PRA in Siamese cats causes gradual blindness — but it’s one of the few hereditary diseases we can design out completely. A simple DNA test and a little planning mean no kitten ever need be born to go blind from it. For clarity, PRA is equally an issue for Siamese, Orientals, Balinese and Oriental Longhairs.
When I first wrote about PRA I was contacted by people here in the UK whose Siamese were affected, which is why I went back and wrote this fuller piece with some practical breeding guidance. The encouraging part is the message at the heart of it: with the DNA test now available, breeding away from PRA really is straightforward.
What Is PRA?
Progressive Retinal Atrophy is a disease of the retina that ultimately causes blindness. The retina contains photoreceptor cells which absorb the light focused on them by the lens and convert it into electrical signals; these travel down the optic nerve to the brain, where they’re perceived as vision. In a cat with PRA those photoreceptors gradually degenerate. Typically sight worsens over time with total blindness around four to five years of age, though there are reports of it not being obvious until much later. It’s well documented in Abyssinians and Somalis, and widely reported in Siamese and Oriental cats.
How PRA Is Inherited
The form we’re concerned with is caused by the rdAc mutation, and it’s a simple autosomal recessive. That means the gene has to be present twice — one copy from each parent — for a cat to be affected and go blind. A cat with a single copy is a carrier: it won’t go blind itself, but it can pass the gene on. Because breeders work with recessive genes every day, this is familiar territory — and crucially, we can test for it.
DNA Testing for PRA
There is now a reliable DNA test for the rdAc mutation. A cat with two copies of the recessive gene will develop PRA and ultimately lose its sight; a cat with one copy is a carrier; a cat with none is clear. Knowing a cat’s status before you breed is the whole game. Testing is available through:
- Langford Veterinary Services (University of Bristol)
- UC Davis Veterinary Genetics Laboratory
Members of clubs registered with Langford can often get a discount on DNA tests through voucher codes from their club — worth asking your breed club secretary. Your vet can also diagnose an already-affected cat by dilating the pupils and examining the retina, but for breeding decisions the DNA test is what you want, because it identifies carriers before any symptoms appear.
Is PRA in Siamese Something New?
No — what’s new is the awareness and the availability of testing, not the disease. People often assume Siamese never went blind in the past, but it’s quite possible that affected cats did gradually lose their sight and simply adapted, leaning on their other senses, with the decline written off as old age. Some cases may have been noticed and never reported back to breeders. I’ll be honest: I’m not personally convinced we have a widespread problem in the breed — but I’m equally sure that without testing and data we’ll never actually know. There have been cats in this country confirmed positive with two copies of the rdAc gene, so it is real and it is here.
Responsible Breeding and PRA
Because the gene has to be present twice to cause blindness, and because we can test for it, we can breed cats that have zero chance of being affected. The table below shows what each type of mating produces — the golden rule is simply that a carrier or an affected cat should only ever be mated to a DNA-clear cat.
| Mating (parents) | Kittens produced | Verdict |
|---|---|---|
| Clear × Clear | 100% clear | ✅ Safe |
| Clear × Carrier | 50% clear, 50% carrier | ✅ Safe — no affected kittens |
| Clear × Affected | 100% carrier | ✅ Safe — no affected kittens |
| Carrier × Carrier | 25% clear, 50% carrier, 25% affected | ⛔ Avoid |
| Carrier × Affected | 50% carrier, 50% affected | ⛔ Avoid |
| Affected × Affected | 100% affected | ⛔ Avoid |
My suggested guidelines follow straight from that table:
- Only breed from cats whose rdAc status is known.
- Carriers should only be mated to clear (negative) cats.
- Positive cats (two copies) should only be mated to clear cats — never to another carrier or positive.
Follow those and no kitten you breed will ever be affected by rdAc PRA. It could be argued that positive cats shouldn’t be bred from at all — and that’s a fair position — but a positive mated to a clear cat produces only carriers, never affected kittens. The danger of trying to purge the gene too aggressively is that you slam the gene pool shut and lose valuable genetic diversity. Until we know how widespread the gene really is, careful selective breeding beats blanket exclusion.
If you’d like to understand the genetics behind this and your cats’ colours, my Siamese colour predictor and genetics article are good companions to this one, and I set the whole topic of health testing in context in my complete guide to breeding Siamese cats.
How Else Can You Help?
Raising awareness is the single most useful thing the community can do. If you have a breeder website or your cat club has one, please link to this article to help spread the word — the more cats that get tested, the clearer the real picture becomes.
Understand your matings before you plan them
My free Siamese colour predictor shows what a given pairing can produce — a handy way to get comfortable with how recessive genes like rdAc pass down a pedigree.
Frequently Asked Questions
Can PRA in Siamese cats be cured?
No, there’s no cure once a cat is affected — the sight loss is permanent. That’s exactly why prevention through DNA testing and careful mating matters so much.
Should I stop breeding from a PRA carrier?
Not necessarily. A carrier mated to a DNA-clear cat will never produce an affected kitten, so a valuable carrier can stay in a breeding programme without risk, helping preserve genetic diversity.
At what age does a PRA-affected cat go blind?
Typically vision declines towards total blindness around four to five years of age, though it can be later. Night blindness is often one of the first signs.
Where can I get my Siamese DNA tested for PRA?
Through Langford Veterinary Services or the UC Davis Veterinary Genetics Laboratory. Club members registered with Langford may be able to get discounted voucher codes via their breed club.
I’ve bred Siamese for many years and care deeply about the breed’s health — more about me and this site here.
