Felipedia

Progressive retinal atrophy (PRA) is a well known and well recognised form of retinal degeneration due to inheritance in many breeds of dogs throughout the world.

It is remarkable that in the cat, to date, only one breed has proven hereditary progressive retinal atrophy and furthermore, that breed, the Abyssinian, has two separate forms occurring at different ages, one caused by a dominant gene and the other by a recessive gene. Retinal degeneration with a suspected hereditary basis has been reported in the Siamese, and investigated. Possible hereditary retinal degeneration in two litters of Persian kittens was recorded in America and abnormal photoreceptor development in two generations of mixed breed domestic cats. However, none of these reports proved an hereditary cause. PRA in the Abyssinian cat was first described in Sweden and Finland and then later in the UK.

In the Abyssinian cat, there are two distinctly separate forms of progressive retinal atrophy. Both are bilateral, symmetrical in the two eyes and progressive, and both have been proven to be hereditary. The first form affects young kittens and the first observable clinical sign is a dilated pupil, or mydriasis, which is obvious in affected kittens when compared with their non-affected littermates at as early as 4 weeks old. The affected kittens also show nystagmus which is variable, intermittent and often rapid in its form. This form of PRA is a rod-cone dysplasia, degeneration of the photoreceptors occurring before maturation. The first observable ophthalmoscopic lesion is present at about 8 weeks old when affected kittens can be distinguished from non=affected littermates, Progression is rapid and obvious differences are present by 24 weeks old. Ophthalmoscopic signs consist of tapetal hyperreflectivity and attenuation of the retinal blood vessels and loss of pigment in the nontapetal region. With further progression loss of tapetal structure occurs several months later, particularly in the area centralis region. Even in advanced cases, with ghost vessels visible ophthalmoscopically and tapetal degeneration evident, cats retain some pupillary light reflex in bright sunlight and there has been no cases of a secondary cataract, which is common with generalised PRA in dogs.


Normal retina	Progressive retinal atrophy (PRA). Note the tapetal hyperreflectivity, vascular attenuation, and optic nerve atrophy

The fundus of an advanced case of hereditary retinal degeneration in a 4-year old Abyssinian cat.	Loss of pigment from nontapetal fundus in an affected kitten

The rod-cone dysplasia in the Abyssinian cat has been shown to be due to an autosomal dominant gene by crossbreeding unrelated females of both mixed and pedigree breeding resulting in affected offspring in the first generation; the proportion of affected and non-affected parents being not significantly different from ratios expected; the production of normal kittens in litters born to parents both of whom are affected.

The other form of PRA in the Abyssinian cat occurs at a later age in young adults, the majority showing clinical signs by 1.5-3 years, occasionally 3-4 years of age. This disease is also bilateral and progressive and advanced retinal atrophy varied from 3-6 years, with similar ophthalmoscopic signs to those described above. Interestingly, genetic analysis of these cases indicated autosomal recessive mode of inheritance, evidence being that all offspring from two affected parents were themselves affected, affect offspring derived from matings between clinically unaffected (carrier) parents, and the expected proportion of normal and affected kittens born to matings between affected parents and carriers.