Night blindness

Simply defined, Appaloosa-specific CSNB involves absent night vision that is genetic, present from birth (congenital), and non-progressive (stationary).

CSNB affects many species of animals including humans and horses. But researchers involved in The Appaloosa Project have discovered an important link in affected Appaloosa horses.

Horses with two dominant LP alleles, or in other words homozygous for the Leopard Complex gene (LP/LP), are affected while heterozygotes (Lp/lp) and true solids (lp/lp) are unaffected.1 A sire and a dam that are each heterozygous for LP (LP/lp typically having spotted coat patterns) have a 25 percent chance of producing an affected foal. Crossing two horses homozygous for LP (LP/LP) will result in an affected foal.

Symptoms: Your Appaloosa may be fine during daylight hours but become fearful, spooky and/or confused in low light conditions. Or, he may injure himself after dark.

Diagnosis: At this time, a DNA test for CSNB and/or LP is not available, although Appaloosa Project researchers have identified a candidate gene that they are currently investigating.2 Therefore at present, the only way to diagnose CSNB is by having a veterinary ophthalmologist test your Appaloosa’s retinal function using an electroretinogram (ERG). The ERG measures the transmission of electrical impulses from the rods (the retina’s photoreceptors that function best in the dark) to the bipolar cells (which you might consider as the next car in the train of structures that convert light into visual images).

Treatments: There is currently no treatment for CSNB. However since Appaloosas with CSNB have been that way since birth, they usually adapt to their condition. As an owner, you need to provide safe surroundings for your Appaloosa at night and when in low-light conditions such as trailers, arenas and barns. Educate yourself and others on how to handle and manage a night blind horse for the horse’s safety as well as your own. Some tips include using safe fencing, providing light in dark conditions, using your voice and touch to convey your presence, following predictable patterns, and not clipping your horse’s muzzle or eye whiskers, as they serve as feelers in the dark.

Editor’s note: Thanks to The Appaloosa Project team members for their help with this article. For more information, see www.appaloosaproject.info. Always consult your veterinarian for advice on diagnosis, treatment and questions about your Appaloosa’s health.
1. Sandmeyer L, Breaux CB, Archer S, Grahn BH (2007) Clinical and electroretinographic characteristics of congenital stationary night blindness in the Appaloosa and the association with the leopard complex. Vet Ophthalmol 10: 368–375.
2. Bellone RR, Brooks SA, Sandmeyer L, Murphy BA, Forsyth G, et al. (2008) Differential gene expression of TRPM1, the potential cause of congenital stationary night blindness and coat spotting patterns (LP) in the Appaloosa horse (Equus caballus). Genetics 179: 1861–1870.

Printed version in the January 2010 issue of Appaloosa Journal.
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