Collie eye anomaly (CEA) is an inherited disorder of ocular development that mainly affects herding breeds with Collie ancestry (Rough and Smooth Collie, Border Collie, Shetland Sheepdog, Australian Shepherd). It is associated with a deletion in the NHEJ1 gene on chromosome 37 (risk allele linked to the locus in several herding breeds) and causes abnormal development of the inner layers of the eye, with choroidal hypoplasia as the main finding. Severity is highly variable, from subclinical cases to blindness, even between littermates, which suggests modifier genes.

Described in Rough and Smooth Collie, Border Collie, Shetland Sheepdog and Australian Shepherd. The specific frequencies of affected/carrier animals per breed are not firmly established in the verified sources; prevalence has decreased where systematic genetic screening is carried out.

- Choroidal hypoplasia (pale patches beneath the retina on ophthalmoscopy)
- Retinal folds
- Optic disc coloboma
- Retinal detachment in severe cases
- Microphthalmia or enophthalmia (rare)
- Many affected animals show no external signs; diagnosis by ophthalmological examination in puppies (ideally 6-8 weeks)

CEA was described clinically in 1969 and its autosomal recessive inheritance was established in the 1970s. Lowe and colleagues (2003) mapped the primary disease locus to chromosome 37. Parker and colleagues (2007) showed, using a multi-breed approach, that all affected dogs share a homozygous 7.8 kb deletion in the NHEJ1 gene (an intronic deletion spanning a conserved protein-binding domain), establishing a single common ancestral allele in herding breeds. Genetic testing made it possible to reduce the incidence, although it persists when breeders remove only affected animals and do not screen carriers.

- Test the entire breeding line before the first mating (NHEJ1 test)
- Do not mate two carriers (25% of homozygotes)
- Affected animals (homozygotes) should not be bred
- Carriers should only be mated to non-carriers
- Carry out ophthalmological screening at 6-8 weeks (window before pigmentation)
- Periodic ophthalmological examination by a certified ophthalmologist
- Keep records of the genetic test in the pedigree
- The 'go normal' phenomenon (hypoplasia that becomes pigmented and is no longer visible) does NOT mean that the dog is healthy or that it does not transmit the disease

The severity of CEA cannot be predicted from the genotype alone: one homozygote may have mild signs while a homozygous littermate is severely affected. The ophthalmological examination at 6-8 weeks is crucial because afterwards pigmentation can mask the choroidal hypoplasia ('go normal' phenomenon). There is no treatment that corrects the structural anomaly; management focuses on environmental adaptations and, in selected cases with retinal detachment, preventive laser surgery if detected early.

1. Lowe JK, Kukekova AV, Kirkness EF, et al. Linkage mapping of the primary disease locus for collie eye anomaly. Genomics. 2003;82(1):86-95. PMID: 12809679
2. Parker HG, Kukekova AV, Akey DT, et al. Breed relationships facilitate fine-mapping studies: a 7.8-kb deletion cosegregates with Collie eye anomaly across multiple dog breeds. Genome Res. 2007;17(11):1562-1571. PMID: 17916641