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. Basset Hounds 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 19 base pair deletion in the gene called ADAMTS17 that causes Primary Open Angle Glaucoma in Basset Hounds is recessive. This means that dogs that carry two copies of the mutation (homozygotes) will almost certainly develop Primary Open Angle Glaucoma during their lives. Dogs that carry a single copy of the mutation (also known as carriers or heterozygotes) will not develop Primary Open Angle Glaucoma 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 Primary Open Angle Glaucoma 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 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 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 Primary Open Angle Glaucoma, 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.