Hereditary dystrophy of cone photoreceptors that causes day blindness (hemeralopia) and photophobia with preserved night vision. Affected dogs show poor vision in bright light and light-avoidance behaviour. It is the canine equivalent of human achromatopsia. It has been described in the German Shepherd and Labrador Retriever (CNGA3 variants, ACHM2 form) and in the Alaskan Malamute and Miniature Australian Shepherd (CNGB3 variant, ACHM3 form).

Limited data. Cases described in specific lines of German Shepherd, Alaskan Malamute and Labrador Retriever; no reliable population frequencies have been published. The Malamute form (ACHM2/CNGB3) is better characterised.

- Day blindness (hemeralopia) from an early age\n- Photophobia and squinting in bright light\n- Searching nystagmus in some cases\n- Preserved vision in dim light conditions\n- Normal fundus in the initial stages\n- Progressive decrease in cone function on ERG

Canine achromatopsia has been studied as a model of the human disease since the end of the 20th century. The Alaskan Malamute form (ACHM3) was associated with a deletion in the CNGB3 gene. The German Shepherd form was associated with a variant of the CNGA3 gene, a paralogue of CNGB3 within the cyclic nucleotide-gated channel of cones. In the Labrador Retriever the causative mutation is also in CNGA3 (c.1931_1933del, p.(V644del)), described by Tanaka et al. (2015). The availability of molecular tests varies according to breed and gene.

- Screen breeding animals of affected breeds before mating when a molecular test specific for the breed's variant is available (CNGA3 in German Shepherd, CNGB3 in Malamute)\n- Do not mate two identified carriers\n- In the absence of a test, avoid repeating matings that have produced affected puppies\n- Confirm the diagnosis with a full ERG (separate assessment of cones and rods) and fundus examination

ERG with a cone (photopic) and rod (scotopic) protocol is the key element of diagnosis: achromatopsia shows a cone defect with preserved rod function. Differentiate it from progressive retinal dystrophies (PRA) that affect both types of photoreceptors and from cortical blindness. The molecular test is variant-specific: the CNGB3 test (Malamute) does not cover the German Shepherd CNGA3 variant and vice versa.

1. Sidjanin DJ et al. (2002) Canine CNGB3 mutations establish cone degeneration as orthologous to the human achromatopsia locus ACHM3. Hum Mol Genet 11:1823-1833. PMID: 12140185
2. Tanaka N et al. (2015) Canine CNGA3 gene mutations provide novel insights into human achromatopsia-associated channelopathies and treatment. PLoS One 10:e0138943. PMID: 26407004
3. Yeh CY et al. (2013) Genomic deletion of CNGB3 is identical by descent in multiple canine breeds and causes achromatopsia. BMC Genet 14:27. PMID: 23601474
4. OMIA:001481-9615 (Achromatopsia-2, CNGA3-related) y OMIA:001365-9615 (Achromatopsia-3, CNGB3-related).