Genetic basis of Collie Eye Anomaly - is a developmental congenital eye defect inherited in an autosomal recessive manner. This means that dogs must inherit two copies of the mutant gene to be affected, while carriers with one copy show no clinical signs but can pass the defective gene to offspring. The primary mutation responsible is a 7.8 kb deletion in the NHEJ1 gene on canine chromosome 37, which disrupts normal eye development. The expression and severity of clinical signs in affected dogs are modulated by several modifier genes, resulting in variable outcomes even among littermates. Research also indicates that the presence of optic disc coloboma, a more severe lesion, may have a polygenic basis involving multiple genes beyond the main mutation. CEA is most commonly seen in herding breeds, particularly Rough and Smooth Collies, Shetland Sheepdogs, Border Collies, and Australian Shepherds, but can occur in other related breeds and mixed breeds with herding ancestry.
Pathophysiology - By definition, all dogs with CEA have choroidal hypoplasia, which is an underdevelopment of the blood vessels (choroid) in the back of the eye. This can be identified by a veterinary ophthalmologist through funduscopic examination as early as 6–8 weeks of age. The retinal pigment epithelium and overlying tapetum develop later, which may obscure the visible changes during fundoscopy, a phenomenon called “go normal”, where the choroidal hypoplasia becomes difficult to detect despite the dog still being genetically affected. In more severe cases, the structural defect extends to include: Optic disc coloboma: congenital defects or "holes" in the optic nerve head where the retinal nerve fibers exit. Retinal hemorrhages that may lead to focal or total retinal detachment. Retinal detachment occurs in approximately 4-5% of affected dogs, leading to loss of vision in the detached areas or complete blindness if the entire retina is affected.
Swelling and hemorrhage inside the eye (intraocular hemorrhage) can also occur in severe cases.
Complications - The clinical presentation varies: Dogs with mild forms show only choroidal hypoplasia, often with minimal or no visual impairment. Dogs with optic disc coloboma may have localized visual deficits. Retinal hemorrhages and focal retinal detachment can cause blind spots. Complete retinal detachment results in blindness in the affected eye and may manifest in puppies or develop spontaneously in adult dogs. The variability and incomplete penetrance of clinical signs reflect the modifying genetic factors influencing disease severity.
Why This Matters to Breeders and Vets
Breeding control: CEA’s autosomal recessive inheritance necessitates DNA testing of breeding stock to identify carriers and affected dogs, preventing matings that could produce severely affected offspring.
Prevalence: Historic data shows very high prevalence in Collies, with reported carrier and affected rates of over 60-70% in some populations, though modern testing has reduced frequency through informed breeding.
Related breeds: Carrier rates are also relatively high in Border Collies, Australian Shepherds, and Shetland Sheepdogs, requiring vigilance in these breeds.
Veterinary diagnosis: Early veterinary ophthalmic exams (6–8 weeks old) are critical to detect choroidal hypoplasia. Awareness of the "go normal" phenomenon helps avoid false negative interpretations in older dogs. Genetic testing complements clinical diagnosis and guides breeding decisions.
Management: There is no treatment to reverse structural defects. Visual impairment is variable, and dogs with mild changes often lead normal lives. Retinal detachments and severe complications require symptomatic management and prompt ophthalmic care.
Summary - Collie Eye Anomaly is a hereditary developmental eye disease caused primarily by a recessive deletion in the NHEJ1 gene, characterized by choroidal hypoplasia with variable severity influenced by modifier genes. Clinical signs appear early but can be difficult to detect later due to retinal development changes that mask lesions. Severe forms include optic nerve coloboma, retinal hemorrhages, and detachment, potentially causing blindness. Genetic testing, combined with early veterinary eye exams, is pivotal for managing disease prevalence within Collie breeds and related herding dogs. Responsible breeding programs based on DNA screening have reduced the incidence of this common inherited ocular disorder, protecting vision and welfare in susceptible breeds.