Congenital Stationary Night Blindness*

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. The variant affecting Tennessee Walking Horses 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. These horses should be evaluated by a veterinary ophthalmologist to confirm diagnosis and discuss management strategies. A recent study published in 2023 evaluated horses from different breeds and detected the CSNB2 allele in additional horse breeds. The highest allel frequencies have been found in pacing Standardbreds (17%, with 3% expected to be affected) and Missouri Fox Trotting Horses (8,4%, with 0,71% expected to be affected).