmissible between horses and humans.

Can SCC spread to other parts of the horse’s body?

Yes, SCC has metastatic potential, though local invasion is typically more of a problem than distant spread in most equine cases. Metastasis to regional lymph nodes is the most common form of spread. Distant metastasis to the lungs, liver, or other organs is possible in advanced or poorly differentiated tumours but is less frequently documented. Lymph node assessment at diagnosis is essential to stage the disease appropriately.

Are certain colours of horse more at risk?

Yes. Grey horses, horses with extensive white markings, Appaloosas, and horses with pink (unpigmented) skin around the eyes, muzzle, and genitalia are at significantly higher risk. Melanin pigment provides protection against UV-induced DNA damage; its absence leaves tissues vulnerable. Darker-pigmented horses are not immune to SCC but are at substantially lower risk for UV-related sites.

How often should high-risk horses be examined for SCC?

High-risk horses — older animals, those with unpigmented periocular or genital skin, and breeds with known predisposition — should have a thorough examination of high-risk sites at every veterinary visit, and owners should perform monthly self-checks of the eyes, sheath, and mucocutaneous junctions. Annual veterinary examination including sheath cleaning and ocular assessment is the minimum recommended. Earlier intervention consistently leads to better outcomes. See also: Equine Panniculitis for a differential diagnosis in nodular skin conditions and Appaloosa breed information regarding their heightened SCC risk.