First identified in humans, achromatopsia also affects dogs, impeding their ability to see in bright light. Our Veterinary Vision Center team explains this concerning condition, also known as day blindness, to help you determine if your dog is affected.
The dog’s eye
To understand achromatopsia, you must first understand the dog’s normal eye anatomy. The retina, which lies along the back of mammalian eyes, contains cells called photoreceptors that process the information when light hits the retina, and send electrical signals to the brain via the optic nerve. Two types of photoreceptors are involved in sight:
- Rods — These photoreceptors are adapted to work in low light, and can detect motion. Rods are concentrated in the outer retina and facilitate peripheral vision.
- Cones — These photoreceptors are adapted to work in mid to bright light, and can detect color.
Dogs evolved as a crepuscular (i.e., active at dawn and dusk) predator species, while humans evolved as a diurnal (i.e., active in the daytime) species, so dogs have a much higher proportion of rods to cones, enhancing their night vision. In addition, dogs have two cone types, whereas humans have three, which means that humans can visualize a wider color spectrum. Dogs mostly see yellows, blues, and greys, while humans can also see reds and greens.
Achromatopsia in dogs
Achromatopsia is a congenital eye disorder that causes improper function of the cones in the dog’s eye. The cyclic nucleotide-gated (CNG) ion channels are key mediators for signal transduction in retinal receptors. CNGA3 and CNGB3 are genes responsible for encoding cone CNG channels, and a CNGA3 mutation causes achromatopsia, causing day blindness, total color blindness, and decreased central visual acuity. Two known CNGB3 gene mutations can lead to the condition:
- Cone degeneration 1 (CD1) — A genomic segment that includes the CNGB3 gene is deleted. This defect is found in Alaskan sled dogs, Alaskan malamutes, miniature Australian shepherds, and Siberian huskies.
- Cone degeneration 2 (CD2) — This mutation involves a nucleotide change in exon six of CNGB3. This defect is found in German shorthaired pointers and Alaskan sled dogs.
Achromatopsia is an autosomal recessive disorder, which means that a dog must inherit two mutated genes to be affected.
Achromatopsia signs in dogs
Clinical signs in dogs typically manifest by 8 to 12 weeks of age when their retinal development is completed. The dog’s cones develop normally, but their inner and outer segments gradually deteriorate when they are no longer functional, resulting in a slow cone loss during their lifetime. Signs include:
- Nystagmus — Affected dogs often develop pendulum nystagmus, in which their eyes move abnormally, constantly from side to side.
- Photophobia — Exposure to light is irritating and painful for these dogs, who may seem afraid of bright light and avoid brightly lit areas.
- Preferring shade — You may notice your dog prefers to be in the shade or inside during daylight hours.
- Difficulty finding objects — Your dog may have difficulty finding toys or other objects during daylight hours.
- Difficulty navigating — Your dog may bump into objects or have difficulty navigating stairs during daylight hours.
- Aggressiveness — Some dogs become fearful or aggressive in unfamiliar settings.
All these signs disappear in low light settings. As your dog ages, their signs will not worsen and their night vision will remain normal.
Achromatopsia diagnosis in dogs
A general ophthalmic examination on a dog affected by achromatopsia will reveal no abnormalities, and detecting cone dysfunction requires an electroretinogram (ERG) or genetic testing. Results* include:
- N/N genotype — Dogs with this genotype do not have achromatopsia caused by these mutations.
- N/CD1 or N/CD2 genotype — Dogs with this genotype do not have achromatopsia caused by these mutations, but they are carriers, and will transmit a cone degeneration variant to 50% of their offspring. Matings between two carriers will likely produce 25% cone degeneration affected puppies.
- CD1/CD2, CD1/CD1, or CD2/CD2 genotype — Dogs with this genotype have achromatopsia caused by these gene mutations.
*N = Normal; CD1 = CNGB3 cone degeneration variant 1; CD2 = CNGB3 cone degeneration variant 2
Achromatopsia treatment in dogs
No treatment is currently available for achromatopsia, but some people advocate dog goggles or colored contact lenses to help these dogs function during daylight hours. Genetic research is being conducted in which a normal CNGB3 gene is administered into the retina of affected dogs, and results have been promising.
Dogs at risk for achromatopsia should be genetically tested to ensure they cannot pass on this mutation before they are bred. If your dog’s vision is impaired in bright light, contact our Veterinary Vision Center team, so we can determine if they have day blindness.