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Ocular Squamous Cell Carcinoma in Haflinger Horses

Introduction

Researchers at UC Davis School of Veterinary Medicine and the Veterinary Genetics Laboratory investigated SCC in the Haflinger breed and determined that a recessive mode of inheritance explains some of the genetic components involved in the development of this cancer. They also discovered a DNA marker that identifies horses at higher risk to develop limbal and/or third eye lid SCC.

Squamous cell carcinoma (SCC) is the second most common type of tumor in the horse and the most frequent tumor of the horse’s eye. Several factors thought to increase risk for this cancer include, among others, UV exposure, pigmentation and genetics. The Haflinger breed has a higher reported incidence for SCC of both the limbus and the third eye lid, indicating that genetic factors play a role in this breed. When originating at the limbus, the region where the cornea meets the white of the eye, SCC can spread into the cornea, and quickly lead to visual impairment and destruction of the eye. Horses homozygous for the risk factor (R/R) are 5.5 times more likely to develop ocular SCC than those with one copy (R/N) or no copies (N/N) of the risk factor. This risk factor does not explain all cases of ocular SCC but it appears to be a major contributor in Haflingers.

The VGL offers a DNA-based test for the only known genetic risk factor for SCC in horses. Owners and breeders of Haflingers can use the DNA test result to identify horses at higher risk and to assist in breeding pair selection. Homozygous horses (R/R) are advised to have routine eye exams performed by a veterinary ophthalmologist for early detection and better prognosis, and are advised to wear a UV protecting fly mask when out during the daylight hours. Breeding homozygotes (R/R) and heterozygotes (R/N) among or to each other should be avoided in order to reduce the chances of producing horses that have a high risk of developing this cancer. The ideal mate in either case is a horse with no copies of the risk factor (N/N).

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Results reported as:

N/N: No copies of the SCC-associated marker. Horse has a lower risk of developing limbal or third eye lid SCC.

N/R: 1 copy of the SCC-associated marker (heterozygous for risk). Horse has a lower risk of developing ocular SCC but is able to pass on the risk factor to 50% of the offspring.

R/R: 2 copies of the SCC-associated risk allele. Horse is about 5.5 times more likely to develop SCC in its lifetime when compared to N/R and N/N. Routine ophthalmological exams performed and use of UV-protecting fly masks during sun exposure are advised.

References:

Bellone, Rebecca R., Jiayin Liu, Jessica L. Petersen, Maura Mack, Moriel Singer-Berk, Cord Drögemüller, Julia Malvick, Barbara Wallner, Gottfried Brem, M. Cecilia Penedo, and Mary Lassaline. "A Missense Mutation in Damage-specific DNA Binding Protein 2 Is a Genetic Risk Factor for Limbal Squamous Cell Carcinoma in Horses." International Journal of Cancer (2017).

Lassaline, M., Cranford, T.L., Latimer, C.A., Bellone, R. (2015) Limbal squamous cell carcinoma in Haflinger horses. Veterinary Ophthalmology 18(5) 404-408. doi:10.1111/vop.12229.

 
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