Retinal dysplasia (RD) is caused by abnormal development of the layer of cells at the back of the eye, the retina, which means that an affected dog’s vision can be affected from when the eyes first open. It has been described in multiple dog breeds and can be subdivided into focal, multifocal, geographic and total retinal dysplasia types. Total RD has been identified in English Cocker Spaniels.
This form of RD in Cocker Spaniels presents as total retinal detachment and severe visual deficits at 6 weeks of age. It is reportedly caused by a genetic mutation in the NDP gene. NDP is on the X chromosome, which means that male dogs will be affected if they carry one copy of the defective gene.
The single nucleotide insertion in the gene called NDP that causes Retinal Dysplasia in Spaniel (English Cocker) is X-linked. This means that females that carry two copies of the mutation (homozygotes) and males that carry one copy of the mutation (hemizygotes) will almost certainly develop Retinal Dysplasia during their lives. Females that carry a single copy of the mutation (also known as carriers or heterozygotes) will not develop Retinal Dysplasia as a result of the NDP mutation, but they will pass the mutation onto about half of any offspring they have. Breeding dogs that will not develop Retinal Dysplasia should be the breeder’s priority, with a reduction in mutation frequency within the whole breed being the secondary, longer-term target.
Carriers should not be bred from, even if they are mated to a dog that has also been tested and is clear of the NDP mutation (i.e. carry no copies of the mutation). If a carrier is mated to a clear dog approximately half of the resulting puppies will either be carriers if they are female or affected if they are male.