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. Petit Basset Griffon Vendeen (PBGV) are at risk of developing primary open angle glaucoma (POAG), the early clinical signs of which can be detected by a veterinary ophthalmologist when dogs are between 3 and 4 years of age. The initial signs are a small, sustained rise in intraocular pressure (IOP) and lens subluxation. Unlike primary closed angle glaucoma, which is the more common form of glaucoma in dogs, there is no pectinate ligament abnormality and the iridocorneal angle remains open until the late stages of the disease. POAG is not painful in its early stages and the slow progression of this disease means that often owners are not aware their dog is affected until they notice their dogs’ eyes have become enlarged (due to the increased pressure) or a vision problem becomes noticeable. POAG is progressive however and the continued rise in IOP will eventually lead to pain and blindness.
The large inversion in ADAMTS17 that causes primary open angle glaucoma (POAG) in the Petit Basset Griffon Vendeen is recessive. This means that dogs that carry two copies of the mutation (homozygotes) will almost certainly develop POAG during their lives. Dogs that carry a single copy of the inversion (also known as carriers or heterozygotes) will not develop POAG as a result of the ADAMTS17 mutation, but they will pass the mutation onto about half of any offspring they have. Breeding dogs that will not develop POAG 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 ADAMTS17 inversion (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 POAG, 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.