Squamous Cell Carcinoma in Horses: Risk Factors, Treatment & Prognosis

Squamous cell carcinoma (SCC) in horses is one of the most common forms of equine cancer. Originating in the squamous epithelial cells that form the outermost layer of the skin and line various internal surfaces, this condition can develop in a wide range of locations on the body.

Squamous cell carcinoma is particularly concerning because it can be aggressive, spreading to surrounding tissues and other areas of the horse’s body if left untreated.

While UV light exposure to unpigmented skin is a major risk factor, SCC is not limited to external locations. Factors such as chronic skin irritation, poor hygiene, viral infections, and genetic predisposition also play roles in its development.

Recognizing symptoms early, such as unusual growths or persistent sores, and seeking prompt veterinary care are essential for managing outcomes. With appropriate preventive measures, including UV protection and regular health monitoring, owners and caretakers can reduce the risk of SCC in their horses.

Squamous Cell Carcinoma in Horses

Squamous cell carcinoma (SCC) is a type of cancerous tumor that affects horses and originates in the squamous epithelial cells. These cells make up the outermost layer of many tissues, including: ^[1]

• Skin
• Respiratory tract
• Digestive tract
• Blood vessels

SCC is associated with UV exposure to areas of skin that are not pigmented, but also occurs in other parts of the horse’s body. ^[2] Left untreated, SCC can be fatal. ^[3] Horses suspected of having SCC require prompt veterinary attention.

Locations on the horse’s body that are susceptible to SCC include: ^{[4][5][6][7][8][9][10]}

• Skin
• Tissues in and around the eye
• Cornea
• Nasal passage or sinus
• Hard palate
• Pharynx/Larynx (throat)
• Perianal/Anal tissue
• Ear canal
• Hoof
• Tongue
• Guttural pouches
• Bladder and surrounding structures
• Head
• Genitals
• Tips of the ears
• Upper respiratory tract
• Lower urinary tract
• Upper digestive tract (mouth, esophagus, stomach)

Squamous Cell Function & Cancer Pathogenesis

SCC develops from squamous epithelial cells. These scaly cells are flat and thin, forming a tight pattern of closely fitted polygons. Squamous epithelial cells make up the outermost layer of the horse’s skin and line various surfaces inside the body, especially areas where skin meets mucous membrane such as around the mouth, nostrils, eyes, and genitalia. ^[1]

The main function of squamous cells is protection. They prevent the body from losing too much moisture through evaporation and they provide a physical barrier between the tissue they line and outside forces. ^[1]

When a horse develops SCC, the cells mutate gradually from normal tissue into a tumor. As these mutations occur, the tumor goes through a series of developmental stages in a predictable pattern. ^[5] This allows veterinarians and laboratories to investigate how far the tumor has progressed (i.e. staging), which provides insight into potential outcomes and treatment options. ^[2]

Squamous cell carcinoma in horses is typically staged using a three-level system, with stages 1 through 3 indicating increasing tumor severity.

Table 1. SCC tumor grading ^[2][5]

Veterinarians use this grading system to guide treatment options for affected horses and predict possible outcomes. ^[2] The likelihood of metastasis (i.e. the cancer spreading) and poor outcomes increase with higher grades. ^[2]

Clinical Signs

In horses, early squamous cell carcinoma (SCC) lesions usually appear as persistent white plaques, raised nodules, warty growths, or non-healing sores. ^[3][8][11] Unlike ordinary wounds, they do not resolve on their own. ^[3]

As the tumor progresses, it often forms a firm mass with a characteristic cauliflower-like surface. ^[3] The tissue may ulcerate and become black or necrotic. ^[3][8] Some lesions develop into deep, crater-shaped defects, whereas others enlarge into bumpy, wart-like structures. ^[8]

Without treatment the tumor can invade surrounding tissues and metastasize. ^[11] Clinical signs vary with location: for example, stallions or geldings with genital SCC may strain to urinate or experience pain during mating. ^[3]

Causes

Like other forms of cancer, the causes of SCC have not been fully characterized, but genetic and environmental factors have been identified. ^[2][4][11]

