Squamous Cell Carcinomas in Horses

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Key takeaways

Squamous cell carcinoma (SCC) is a malignant skin cancer that arises from the superficial layer of the skin. 

  • SCC appears as a single, raised mass that often ulcerates and may bleed or as soft flesh-colored masses that mimic “proud flesh”
  • Although SCCs can arise anywhere on the skin, they are more frequently present in poorly pigmented and poorly haired areas (i.e., around the eyes, nose, lips, anus, and genitalia)
  • SCCs are typically diagnosed by cytology and biopsy of the mass
  • Treatment can include radiotherapy, chemotherapy, cryotherapy, and surgical removal
  • While the prognosis depends on location and how aggressive the tumor is, a better prognosis is achieved with prompt diagnosis and aggressive treatment
  • Preventive measures such as limiting exposure to ultraviolet (UV) light, especially in white-coated horses, may help prevent the development of SCC
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A closer look: Squamous Cell Carcinomas in Horses


SCCs often present as solitary, flat-to-raised masses on the skin, most commonly around the eyes, nose, lips, anus, and genitalia. These masses can also be:

  • Ulcerated (skin loss)
  • Red
  • Bloody
  • Crusty/scabbed
  • Flesh colored (appear similar to “proud flesh”)

Since SCCs often ulcerate, secondary infections are common. Secondary infections of these tumors may result in:

  • Foul smell
  • Oozing yellow discharge

Risk factors


SCCs are a common skin tumor in horses. As these tumors are malignant, and delayed treatment can result in long-term consequences, seeking veterinary consultation as soon as possible is advised. The best prognoses often follow prompt diagnosis and aggressive treatment.

Horses with white markings or white coats and older horses are at an increased risk. UV light exposure and chronic skin wounds are predisposing factors.

Possible causes


Similar to many cancers, the underlying cause of SCCs often cannot be determined.

Exposure to UV light is a risk factor for the development of SCCs, especially in nonpigmented (e.g., white coat or markings) horses.

Chronic wounds and burns are also a predisposing factor for SCC reported in horses.

Cutaneous papillomavirus infection may also develop into squamous cell carcinomas.

Main symptoms


Severity of symptoms depends on the location and invasiveness of the mass.

Rarely, these tumors are reported in the guttural pouch.

Testing and diagnosis


Diagnosis of SCCs is through physical examination, cytology, and often biopsy of the mass. Biopsy is especially important to determine how malignant the tumor is, and to guide treatment. Additional diagnostic imaging (X-ray, ultrasound, CT) may also be recommended.

Steps to Recovery


Treatment depends on the location of the tumor, but may include one or a combination of:

  • Surgical removal
  • Cryotherapy
  • Chemotherapy
  • Radiation therapy
  • Anti-inflammatories
  • Immunotherapy

As SCC is an invasive and malignant cancer, the condition persists and often worsens without aggressive treatment. Prognosis ultimately depends on location, how malignant the cancer is, and if the cancer has metastasized to other tissues. If the cancer is completely removed before metastasis, horses have a good prognosis. Tumors may recur even after tumor removal, so careful monitoring is required to identify any new masses early.

Tumors affecting the eyelids have a poorer prognosis, and are more likely to metastasize or invade the tissues surrounding the eye. In some cases, complete removal of the eye is required to control the spread.

Prevention


As the exposure to UV light is a risk factor in the development of SCCs, providing shaded areas for horses is an important preventive measure, especially in white-coated horses. SCCs are not contagious.

Are Squamous Cell Carcinomas in Horses common?


Although SCCs are uncommon overall, they are a common cancer of the skin in horses.

Typical Treatment


  • Cryotherapy
  • Radiation
  • Chemotherapy
  • Surgical removal
  • Anti-inflammatories
  • Immunotherapy

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