PRA eli Progressive Retinal Atrophy on silmän verkkokalvoa rappeuttava sairaus jota on löydetty mm. Abessinialaisilta ja Somali kissoilta. PRA johtaa kissan sokeutumiseen ajan kanssa.
Vuonna 2007 valmistui kauan odotettu DNA -testusmenetelmä PRA rdAc-mutaation varalle.
Kaikilla kasvattajilla on nyt mahdollisuus testauttaa kasvatukseen käytetyt kissat kyseisen mutaation varalta ja vanhat, nuorelta kissalta epätarkkoina otetut peilaukset jätetään pistokokeiksi vahemmille kissoile muiden harvinaisten PRA muotojen varalle.
Silmäpeilausmenetelmällä ennen 31.12.07 testattujen kissojen tulokset ovat kuitenkin voimassa vuoden ja käyvät testausmenetelmästä.
Muistathan pentua hankkiessasi, että peilausmenetelmä ei ole varma, ja DNA -testillä terveeksi testatun vanhemman tulos on varmin sen suhteen että tuleva pentusi ei ole rdAc -mutatiota tulevaisuudessa sairastava.
Koska PRA- DNA testi on varsin uusi tulokas, voi vielä eteen tulla tapauksia joissa joudutaan käyttämään astutukseen terveen kanssa kantajaa. Kantajan sekä terveen kissan yhdistäminen ei tuota sairaita jälkeläisiä. Näin toimitaan yleensä vain tilanteissa joissa todetaan kasvatusmateriaalin olevan geneettisesti niin arvokasta, että sen käyttämättä jättäminen on suurempi paha, kuin terveeseen kissaan yhdistäminen.
Suomen PRA tilanne on suhteellisen hyvä ja sairaaksi todettuja yksilöitä on tullut esille ainostaan muutama.
PK-anemia on saatu vapaaehtoisella testuksella DNA -testin tultua pyyhkäistyä pois nopeasti, oletamme näin tapahtuvan myös PRA rdAc -mutaatiolle.
Kaikki Bastian's kasvatukseen käytettävät naaraat tai niiden vanhemmat ovat testattu DNA -testuksella terveiksi PRA rdAc -mutaation varalta.
Kuu-Katjaana
Progressive retinal atrophy, or PRA as it is frequently termed, is a long recognised, hereditary, blinding disorder. PRA is a disease of the retina. This tissue, located inside the back of the eye, contains specialised cells called photoreceptors that absorb the light focused on them by the eye's lens, and converts that light, through a series of chemical reactions into electrical nerve signals. The nerve signals from the retina are passed by the optic nerve to the brain where they are perceived as vision. The retinal photoreceptors are specialised into rods, for vision in dimlight (night vision), and cones for vision in bright light (day and color vision). PRA usually affects the rods initially, and then cones in later stages of the disease.
PRA, as an hereditary condition in the dog and showing many similarities to retinitis pigmentosa in man, has been known since the early part of this century. However, reports of hereditary PRA in the cat are rare. Magnusson first described PRA in the dog, in Sweden 1909 in the Gordon Setter, and called it retinitis pigmentosa. Since that time it has been recorded in many breeds of dog and in several countries. PRA was divided into two forms, generalized and central, the former being further subdivided into three types, according to whether the rods and/or the cones degenerate prior to maturation.
Advanced generalized retinopathy in the cat, of unknown aetiology, was recorded by Rubin. Retinal degeneration in the Siamese breed was suspected of an hereditary basis and this aspect has been further studied more recently. An hereditary retinal degeneration in two litters of Persian kittens was described in 1973 by Rubin and Lipton. Feline central retinal degeneration was at first suspected of having an hereditary origin, but later was proved to be due to deficiency of the sulphur-containing amino-acid, taurine.
