Congenital Stationary Night Blindness (CSNB2)

Congenital stationary night blindness (CSNB) is an inherited condition in which affected individuals are unable to see in low light or dark conditions. Horses with CSNB are born with this condition, and it is non-progressive (it does not get worse with age). Some typical signs of CSNB include apprehension to enter unfamiliar places in dark conditions, having trouble finding feed or water buckets at night, or getting injured at night. Oftentimes CSNB in horses is undetected by the handler or owner. CSNB is definitively diagnosed by a dark-adapted electroretinography (ERG) test conducted by a veterinary ophthalmologist.

Similar to what is known in humans and other animals, there are likely several different genes that contribute to this disease in horses, and these genes are likely breed-specific. For example, horses homozygous for the variant that causes leopard complex spotting in Appaloosas and related breeds are night blind. However, other breeds without this white spotting pattern (e.g. Thoroughbred, Paso Fino, and Tennessee Walking Horse) have also been reported to have CSNB, and thus variants in different genes likely explain CSNB in these other breeds.

Researchers from the VGL's equine genetic research team (including Dr. Bellone and her undergraduate intern Izzie Hack) teamed up with equine ophthalmologist Dr. Brian Gilger and his residents, Drs. Oh and Crabtree, from North Carolina State to investigate the genetics of this disease in Tennessee Walking Horses. In 2019, a case study involving a night blind Tennessee Walking Horse identified a variant in a gene with a pivotal role in cell signaling low light vision conditions. This identified variant is therefore suspected to cause CSNB in this breed.

The variant follows a recessive mode of inheritance, meaning that only horses with two copies of the variant are affected. Based on population screening, it is estimated that one in a hundred Tennessee Walking Horses are homozygous for this variant and thus likely night blind. Genetic testing for this variant can help to identify these horses. Horses homozygous for this variant should be evaluated by a veterinary ophthalmologist to confirm diagnosis and discuss management strategies. Testing can now be performed to also identify horses with the variant and avoid matings between carriers, which could produce affected foals.

The scientific study describing the identification of the genetic variant for CSNB in the Tennessee Walking Horse is available here. Recent research at the VGL confirmed this to be a causal variant in this and other breeds and, when the study is published, additional information will be added here.