One major contributing factor to SCC in horses is overexposure to UV light. ^[2][4] UV light has mutagenic properties, leading to changes in the genes that control cell growth and repair damaged DNA. These mutations allow cells to grow unchecked, which can result in tumors. ^[2]

Skin with no pigment is more vulnerable to these mutations because they absorb more UV light than skin with high concentrations of melanin, which disperses UV radiation. ^[2] Although the incidence of SCC on non-pigmented skin is much higher, pigmented skin can also be affected. ^[5]

Chronic inflammation is another factor in SCC. There appears to be a relationship between a high level of inflammatory markers and this condition. ^[2] There has also been a clear link made between SCC and equine papillomavirus (EPV-2), although not all horses with SCC test positive for EPV-2. ^[2][11]

Additionally, there appears to be a genetic component to this condition. Certain breeds and lineages have a higher prevalence of SCC. ^[2]

Health Factors

In addition to environmental and genetic factors, there are some predisposing health factors that influence the development of SCC in horses, including: ^[2][4][5]

• Disruption to enzymes involved in blood vessel formation
• Chronic keratosis
• Exposure to certain chemicals
• Chronic skin irritation
• Burns
• Poor wound healing
• Parasitic skin disease such as habronemiasis and onchocerciasis

In the case of SCC near the eyes, factors include: ^[5]

• Chronic conjunctivitis
• Chronic nasolacrimal duct obstruction
• Eyelid deformities

SCC is the most common type of tumor to affect the genitalia of male horses. Penile SCC tumors tend to be aggressive, especially if they develop in a young horse. ^[4] Female horses may also develop SCC in or around their genitalia. ^[4]

Penile SCC may be caused by a number of different factors, including: ^[2]

• Accumulation of smegma
• Chronic irritation
• Balanoposthitis (inflammation of the glans penis and prepuce)

Risk Factors

Squamous Cell Carcinoma is a common tumor found in horses. ^[2][7] Horses of all ages are at risk, but middle aged to older horses are at greater risk. ^[2][3][5][6]

SCC is most often associated with unpigmented skin, but can be found on any skin color or type and in other tissues. ^[2] Horses that have been chronically exposed to high levels of UV light are at greater risk. ^[2]

Additional risk factors for SCC in horses include: ^[3][6]

• Chronic skin irritation
• Castration
• Chronic inflammation
• Coat colour
• Poor genital hygiene
• Infectious agents
• Papilloma
• Trauma
• Burn scars
• Chronic non-healing wounds

Certain horse breeds are more likely to develop certain types of SCC tumors. Below is a breakdown of the types of SCC associated with at-risk breeds.

Table 2. Horse breed predisposition to types of squamous cell carcinoma. ^[3][4][6]

Severity

The severity of SCC depends on the characterization of the tumors, specifically their: ^[2]

• Location
• Grade
• Aggressiveness/Spread

Some SCC tumors are slow-growing and benign, while others are aggressive and grow quickly. ^[4]

Veterinarians perform a “staging workup” after diagnosing SCC in horses. This process determines what grade the tumor has reached. The higher the grade, the more severe the SCC. ^[2]

Grades 1 and 2 tumors are easier to treat and often have a better prognosis. Grade 3 tumors are more severe as they have spread to other tissues in the body (metastasized). ^[2]

Even with treatment, severe cases of SCC tend to recur. When SCC recurs or spreads through the body, the prognosis worsens, although treatment may still be an option in some instances. ^[2]

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Diagnosis

Early diagnosis and treatment create the best chance of a positive outcome in cases of equine SCC. ^[4] Horses that are suspected of having SCC require prompt veterinary attention.

Diagnosis of SCC localized to the outside of the body is usually based on a biopsy of the affected area. A sample of the tumor is sent for laboratory testing to confirm the presence of SCC cells. ^[4][8]

“If caught in its initial stages, Squamous Cell Carcinoma can often be successfully treated or managed. Horse owners should monitor for any unusual growths or changes in the skin, especially in at-risk breeds like Appaloosas and Paints, and consult their veterinarian promptly if they have concerns.”

