Progressive retinal atrophy

PRA results from mutations in genes essential for photoreceptor function, leading to progressive degeneration of rods and cones. Rods are affected first, causing night blindness (nyctalopia), which often goes unnoticed until the disease advances. As cones degenerate, daytime vision deteriorates, and affected dogs may develop cataracts and increased reflectivity of the tapetum lucidum. Clinical signs include reluctance to navigate in dim light, bumping into objects, and dilated pupils. The age of onset varies: early-onset forms (e.g., rod-cone dysplasia in Irish Setters) appear by 6–8 weeks, while late-onset forms (e.g., progressive rod-cone degeneration) may not manifest until middle age.¹

Most PRA mutations are inherited in an autosomal recessive pattern, meaning a dog must inherit two copies of the mutant gene to be affected. However, some forms are X-linked (e.g., in Siberian Huskies) or dominant (e.g., in English Mastiffs). Over 30 genes have been implicated, including PDE6B, PRCD, RPGRIP1, and CNGB1. Breed-specific mutations are common: for example, the PRCD mutation is found in many breeds, while RPGRIP1 is associated with PRA in Dachshunds. Genetic testing allows breeders to identify carriers and avoid producing affected puppies, significantly reducing disease prevalence.²

Diagnosis is based on ophthalmic examination, including fundoscopy, which reveals retinal thinning and vascular attenuation. Electroretinography (ERG) can detect functional loss before visible changes, and genetic tests confirm the specific mutation. There is no cure for PRA; management focuses on supportive care, such as maintaining a consistent environment and using scent-based cues to help blind dogs navigate. Antioxidant supplements have been studied but lack proven efficacy. Research into gene therapy, particularly for RPGRIP1-related PRA, has shown promise in animal models, with some clinical trials underway.³

PRA is not a single disease but a group of over 20 distinct genetic disorders, each with unique onset and progression. Some breeds, like the Tibetan Terrier, have a form that spares cones initially, preserving day vision longer. In the Cardigan Welsh Corgi, a dominant mutation in RHO causes a slowly progressive form. PRA also occurs in mixed-breed dogs, often due to recessive alleles from multiple breeds. Interestingly, a mutation in CNGA1 in Shetland Sheepdogs causes a form that is allelic to human retinitis pigmentosa, making dogs valuable models for human disease. Additionally, some breeds, such as the Basenji, have a form that is not associated with any known gene, highlighting the genetic heterogeneity.⁴

Photoreceptor

A specialized neuron in the retina that converts light into electrical signals.

Rod

A photoreceptor cell responsible for vision in low light.

Cone

A photoreceptor cell responsible for color and high-acuity vision.

Tapetum lucidum

A reflective layer behind the retina that enhances night vision.

Electroretinography

A test that measures the electrical responses of photoreceptor cells.