Hereditary eye diseases
Progressive Retinal Degeneration (PRD / PRA) refers to a group of genetic (hereditary) retinal diseases that cause damage to the photoreceptor (photosensitive) retinal cells – rods and cones, leading to their degeneration and ultimately death.
The first signs of the disease that appear in the animal are a violation of twilight (night) vision, the affected animal is poorly oriented in the dark. Further, daytime vision is gradually deteriorating. Owners also pay attention to the constantly dilated, poorly responsive to the pupils of their pet. The dog’s eyes in the dark, instead of the usual bright green, begin to “glow” yellow or even silver. In some dogs with progressive retinal degeneration, bilateral cataract begins to develop, the owners notice that the dog’s eyes become white-blue, cloudy (this is especially noticeable with dilated pupils). These signs, depending on the early or late form of the disease, most often begin to appear at the age of 1 to 5 years, but the disease can manifest itself at any age.
The death of photosensitive cells (rods and cones) causes gradual atrophy (destruction) of the retina. The end result of the disease is complete blindness. It was established that the earlier the disease manifests itself in the animal, the faster the complete loss of vision occurs. To date, treatment for progressive retinal degeneration does not exist, the affected animal is doomed.
The disease is genetic (hereditary), i.e. transmitted from parents to offspring. With the exception of the Siberian Husky and Samoyed Husky (X-linked inheritance), the inheritance of the disease is autosomal recessive. The mutant gene that causes this disease is located in a specific, so-called “prcd locus (region)” of the non-sex chromosome. Genetic studies using DNA marker tests allowed us to divide dogs into three groups:
1. Normal homozygous (AA) – in a healthy dog, both copies of the gene are the same (do not have mutations).
2. Heterozygous carriers (Aa) – one copy of the gene is normal, the other is mutant.
3. Homozygous affected (aa) – both copies of the gene are affected.
The disease appears only in affected (aa) animals (in which both copies of the gene are mutant). Animal carriers (Aa) themselves do not get sick, but transmit the mutant gene to their offspring, contributing to the hidden spread of the disease.
CPRA is characterized by pigmented changes in the coverage of the fundus (tapetum lucidum), due to hypertrophy and migration of pigment epithelial cells. Degeneration of photoreceptors is secondary to changes in pigment epithelium. The disease progresses slowly, and some dogs retain vision.
Unlike GPRA, sick dogs are better seen in twilight than in bright daylight. Peripheral vision remains normal until the final stage of the disease, due to which dogs see moving objects, although they stumble on immobile. In addition, dogs with CPRA have difficulty seeing objects at close range.
The central and generalized forms differ in their ways of inheritance. In CPRA, it is dominantly autosomal (i.e., non-sexually linked), probably with incomplete gene penetration – this means that even with its presence, the trait for which it is responsible may not appear. The area of the disease is within Europe, which makes researchers suspect environmental effects or nutrition rather than heredity on animals. Dr. Aguirre reports that “until now, unpublished evidence from England suggests that the cause of the disease is a lack of antioxidants in food.”
In most dogs, CPRA appears at the age of two to three years, and is diagnosed, as a rule, by four years (after an ophthalmoscopic examination). When examining the eye at the beginning of the disease, brown pigment spots are found in the fundus, and later, merging plaques. In sick dogs, as a rule, there is a perplexed expression, since the loss of central vision is compensated by peripheral.
Diagnosis of PRA: A PRA lesion is very typical of a Labrador. Perhaps changes already exist at the age of nine months, but do not manifest themselves, and therefore pass unnoticed. Currently, there are three methods for examining the eye in a dog: using a slit lamp microscope, an ophthalmoscope, and an electroretinogram.
Using a microscope with a slit lamp, only the anterior part of the eye is examined, and since the fundus remains inaccessible, it is impossible to diagnose PRA in this way. Nevertheless, this is an indispensable way to diagnose diseases of the lens and cornea.
For examination of the retina, two types of ophthalmoscopes are used – controlled, or manual, and uncontrolled (stationary). Managed has limitations.