It is surprising that there are so few accounts of hereditary retinal degeneration in the cat with the amount of pedigree cat breeding that occurs and the multiplicity of reports of proven hereditary retinal degeneration in the dog. However, a form of progressive retinal atrophy was diagnosed in the Abyssinian cat in Sweden in 1977, and, subsequently, elsewhere in Europe. In this autosomal recessive disease, the first clinical signs become apparent at approx. 1,5 - 2 years of age. A research project was undertaken to present a genetic, clinical, electrophysiological and morphological analysis of the disease. A preliminary report on progressive retinal atrophy of hereditary origin in the Abyssinian cat has been presented by Narfström in 1982.
In England, a condition of similarly early onset has been described in the Abyssinian cat, and breeding studies involving 14 litters have confirmed an autosomal dominant mode of inheritance. The first sign of disease is an increase in pupil size in moderate daylight between 2-3 weeks of age. The pupillary light reflex, although sluggish, persists even late in the disease. Feline retinal dystrophies of much earlier onset have also been reported in the United States in the Persian breed and in three generations of domestic cats of mixed breeding.
Diagnosis of PRA is normally made by ophthalmoscopic examination. This is undertaken using an instrument called an indirect ophthalmoscope, and requires dilatation of the cat's pupil by application of eyedrops. Boradly speaking all forms of PRA have the same sequence of ophthalmoscopic changes: increased reflectivity (shininess) of the fundus (the inside of the back of the eye, overlain by the retina); reduction in the diameter and branching pattern of the retina's blood vessels; and shrinking of the optic nerve head (the nerve connecting the retina to the brain). These changes occur in all forms of PRA, but at different times in the different forms. Usually by the time the affected cat has these changes there is already significant evidence of loss of vision.
Confirmation of the diagnosis can be undertaken by electroretinography. This is an electrical measurement of retinal function somewhat similar to an electrocardiographic test of heart function, but with two differences: the electroretinogram (ERG) can only be recorded as a response to a flash of light and accurate recording of the ERG requires that the cat be anesthesized. In all cats showing clinical evidence of PRA, the ERG is severely diminished or extinguished.
Other causes that might lead to blindness:
Taurine deficiency retinopathy: cats of any breed fed a
diet deficient
in the amino acid taurine (eg. dog food) may develop retinal
degeneration. Ophthalmoscopically this has a characteristic appearance:
it always begins in the area centralis and may or may not progress.
Ophthalmoscopic evidence of retinal degeneration develops only after
the cat has been on a deficient diet for several months. The diagnosis
here relies on history: check for any affected relatives, evidence of
abnormal diet, or even test-mating. Vitamine A and E deficiency
were
also mentioned to be a possible cause for retinal degeneration as were
toxoplasmosis, feline
infectious peritonitis, feline leukemia virus,
tuberculosis and oculomycoses. Idiopathic retinal
degeneration may
occur with some frequency. But histologically these cases seem to be
different to PRA.
References:
- Retinal Degeneration in the Dog and Cat, Veterinary Clinics of North
America:
Small Animal Practice, Vol. 20, No. 3, May 1990;
- Progressive Retinal Atrophy in the Abyssinian Cat by K.C. Barnett, in
Journal of Small
Animal Practice, 1982, 23, 763-766;
- Six cases of progressive retinal atrophy in Abyssinian cats, Journal
of Small Animal
Practice, 25, 415-420;
- Progressive retinal atrophy in the Abyssinian cat: An update by
Narfström, in Veterinary
Records, 112, 525-526;
Progressive retinal atrophy in the Abyssinian cat, Clinical
characteristics by Narfström,
in Investigative Ophthalmology & Visual Science, 26, 193, 1985;
- An Early-Onset Retinal Dystrophy with dominant Inheritance in the
Abyssinian cat by
Curtis, Barnett and Leon in Invest. Opht. & Visl Sc. 28,
131-139;
- Taurine deficiency retinopathy in the cat by K.C. Barnett in J. of
small Animal Practice, 1980, 21, 521-534;