Due to decreasing use over the years, I have decided to disable the forum functionality of the site.

Forums will still be available to view but new posts are no longer allowed.

Colour related blindness?

Danni July 27, 2011 July 27, 2011

Forums

A friend sent me these pictures as the foal is by her stallion, wondering if silver was the culprit, being there is a lot of hype on the internet about not breeding silver horses together, foals are blind etc.. Splash was the obvious culprit in my mind, but I've never heard that splash would caused the retina to detach, which is what the vet found, so the foal was put down. The foal has two blue eyes, is blind in one eye and the vet thought about 90% loss of vision in the other eye. Further inspection found the retina detaching from the eye. Someone thought the foal had overly 'poppy' eyes, but then welshies have pretty poppy eyes anyway. The owners are trying to work out what caused it. So I think they are going to test for silver, after reading things about that. But realistically silver is so rare in the welshies anyway I doubt that's the culprit. Which brings me to splash, I figure the blue eyes and blindness are splash. But then why was the eye detaching? Do you thinking the blindness and detaching retina was totally not related to the blue eyes? Just curious on people's thoughts! Cheers Danni [img]http://img832.imageshack.us/img832/286/pennylanemollyanddam.jpg[/img] [img]http://img21.imageshack.us/img21/5030/pennylanemolly.jpg[/img]

Daylene Alford Wed, 07/27/2011 - 18:00

I think it's probably unrelated. Silver doesn't cause the retinas to detach it causes ulcers in the eye (I think that is right correct me if I'm wrong) and there are to many blued eyed splash horses that if splash were to blame blind blue eyed horses would be more common.

Danni Wed, 07/27/2011 - 19:25

Thanks, I didn't think silver could cause effects that bad, but wasn't sure if splash looking horses had been reported as blind before

Threnody Wed, 07/27/2011 - 22:53

Homozygous Silver causes detached retinas 10% of the time. It's rare but does happen. Heterozygotes get cysts that don't affect the sight. Poor little thing. :(

Monsterpony Thu, 07/28/2011 - 17:19

Silver could be the culprit. Homozygous silver causes multiple congenital ocular anomolies (MCOA). Many homozygous silvers have retina changes due to detachment and reattachment of the retina during embryogenesis; this can be seen in black lines around the edges of the retina and ridges within the retina. The most common reason for vision loss is luxated lens though.

I have never heard of a case of vision loss due to splash white.

Daylene Alford Thu, 07/28/2011 - 17:25

That is interesting I knew silver caused eye problems but I didn't realize they could be so extensive. Will you keep us updated when the test results get back?

Monsterpony Fri, 07/29/2011 - 00:28

GRAPHIC IMAGE WARNING
This is an image of the cysts that are found on all silver horses (Zz and ZZ)
[IMG]http://i83.photobucket.com/albums/j294/…]

Cyst eye on left, normal eye on right
[IMG]http://i83.photobucket.com/albums/j294/…]

Another thing seen in MCOA is called cornea globosa, which means the cornea is enlarged. This does not appear to cause visual deficits.
[IMG]http://i83.photobucket.com/albums/j294/…]

Danni Fri, 07/29/2011 - 20:13

Oh they were your pictures I think I've seen before, I do remember the cysts, they don't ever cause blindness though do they? So cysts are the normal silver thing, I think that's in my brain well and truely now, and accepted. Still having trouble getting my head around all the other more severe defects though, and really how common they are in other breeds. It's hard when I know of some many silvers and have never heard of eye problems, to think of it as a real problem, if that makes sense. The cornea globosa isn't related to the cysts? Just a 'thing' that happens to some of them and gives them that poppy eyed look?

Yeh I'll let you know what the test comes back as, I mean both ponies are chestnut based a fair way back so silver could hide and even though it's still so rare in the welshies, it would be just the luck to have two rare silver carriers not only have a homozygous silver foal but have one with the rare defects as well! It would go along way in helping convince me of the seriousness of silver breeding.

