Congenital Stationary Night Blindness (CSNB2)Quick Summary
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Phenotype: An inability to see in low light / dark conditions from birth. Signs of CSNB may include apprehension of unfamiliar places in low light conditions, having trouble finding feed or water buckets at night, or getting injured at night.
Mode of Inheritance: Autosomal recessive
Alleles: N = Normal, CSNB2 = Congenital stationary night blindness variant
Breeds appropriate for testing: Tennessee Walking Horse, Standardbred and Missouri Fox Trotter
Explanation of results:
- Horses with the N/N genotype do not have the genetic variant causing congenital stationary night blindness and cannot transmit the variant to any of their offspring.
- Horses with the N/CSNB2 genotype carry one copy. They will have normal vison but are predicted to transmit the variant to 50% of their offspring, and if mated to another carrier, there is a 25% chance the foal will have CSNB.
- Horses with the CSNB2/CSNB2 genotype have two copies of the genetic variant associated with congenital stationary night blindness and are likely night blind. Examination by a veterinary ophthalmologist is recommended. All offspring produced in matings with this genotype will have at least one copy of the variant.
Sample Collection
Horse DNA tests are carried out using cells from the roots of a hair sample (roughly 20-30 hairs).
1. Grab about 10 hairs at the base.
2. Wrap the hairs around your finger and give it a quick pull.
3. Check the ends to make sure the pulled hairs have roots.
4. Repeat the process until you have collected about 20-30 hairs with intact roots.
5. You can choose different places on the mane or tail. NOTE: For foals, we recommend pulling all hairs from the tail only.
6. Tape the hairs to the submission form and fold the form along the dotted line to protect the sample. Do not use ziploc bags as they can cause condensation that allows mold to grow on the hair.
7. Place the folded form containing the sample in a paper envelope and mail it to the laboratory.
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 missense variant in the glutamate metabotropic receptor 6 (GRM6 c.533C>T, p.(Thr178Met)) gene. GRM6 is a a gene with a pivotal role in cell signaling at low light vision conditions. This identified variant is therefore suspected to cause CSNB in this breed. Since it is now the second variant thought to cause CSNB in horses, it has been termed CSNB2.
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.
A recent study published in 2023 by VGL researchers provided further evidence that the CSNB2 allele causes congenital stationary night blindness (CSNB) in the Tennessee Walking Horse and other breeds in an autosomal recessive manner. The study evaluated 3,518 horses from 14 breeds and researchers detected the CSNB2 allele in nine additional horse breeds (Table 1). Researchers found the allele frequency of CSNB2 ranged from 0.0010 in Quarter Horses to 0.17 in pacing bred Standardbreds. 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. Ocular examinations, including electroretinography (ERGs), were performed on three CSNB2/CSNB2 homozygous horses and found all three to be affected by CSNB (one Standardbred, one Missouri Fox Trotting Horse, and an additional Tennessee Walking Horse). Additionally, seven CSNB2/N heterozygous horses and nine N/N horses underwent the same examinations and ERGs and were determined to be unaffected by CSNB. These findings support that two copies of CSNB2 cause CSNB in horses.
Table 1. CSNB2 genotypes and allele frequencies from the investigated horses, and the expected carrier and CSNB-affected frequencies for each breed.
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CSNB2 allele frequencies and expected number of CSNB homozygotes (affected) by breed |
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Breed |
n |
CSNB2 Allele Frequency |
Expected CSNB Affected Frequency |
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Miniature Horse |
99 |
1.0% |
0.01% |
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Missouri Fox Trotting Horse |
89 |
8.4% |
0.71% |
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Morgan |
330 |
1.4% |
0.02% |
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American Quarter Horse |
486 |
0.10% |
0.0001% |
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Racking Horse |
58 |
10% |
1.1% |
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Rocky Mountain Horse |
74 |
0.68% |
0.005% |
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American Saddlebred |
92 |
0.54% |
0.003% |
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Spotted Saddle Horse |
55 |
10% |
1.0% |
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Standardbred (pacer) |
110 |
17% |
3.0% |
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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 the Tennessee Walking Horse and other breeds and the full publication can be reviewed here.
