The Modern Equine Vet
October 2023
Vol 13 Issue 10 2023
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News Notes

Genetic Cause of Night Blindness Confirmed in Tennessee Walking Horses

By Liza Crissiuma Gershony

New collaborative research led by Rebecca Bellone, PhD, director of the UC Davis Veterinary Genetics Laboratory (VGL) provided further evidence that the CSNB2 allele causes congenital stationary night blindness (CSNB) in the Tennessee Walking Horse.

Researchers also detected the CSNB2 allele in 9 other horse breeds and confirmed that Standardbred and the Missouri Fox Trotting Horse who were homozygous for CSNB2 (those with 2 copies of the CSNB2 allele) also suffered night blindness, which refers to the inability to see in low-light conditions, such as at night or at dusk.

Affected horses may have difficulty finding feed buckets and navigating at night and are prone to injuring themselves. No treatment is available for CSNB, but affected horses can be managed by providing a light source to help them navigate at night.

The study built upon previous work that identified a single nucleotide polymorphism (SNP) in the glutamate metabotropic receptor 6 (GRM6) gene as a likely causal of CSNB in a Tennessee Walking Horse. This variant was called CSNB2 because it is the second variant identified to cause CSNB in horses; the first of which was LP.

The recent study confirmed that CSNB2 is inherited as an autosomal recessive trait, meaning that only homozygous horses (CSNB2/CSNB2) are affected by CSNB, and heterozygous or “carrier” horses (CSNB2/N) are not affected. The researchers performed ocular examinations including electroretinography (ERGs) on 3 CSNB2/CSNB2 homozygous horses and found all 3 to be affected by CSNB (one Standardbred, one Missouri Fox Trotting Horse, and an additional Tennessee Walking Horse). Additionally, 7 CSNB2/N heterozygous horses and 9 N/N horses underwent the same examinations and ERGs, and were determined to be unaffected by CSNB. These findings support 2 copies of CSNB2 are causative for CSNB in horses.

Researchers found the allele frequency of CSNB2 ranged from 0.0010 in Quarter Horses to 0.17 in pacing bred Standardbreds (Table 1). Overall, the highest allele frequency was found in pacing Standardbreds, with more than 1 in 4 (29%) pacing bred Standardbreds expected to be carriers of the CSNB2 allele, and 1 in 33 (3.0%) expected to be affected by CSNB. The CSNB2 was only identified in the pacing subpopulation of Standardbreds, and not in the trotting subpopulation.

“Many Standardbreds race at night. It is currently unknown if CSNB2 impacts racing performance. Therefore, routine testing for CSNB2 is recommended for pacing Standardbreds and careful management decisions need to be considered for those that test homozygous (CSNB2/CSNB2)” advised Dr. Bellone, a professor in the Department of Population Health of Reproduction and equine geneticist who has been studying the genetics of ocular disorders in horses for nearly 20 years.

This is the first report with clinical confirmation of CSNB in Standardbreds and Missouri Fox Trotting Horses, suggesting the condition has been significantly underdiagnosed.

“Previous studies have indicated that other breeds aside from LP spotted horses had night blindness, but this is the largest across breed study to investigate this disorder in horses and highlights the need for further study,” said Sara Thomasy, DVM, PhD, DACVOa professor of Comparative Ophthalmology at University of California, Davis. MeV

The study was supported by funding from the United States Trotting Association and the UC Davis Center for Equine Health. Affected horses should receive an ocular examination including electroretinography (ERG) performed by an ACVO board-certified veterinary ophthalmologist (https://www.acvo.org/ophthalmologist-search).

For more information:
Esdaile E, Knickelbein KE, Donnelly CG, et al. Additional evidence supports GRM6 p.Thr178met as a cause of congenital night blindness in three horse breeds. Vet Ophthalmol. 2023 Oct. 10.
https://onlinelibrary.wiley.com/doi/10.1111/vop.13151