Monsterpony Fri, 07/29/2011 - 23:10

[quote=Danni]Oh they were your pictures I think I've seen before, I do remember the cysts, they don't ever cause blindness though do they? [/quote] If the cysts were really big, they might cause distortion of vision or maybe shadow-esque movement, but probably not something a person would be able to positively determine. Dr. David Ramsey has a great website with many more and better images of MCOA. http://www.eye-vet.com/silverdapple/
[quote=Danni]The cornea globosa isn't related to the cysts? Just a 'thing' that happens to some of them and gives them that poppy eyed look?[/quote] They are unrelated (except for the caused by the same mutation). Basically, it is hypothesized that the MCOA mutation causes changes in the eye during embryogenesis so that it does not develop perfectly. In cornea globosa, the cornea is too big. You can also get glaucoma due to the angle between the iris and cornea forming incorrectly. Retinal dysplasia (misshapen retina) is common because the retina of a ZZ horse detaches and reattaches during eye formation. MCOA occur because the eye is not forming correctly, hence why there are so many different anomalies seen and why every horse can be different in severity.

Danni Sat, 07/30/2011 - 02:30

[quote=Monsterpony]Retinal dysplasia (misshapen retina) is common because the retina of a ZZ horse detaches and reattaches during eye formation. [/quote]

But there has never been an actual study of tested homozygous silvers I didn't think? It seems it just been assumed that the worst cases were homozygous silvers? Which of course may not be a wrong assumption, but I do wonder why the more severe abnormalities aren't as noticeable in other silver breeds. Ramsey seems to have done a good study on what eye deformities he saw, but hasn't at all helped the inheritance of it. With things like a supposed Belgian in there that's assumed to be silver just because it had eye deformities doesn't really help his case. Maybe it's a combination of what Ramsey says and that other study saying it could just as likely be a common ancestor. The RMH's have inadvertently bred for it and made it worse?

[quote=Monsterpony]MCOA occur because the eye is not forming correctly, hence why there are so many different anomalies seen and why every horse can be different in severity.[/quote]

Ok, that makes sense, everything you say makes me want to run a mile from silvers, which I find very depressing. The stubbon part of me really wants to know though why say in Australia, silvers aren't at all known for eye problems, and "Taffies" have been extemely popular in our history, even if they went out of popularity for a while. You'd think someone would notice even if it was just happening the 2% or whatever it is. And if a breed like the Comtois has ever noticed a higher proportion of problems, and if they are as bad as the RMH's, same with the American Shetlands, and American Mini's etc..

Maigray Sat, 07/30/2011 - 09:04

Don't ever discount the human factor. Once you get involved in a breed, I have always found that certain problems or issues come out of the woodwork that you never knew existed.

Monsterpony Sat, 07/30/2011 - 17:53

The studies Dr. Ramsey did were before the silver mutation was identified so it couldn't be proven at that time. But if you read the full papers, a lot of research was done on familial inheritance patterns to determine as best as possible the zygosity of the subjects. That being said, there was one family line in particular that was determined to have homozygous silvers, but without notable MCOA, implying that there may be a protective gene that reduces the incidence of MCOA. Also, since the silver test came out, more research is being done. A tested homozygous silver Icelandic (in Iceland with only Icelandic breeding) was found to have MCOA. Given that the Icelandic gene pool has been closed off from other breeds for over 1000 years, it would be hard to put more blame on the occurrence of MCOA being because RMHs were unintentionally selected for it. I have also talked to owners of and examined silver shetlands and minis that do have MCOA. The images I posted above are from minis.

So basically, I am waiting for the research on MCOA to come out now that there is a test for silver to make a definitive answer.

And, yes, I do realize I contradicted myself more than once in the above paragraph. I am my own devil's advocate :D

Threnody Sat, 07/30/2011 - 18:08

I just want to say thank you so much for sharing the photos. Those are the clearest images of cysts caused by MCOA that I've ever seen.

Daylene Alford Sat, 07/30/2011 - 18:19

Would someone be kind enough to draw a circle around or an arrow pointing to the cysts? I have no idea what I'm supposed to be seeing.

Monsterpony Sat, 07/30/2011 - 23:47

MORE GRAPHIC IMAGES WARNING!

Sorry, I didn't even explain what you are looking at. The images are looking at the eye from the back. I used this (open source from wikipedia) illustration of a human eye to try to explain. Basically, I cut the eye open across the red line and then folded it back to invert the shape. The lens is now popping out into the area that would be the back of the eye. The ciliary bodies are what hold the lens in place and change the shape of the lens as you focus at different distances. The cysts are little clear sac-like pockets of fluid that develop on the ciliary bodies (yellow circle in the illustration). As you can see, the cysts are out of range of the lens, hence why they wouldn't cause visual impairment.

