belgian horses
Squamous cell carcinoma (SCC) is the most common ocular cancer in horses. Belgians appear to be at a higher risk of developing SCC than some other breeds.

To confirm a genetic link, Kelly Knickelbein, VMD, an ophthalmology resident at the University of California, Davis (UC Davis) Veterinary Medical Teaching Hospital, studied ocular SCC in Belgians and presented her findings at the 2018 American Association of Equine Practitioners Convention, held Dec. 1-5 in San Francisco, California.

Squamous cell carcinoma tumors around the eye most commonly affect Haflingers and Belgians but are also reported in Clydesdales and other draft breeds, Quarter Horses, Appaloosas, Thoroughbreds, and others. Based on retrospective studies, said Knickelbein, chestnut horses appear to be at higher risk of developing ocular SCC than horses with other coat colors. She said researchers speculate this is because their coats lack a photoprotective pigment that prevents DNA damage associated with UV exposure.

In a recent study of Haflingers, Rebecca Bellone, PhD, and Mary Lassaline, DVM, PhD, also from UC Davis, identified a genetic risk factor for ocular SCC within that breed. It occurs in a variant of a gene called damage-specific DNA binding protein 2 (DDB2), which codes for a protein involved in DNA repair following ultraviolet (UV) damage. They found that nearly 80% of affected Haflingers were homozygous (had two copies) for the DDB2 genetic variant. So Knickelbein and colleagues investigated whether having the DDB2 variant or having a chestnut coat were risk factors for ocular SCC in Belgians.

Knickelbein said the results of the study show that Belgians have the same genetic risk factor for SCC as do Haflingers. Belgian horses homozygous for the DDB2 variant are four times more likely to develop ocular SCC than those that are not homozygous.

“There was a trend that homozygous horses developed SCC at a younger age,” she added, “but it was not statistically significant.”

Knickelbein recommended that owners of Belgians have their horses genetically tested (a genetic risk test is available at the UC Davis Veterinary Genetics Laboratory) and use that information to guide breeding and management decisions. Additionally, she recommended stabling horses during peak sunlight hours and outfitting them with UV-protective fly masks to help reduce ocular SCC risk. She also urged owners of high-risk horses to have their veterinarians perform frequent eye exams.