Primary glaucoma results from reduced drainage of the fluid (aqueous humour) that is produced within the eye, resulting in a build-up of intraocular pressure (IOP) which, in turn, damages the optic nerve and leads to pain and blindness. Primary closed angle glaucoma (PCAG) in dogs is associated with a risk factor known as pectinate ligament abnormality (PLA), a form of goniodysgenesis, and affects several breeds of dog, including the Border Collie. PLA is the term given to an abnormality of the drainage angle within the eye and can only be detected by a veterinary ophthalmologist, using a technique called gonioscopy. Although severe PLA (or goniodysgenesis) is a risk factor for PCAG it does not directly cause any clinical signs that the dog or its owner will notice. PCAG, on the other hand, that is associated with severe PLA, is painful and potentially blinding if not treated immediately. The onset of PCAG can be very rapid (overnight) and results in an extremely painful eye that might be red and/or appear cloudy. The pupil will not constrict when a light is shone into it and the affected dog will demonstrate signs of pain, including a squinting or watering eye, being head shy or tilting its head.
The variant in OLFML3 that is associated with severe pectinate ligament dysplasia (PLA) and primary closed angle glaucoma (PCAG) in Border Collies appears to be recessive. This means that dogs that carry two copies of the mutation (homozygotes) have a very high chance of developing PCAG during their lives. Dogs that carry a single copy of the variant (also known as carriers or heterozygotes) are unlikely to develop PCAG as a result of the OLFML3 variant, but they will pass the mutation onto about half of any offspring they have. Breeding dogs that will not develop PCAG should be the breeder’s priority, with a reduction in mutation frequency within the whole breed being the secondary, longer-term target.
Carriers can be bred from safely, provided they are mated to a dog that has also been tested and is clear of the OLFML3 variant (i.e. carry no copies of the mutation). If a carrier is mated to a clear dog approximately half of the resulting puppies will also be carriers, so should be tested themselves prior to breeding. Breeding carriers to tested, clear dogs is safe, in terms of avoiding dogs affected with PCAG, and will help to maintain the genetic diversity of a breed. It is therefore encouraged, particularly in the first few generations following the availability of a new genetic test, so that other desirable characteristics and traits can be preserved before the frequency of the disease mutation within the breed is gradually reduced.
Genetic tests are only able to detect specific mutations or variants that have been previously identified. New eye diseases emerge within breeds from time to time and for this reason we recommend that all breeding dogs have their eyes clinically examined by a veterinary ophthalmologist within the twelve months prior to breeding, regardless of the availability of genetic tests for eye diseases that are known to affect the breed. This will help identify new eye diseases before the become widespread. We also recommend that breeding dogs have their eyes examined when they reach eight years of age, to detect any late onset eye diseases that had not developed at the time they were bred.