[IMG]http://i83.photobucket.com/albums/j294/…]

Now, on the actual image, the cyst is the clear 'bubble' circled in red on the edge of the thick black line surrounding the clear lens; the thick black line being the ciliary bodies. The greenish yellow area is part of the retina (tapetum lucidum to be specific).

[IMG]http://i83.photobucket.com/albums/j294/…]

On this image, both eyes are just cut in half without being inverted. The cyst is again circled in red. I am not positive if the fact that the retina (circled in yellow here) reaches all the way to the cilliary bodies is due to silver or if it is a normal variation in retina placement. The normal eye is that of a black tobiano so definitely no chance of MCOA due to silver. There is what looks like it could be a cyst on the lower right of the normal eye, but it is actually where two layers of the eye had started to separate.

[IMG]http://i83.photobucket.com/albums/j294/…]

This is a view of the normal eye (left) and silver eye (right). As you can see, the silver eye is shaped differently in general.

[IMG]http://i83.photobucket.com/albums/j294/…]

This is the mini that donated the silver eye for the images. She was euthanized for other reasons and I was very kindly given permission to examine and photograph the eyes.

[IMG]http://i83.photobucket.com/albums/j294/…]
[IMG]http://i83.photobucket.com/albums/j294/…]

Daylene Alford Sun, 07/31/2011 - 08:56

Thank you for doing that. I thought that might be what I was supposed to be looking for but I wasn't sure.

I circled two additional spots on the affected eye. Are they additional cysts or something else?

JNFerrigno Mon, 08/01/2011 - 12:17

So to confirm all Silver horses, heterozygous and homozygous, have cysts. The effects of these on hetero horses isn't known because it's hard to assess? Homo horses can have blindness but not all the time?

Monsterpony Mon, 08/01/2011 - 14:57

Cysts shouldn't cause any noticeable effects as they occur outside of the vision pathway. Homozygous silvers, in rare cases, have appreciable visual deficits based on anomalies. Luxated lenses and retinal detachment are likely the only ones to cause congenital blindness, though other anomalies can lead to the reduction in vision and eventual blindness.

Threnody Mon, 08/01/2011 - 18:21

Also about 12-18% of heterozygous silvers will be silent carriers with no cysts themselves but still have the disorder and pass it on to their offspring.

Monsterpony Mon, 08/01/2011 - 20:49

Well, at least you can't see the cysts, but that doesn't necessarily mean they don't have them. You actually have to look around the iris to see them and it is not easy.

JNFerrigno Tue, 08/02/2011 - 00:16

If you were to give it a percentage. How common do you feel blindness in Silver horses is?

I ask because I am doing some genetics for an equine game and we will be adding in things like this.

Danni Tue, 08/02/2011 - 01:04

[quote=JNFerrigno]If you were to give it a percentage. How common do you feel blindness in Silver horses is?[/quote]

From what I've seen, maybe 0.0001% LOL! Actual blindness seems extremely rare, but various changes in the eye aren't?

Monsterpony Tue, 08/02/2011 - 13:06

I think Dr. Ramsey found vision deficits in the single digit percentages in homozygous individuals. Again, I wish the new research would get published taking into account the silver mutation.

JNFerrigno Tue, 08/02/2011 - 13:32

Alright, thanks guys! That works, keeping it a low percentage in the game. But still enough to be aware in your breeding program. We want to teach people how to breed things out of their lines as well, so genetic disorders are great for that.

Danni Tue, 08/16/2011 - 23:37

Hey MP,

I told the people with the blind foal that you were doing your thesis on colour related problems and they said to show you the vet report to see what you thought. No colour test results back yet.
[img]http://www.gypsyhorsesaustralia.com/mis…]

Bit hard to read, I can type it out if you'd prefer. It said she had depigmentation of the fundas? Whatever that means. Didn't seem to have any cysts from what I can read. Anyway they'd love to know if you can draw any conclusions from their observations. The vet wants to know too now apparently LOL! You might be come an Australia vet consult ;)