What is PRCD -PRA?
Progressive rod-cone degeneration (PRCD) is an inherited form of late-onset progressive retinal atrophy (PRA). Progressive retinal atrophy is the common name for a family of hereditary retinal dystrophies commonly leading to blindness. PRA is found in more than 100 dog breeds but the genetic background varies among breeds and populations. prcd-PRA is a very common form of PRA. This disease is caused by the degeneration of photoreceptor cells of the retina. The disorder is inherited in an autosomal recessive manner.
PRCD-PRA affects the photoreceptor cells in the eye involved in both night and day vision. The cells of the retina involved in low light vision, known as rods, are affected first, resulting in night blindness. Subsequently, the bright light photoreceptors known as cones, which are important for color vision, are also affected, resulting in daytime visual deficit. The age of onset and rate of progression vary among breeds, but retinal changes can be identified by screening performed by a veterinary ophthalmologist from adolescence to early adulthood. Most PRCD-affected dogs have noticeable visual impairment by 4 years of age, typically progressing to complete blindness.
Progressive retinal atrophy (PRA) is an inherited eye disease that leads to blindness in dogs. PRA is a non-painful condition that tends to progress slowly over time, often starting with a decreased ability to see well at night. While there is no cure available right now, most dogs adapt well to their vision loss and continue to have a good quality of life.
Several different genetic variants can cause PRA, but all of them lead to the same outcome: cell degeneration in the retina, which is located in the back of the eye. There are two types of cells in the retina: rods and cones. Rods assist with night vision and motion detection, whereas cones help with day vision and color detection. Dogs with degenerative PRA are born with normal rods and cones, but the cells start to break down sometime during adulthood. This slowly progresses into blindness, but the progression and age at which the majority of vision is lost will vary depending on the breed and the individual. In true PRA, rods will die before cones, causing night vision to be affected first. When cones degenerate first, the condition is called achromatopsia, or day blindness. A less common form of retinal disease occurs in puppies who are born with rods or cones that never develop properly, which leads to blindness much more quickly. These conditions are referred to as retinal dysplasia rather than retinal degeneration, but may still be called PRAs at times. Eventually, all genetic variations of PRA progress to complete blindness.
These signs may not be apparent until later in the disease, as dogs will naturally acclimate to their vision loss. Signs may be more noticeable at night or when the dog is in a new environment. Clinical signs may include the following:
Abnormally reflective eyes when a light shines on them
Reluctance to enter a dark room or go outside at night
Hesitance to go down stairs
Bumping into door frames or clumsiness in new surroundings
Cataract formation in both eyes
PRA is diagnosed with an eye exam focusing on the retina. Both eyes are equally affected by PRA, and dogs may also develop cataracts in the late stages of the disease. Your veterinarian may refer you to a veterinary eye specialist to confirm a diagnosis with additional testing, such as electroretinography (ERG), especially if cataracts block the retinas from view. It is important to differentiate PRA from other forms of blindness in dogs, since other diseases may cause pain or additional systemic side effects.
Normal: This dog has tested normal/clear for the mutation known to cause Progressive Retinal Atrophy (PRA). It can only transmit the normal/clear gene to its offspring.
Carrier/Low Risk: This dog has tested as a carrier/low risk for the mutation known to cause Progressive Retinal Atrophy (PRA). This means the dog has one normal/clear copy and one mutated copy of the gene. Either the normal/clear copy or the mutated copy of the gene can be transmitted to its offspring.
Affected/High Risk: This dog has tested as affected/high-risk for the mutation known to cause Progressive Retinal Atrophy (PRA). It is at risk for developing clinical symptoms of PRA at some point in its lifetime. It can only transmit the mutated copy of the gene to its offspring.
PRA management focuses on helping dogs avoid injury and feel safe in their environment as their vision declines. Steps to take include:
Adding supplemental lighting inside and outside the house — this may help in the early stages when only night vision is affected
Keeping furniture in the same arrangement
Using safety gates to block access to stairs
Guiding them with a leash when in unknown areas
Training with verbal commands (prior to vision loss)
Gene therapy is a prospective treatment for some specific types of canine PRA, but at this time, it has only been used in research settings, and it is not widely available. While there are no approved treatments for PRA, specific antioxidant supplements may improve retinal function and help to delay the formation of cataracts.
Unfortunately, nearly all dogs with PRA will eventually go completely blind. Dogs tend to acclimate well to vision loss, since most cases of PRA progress slowly, and affected dogs can go on to live otherwise healthy lives.
Australian Cattle Dogs are one of the breeds that are known to have genetic variants that put them at a higher risk for PRA.
DNA testing is available for all known variants, but researchers have not yet discovered all the variants that cause PRA. The existing tests can detect if a dog is at risk of developing PRA, or if they are merely a carrier of a variant but not at risk of developing the disease. One of the most common forms of inherited PRA is called progressive rod-cone degeneration (PRCD).
Since this condition is recessive, two copies of the variant must be present to consider a dog at-risk. Dogs with two copies of the PRCD variant will almost always develop vision loss as an adult. There are also a few known variants that are called dominant variants (requiring only one copy) or X-linked variants (requiring one copy for males and two copies for females).
However, some forms of PRA do not have any known genetic variants — meaning that a dog could still develop the disease even if they test negative for the known variants. Dogs with PRA and dogs that test positive for known PRA variants should not be used for breeding without prior genetic testing and careful consideration of mate selection.