— Dr. Jennifer Skaggs, DVM
Equine Veterinarian

Internal cases also require a tissue biopsy to confirm diagnosis. Localization of internal tumors is guided by diagnostic tools that may include: ^[6]

• Gastroscopy: A small camera is passed through the horse’s nostril into the stomach
• Rectal examination: The rectum and anus are palpated for masses
• Abdominocentesis: A sample of fluid is taken from the abdominal cavity and sent for testing
• Diagnostic imaging: Ultrasonography is the primary method of imaging used to localize SCC
• Blood tests: May indicate the presence of cancer markers and help rule out other conditions

Treatment & Prognosis

The preferred treatment for squamous cell carcinoma in horses is surgical removal of the tumor. The aim of excision is to remove all of the tumor tissue with clean, wide margins, typically including 1–2 cm of healthy tissue beyond the visible lesion. Taking wide margins tries to ensure any microscopic cancer cells in adjacent tissue are eliminated and reduces the risk of recurrence. ^[2][4][11]

In some cases, adjunct treatment may also be recommended. Possible options include: ^{[2][4][7][11][12]}

• Radiation therapy
• Chemotherapy
• Photodynamic therapy
• Cryosurgery
• Radiofrequency hyperthermia
• Nonsteroidal Anti-inflammatory Drugs (NSAIDs)

In cases of penile SCC, removal of part or all of the prepuce or the penis may be necessary. ^[3][12]

The prognosis for a horse with SCC is unpredictable. ^[5] Even with treatment, recurrence rates are high, particularly in cases of tumors near the eyes. ^[2] The prognosis is graver for horses where SCC has metastasized. ^[2]

Horses with SCC of the stomach have a poor prognosis. This form of the disease does not usually get diagnosed before it has advanced to significant metastasis. ^[6]

Prevention

Since the causes of SCC are not fully understood, prevention is limited. However, like other forms of cancer, there are environmental and management strategies that can reduce risks for at-risk horses.

Prevention strategies include:

• Limiting exposure to UV light: Providing shade in turnout areas and using protective gear like UV-blocking fly masks or sheets can help shield vulnerable areas, especially in horses with light-colored or non-pigmented skin. Adjusting turnout times to avoid peak UV radiation hours (10 a.m. to 4 p.m.) can further reduce risk.
• Using sun protection: Applying horse-safe sunscreen to depigmented or pink areas, or using UV protecting face masks, offers additional protection against UV damage.
• Monitoring for abnormal growths: Regular cleaning and inspection of the skin can help identify potential issues early.
• Maintaining hygiene: Keeping the genitals clean is an important consideration, particularly for male horses, as smegma buildup has been associated with SCC of the penis and prepuce. Routine cleaning with mild, vet-approved solutions can help minimize irritation and inflammation, and provides an opportunity to monitor for any changes.
• Reducing viral exposure: Adhering to robust biosecurity and quarantine protocols reduces transmission of viruses like EPV-2, which is associated with SCC in horses, as there is no vaccine currently available.
• Managing existing skin conditions: Chronic irritation is associated with SCC. Ensure pre-existing skin conditions like sweet itch and rain scald are managed proactively to avoid persistent skin irritation.

Finally, providing a well-balanced, forage-first diet supports overall equine health and wellness. Staying up-to-date with routine care including veterinary check-ups, vaccinations, deworming, dental floatation and farriery provide the best chance of early detection and optimal outcomes.

Frequently Asked Questions

Here are some frequently asked questions about squamous cell carcinoma (SCC) in horses:

Summary

Squamous cell carcinoma is the most common form of skin cancer in horses. SCC affects squamous epithelial cells, which are the outermost layer of cells in certain tissues including skin and mucous membranes.

• SCC is most commonly found in the skin, especially near mucous membranes such as the eyes, the mouth, and genitalia.
• SCC tumors are graded on a scale of 1 to 3. Each grade represents increasing seriousness of tumor development.
• Overexposure to UV light, especially on non-pigmented skin, is the greatest risk factor for developing SCC.
• Other risk factors include equine papilloma virus infection, chronic inflammation, trauma, and genetic predisposition.
• The preferred treatment is surgical removal of the tumor. Other treatments include radiotherapy, chemotherapy, and cryosurgery.
• Prognosis depends on the location and grade of the tumor and the overall health of the horse. Early detection ensures the best possible outcome.