Ocular Squamous Cell Carcinoma in Specific Equine Breeds

Researchers found some horse breeds have a higher incidence of ocular squamous cell carcinoma.

A genetic aberration has been identified in Halflingers (pictured above), Belgian Draft horses, Rocky Mountain horses and to a lesser degree in Percherons and Appaloosas. iStock/Blanca Grueneberg

Squamous cell carcinoma is one of the more common cancers found in horse eyes and adnexa. Exposure to ultraviolet light is thought to amplify the risk of development of this carcinoma through damage to DNA as well as disruption of a tumor-suppressor protein, p53. Another genetic risk factor has to do with the DDB2 gene, which “binds to specific dimers to be able to recruit a protein complex that repairs UV damage.” The presence of p.Thr338Met interferes with binding of DDB2 to DNA with the end result that horses with this genetic code are more predisposed to developing ocular squamous cell carcinoma (SCC).

A study investigated horse breeds that tend to have a higher frequency of ocular SCC. [Crausaz, M.; Launois, T.; Smith-Fleming, K., et al. DDB2 Genetic Risk Factor for Ocular Squamous Cell Carcinoma Identified in Three Additional Horse Breeds. Genes Dec 2020, vol. 11, 1460; doi:10.3390/genes11121460].

This genetic aberration of the DDB2 gene has been identified in Haflingers, Belgian Draft horses, Rocky Mountain horses, and to a lesser degree in Percherons and Appaloosas. The study looked for the allele in three horses represented as Belgian Warmbloods, Connemara ponies and Holsteiners—genotyping confirmed DDB2 homozygosity in these three horses.

Over six years of horses presented to the Bailly Vétérinaires Clinique du Lys in France, the clinic identified an incidence of 17% of SCC in Connemara ponies, 17% in Haflingers and 17% in Selle Francais horses. 

At the University of Illinois Veterinary Teaching Hospital, the majority of SCC cases were seen in Paint horses (36%), Quarter Horses (18%) and Haflingers (6%). 

The authors stated: “Due to the presence of the risk allele in Holsteiner and Belgian Warmblood breeds, this variant may be an important genetic risk factor for ocular SCC in multiple warmblood breeds.”

The influence of hormones was also evaluated with the finding that 66.7% of ocular SCC cases are geldings. One proposal suggested that lack of circulating androgens and estrogens in geldings contributes to ocular SCC, yet this is not yet substantiated through scientific studies.

As yet, studies correlating non-pigmentation have not been done, but horses with chestnut color and lacking periocular pigmentation seem to have higher numbers affected with SCC.

In summary, the authors advised, “Establishing breed-specific recommendations with regard to genetic testing would allow for the best use of this genetic test for mate selection in order to decrease the incidence of disease and identify which horses are at highest risk of developing ocular SCC so they can be monitored for early detection, increasing the likelihood of a more favorable outcome.”

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