Hereditary cone-rod dystrophy described in the Glen of Imaal Terrier. It initially affects retinal cones and, more slowly than in other PRAs, rods, producing loss of daytime vision that progresses towards blindness. The course is relatively slow and some dogs retain residual vision for years. It is inherited in a recessive manner.

Specific to the Glen of Imaal Terrier, a breed of small numbers. No reliable carrier frequencies are published; the inbreeding typical of the breed favours the concentration of the allele in certain lines.

- Initial loss of daytime vision
- Tapetal and pigment epithelium changes in the ocular fundus
- Slower progression than other PRAs
- Preservation of residual vision for years in some dogs
- Not painful

The crd3 form was characterized in the Glen of Imaal Terrier as a molecularly distinct entity among canine cone-rod dystrophies. It was associated with a variant in the ADAM9 gene (metalloprotease) that, in other mammals, produces retinal degeneration by altering the matrix of the pigment epithelium. The identification of the variant enabled the development of a genetic test used in the breed's breeding programmes.

- Test breeding animals before mating
- Do not mate two carriers: 25% risk of homozygotes
- A carrier can be mated to a clear animal; test offspring intended for breeding
- Due to the variability of expression and late onset, a homozygote without signs is still a risk breeder: manage it as affected
- Prioritize the genetic diversity of the breed when replacing carriers

The variability of expression and late onset mean one should not rely on the ocular phenotype: a genotypically affected dog may appear healthy at a single examination. Differential diagnosis with other late PRAs and with toxic or deficiency retinopathies. The genetic test is the tool of choice for breeding management.

1. Kropatsch R, Petrasch-Parwez E, Seelow D, et al. (2010) Generalized progressive retinal atrophy in the Irish Glen of Imaal Terrier is associated with a deletion in the ADAM9 gene. Mol Cell Probes 24:357-363. PMID: 20691256
2. Goldstein O, Mezey JG, Boyko AR, et al. (2010) An ADAM9 mutation in canine cone-rod dystrophy 3 establishes homology with human cone-rod dystrophy 9. Mol Vis 16:1549-1569. PMID: 20806078