Squamous Cell Carcinoma
Squamous cell carcinoma (SCC) is the second most diagnosed tumor in horses. It can develop anywhere, but is usually identified in non-pigmented, or lightly pigmented, areas of the head and neck, eyes, and genitals.
Squamous cells are flat, thin cells that resemble fish scales. They have important roles in absorption, protection, and transportation of substances and are found in skin and mucous membranes. This includes the linings of organs such as the stomach, kidneys, and lungs.
Locally invasive and usually slow to spread, SCC may appear as open, scaly, or crusted areas and can progress to non-healing wounds and large tumors. Spread to other parts of the body, or metastasis, is common, especially to lymph nodes. Tumors in the gastrointestinal tract and on the penis are often highly malignant. Although it can affect horses of any age, SCC is most often reported in horses older than 10 years of age.
Chronic exposure to ultraviolet (UV) light is a well-known risk factor and accounts for the predisposition to SCC formation in non-pigmented skin that is regularly exposed to the sun. Another risk factor for SCC is chronic inflammation or irritation, such as a chronic wound. In recent years, researchers have established a link between equine papillomavirus-2 (EPV-2) and genital SCC lesions. There are also genetic links to the development of some SCC.
As with other types of cancer in horses, treatment for SCC usually consists of surgical excision with wide margins around the affected area. However, this is not always possible depending on the location of the tumor, and recurrence rates can be high even following apparently successful surgery. Alternative primary or supportive secondary treatment can include radiation therapy, chemotherapy, or photodynamic therapy.
Ocular Squamous Cell Carcinoma
Ocular SCC is the most common cancer of the equine eye. The tumor arises in the outermost layer of skin of the eyelids, conjunctival, or corneal cells. Tumors may grow rapidly and can spread to invade adjacent tissues, causing visual impairment and destruction of the eye.
Early detection of tumors is correlated with the most successful outcomes. Unfortunately, many tumors are large by the time they are noticed, especially if they are located on the third eyelid, which can be difficult to see. The longer the tumor is present, the more likely it will invade surrounding tissues, and the more difficult it becomes to treat.
Researchers at UC Davis, with support from the UC Davis Center for Equine Health (CEH), discovered a variant in the damage-specific DNA binding protein 2 (DDB2) gene and determined that it is a causal risk factor for ocular SCC in several breeds, including Haflingers, Belgians, and Rocky Mountain Horses. The function of this gene is to repair DNA damaged by UV light. Horses with two copies of the recessive variant (homozygous) are at increased risk of developing ocular SCC compared to horses with only one copy (heterozygous) or no copies of the variant. This variant does not explain all cases of ocular SCC, but it appears to be a major contributor in the identified breeds.
Owners can use the DNA test available through the UC Davis Veterinary Genetics Laboratory to identify horses at higher risk for ocular SCC, take precautions to protect them, and selecting mating pairs to breed horses that are at lower risk.
Horses homozygous for the variant should have routine eye exams so tumors are caught early. Additionally, high-risk horses should wear UV-protective fly masks and be stabled during peak sunlight hours. Breeding homozygotes and heterozygotes among or to each other should be avoided to reduce the chances of producing horses that have a high risk of developing SCC. The ideal mate in either case is a horse with no copies of the risk factor (N/N).
Treating Ocular SCC in CEH Teaching Herd Horse O'Linda
A routine eye examination identified abnormal tissue in the right eye of CEH’s 25-year-old Thoroughbred mare O’Linda. The tissue was removed by UC Davis veterinary ophthalmologists and a sample was sent to a laboratory for histological analysis. The surgery site was treated with photodynamic therapy. The diagnosis was confirmed as SCC. The surgery was successful, with clean margins, and O’Linda recovered well. UV light likely played a role in tumor development in this case. Testing at the UC Davis Veterinary Genetics Laboratory indicated that O'Linda does not have any copies of the SCC genetic variant. Recurrence is unlikely, but we make sure that she is not exposed to excessive UV light that could lead to new tumor formation.