Progressive Retinal Atrophy Dominant (Mastiff Type)

Test Overview:

Progressive retinal atrophy (PRA) is a collection of inherited diseases affecting the retina that cause blindness.  Each breed exhibits a specific age of onset and pattern of inheritance, and the actual mechanism by which the retina loses function can vary.  The result of almost all types of PRA is similar - generally an initial night blindness, with a slow deterioration of vision until the dog is completely blind.  The age at which the dog becomes fully blind also varies, depending on the genetic disruption present and the breed. Affected eyes are not painful, unless complicated by a secondary problem, such as cataract or uveitis (inflammation due to a leaking cataract). Progressive retinal atrophy (PRA) has been classified in several different ways.  The simplest of these is by age of onset.  Early onset PRA occurs when the affected dog is night blind from birth, and generally is completely blind between 1 - 5 years of age.  Late onset PRA is where the dog is night blind at some time over 1 year of age, and full blindness will occur at a somewhat later stage in life.  Another is by the type of genetic abnormality causing the PRA.  PRA may be inherited by recessive, dominant or sex-linked mechanisms in dogs. For many types of PRA in many breeds a DNA test is now available to allow for easy screening for the disease.  Despite the complexity of the disease and its many forms, ultimately all forms have one thing in common – degeneration of the retina causing progressive loss of vision. DNA tests are not yet available for all affected breeds.  And because breeds may also be prone to several forms of PRA (and not all may have a genetic test available)  examination of the retina by a veterinary ophthalmologist remains a mainstay of the diagnostic testing regimen.   In some breeds with a late onset PRA, serial eye examinations may be required before the signs of retinal degeneration become apparent. The electroretinogram (ERG) is a diagnostic test that the veterinary ophthalmologist may choose to use in some cases and is a very sensitive method of detecting loss of photoreceptor function.  An ERG can be a very good screening test for puppies that may have an early onset form of PRA. The mastiff is affected by a late onset form of PRA that is the only known form of the disease to be inherited in an autosomal dominant mode.  It is hence abbreviated as AD-PRA.  This form of PRA has also been found in the bullmastiff.  No abnormality is seen in the photoreceptors at the age of 3 - 4 months (i.e. the ERG at this time is normal) but by the age of 13 months the photoreceptor function is abnormal.  These dogs have a single nucleotide transcription error in the rhodopsin (RHO) gene, which leads to focal areas of photoreceptor degeneration in the retina.  With this disease, exposure to bright light greatly increases the rate of photoreceptor damage and degeneration, and photoreceptors may be completely lost over a period of less than a month.  If exposure to light is kept to the lowest levels (i.e. dim light only), damaged photoreceptors may be repaired over a period of weeks to months.  Hence the loss of vision may occur at a variable age between 6 months to a number of years, and can be partly dependent on exposure to light. Because this type of PRA is dominantly inherited, there are no carrier animals.  Any animal inheriting one copy of the trait will be affected by the disease.  The T4R mutation is known, and a genetic test is available for this disease.  Animals test as clear or affected.  It is recommended that affected animals are not used for breeding, as they will result in the production of affected offspring. As with all dogs suffering from PRA, there is no cure.  Dogs generally adapt quite well to blindness - especially when it develops gradually - as long as their surroundings remain familiar (e.g. furniture does not get rearranged, they do not move house etc).  They should always be kept on a lead outside the yard, and care should be taken not to startle them.  Balls containing bells (as an example) can be used as toys for mental stimulation.