Saturday, November 20, 2010

Behind Blue Eyes: Congenital Stationary Night Blindness


Tommy, can you see me?
Can I help to cheer you?
— Pete Townshend (The Who) from the 1969 album, "Tommy"

One evening soon after Daltrey came home, I was spreading Fly Eliminators in the back pasture. The sun had set, but I could still see clearly. As I headed back to the house, I noticed Daltrey standing close to the culvert that bridges the front and back of the property. It seemed odd, because at that time, he still stuck pretty close to his nannies, who were nowhere to be seen. In any case, he wasn’t grazing, just standing there, about 50 paces away. I greeted him in a normal tone of voice, and the little guy jumped straight up. He obviously hadn’t seen or heard me coming, although he was looking right at me.

If you followed Heather’s genetics posts, you know that Daltrey exhibits a number of color and pattern traits gone wild. Seems like if there was any chance of a pattern gene expressing itself, it did in him. We’ve known since his birth that Daltrey is congenitally hearing impaired to some degree, related to his extreme splashed white pattern (read about splashed white here, and look in the right column for links to all the color/pattern posts). That’s a post for another day. The splashed white pattern is also responsible for Daltrey’s startling blue eye color, which does not affect his sight.

However, Daltrey is also homozygous for leopard-pattern complex (LP/LP), which was directly tied to congenital stationary night blindness (CSNB) in a 2005-2006 study. Appaloosas that are heterozygous for LP (LP/lp) are not affected by CSNB. Horses that are true solid, non-characteristic Appaloosas, with no dominant copies of LP (lp/lp), are also not affected by CSNB.

Click on chart to enlarge.
Daltrey is best represented by the bottom row, third from left.
Poco is bottom row, second from right.

So, coat patterns that indicate the presence of two dominant copies of LP are associated with CSNB. The amount of white patterning they display is not important, except that when they have moderate to large amounts of patterning, they are easier to classify correctly as being homozygous for LP. Horses homozygous for LP born with no coat patterning are also night blind.

There may be a variety of causes for CSNB in horses, but we’re looking only at the type found in Appaloosas. CSNB has been detected in other breeds (Paso Fino, Thoroughbred, Quarter Horse, Belgian), none of which were Paint or Appaloosa patterned.

Night blindness is not a single disorder. A horse can have a form of night blindness and not have CSNB (uveitis is the most common other cause). In general, the term CSNB is used to describe impaired/absent night vision, which is present at birth, inherited, and non-progressive.  Incidentally, the "stationary" part of the name refers to the fact the condition is non-progressive, and has nothing to do with whether or not the horse is standing still. CSNB-affected Appaloosas usually have normal day vision.

Because we typically don’t interact with our horses after dark, we might not ever see some of the symptoms, such as anxiety or closely following another horse at night. Under normal circumstances, affected horses cope with the condition because they're born with it. They just think that's the way the world works: at night, they can't see. The world is completely black, unless the horse looks straight at the moon or some other light source that is bright enough to appear as a light spot in its field of vision. Poco has it, though he is either not impaired to the degree that Daltrey is, or he is just that much more accustomed to it because he's older (and his hearing is fine).

There is nothing structurally wrong with the eyes of horses with CSNB. The problem occurs during the process of transmitting information from the eyes to the horse's brain. Cone-shaped photoreceptors transmit color and day vision, while rod-shaped photoreceptors are responsible for vision in the dark. In horses with CSNB, the information normally carried by the rod-shaped photoreceptors is dropped or not transmitted properly.

The study linked CSNB in Appaloosas to the leopard complex, but that doesn’t explain how two apparently independent genetic features are actually related. Research in 2008 proved both CSNB and leopard pattern in Appaloosas are caused by a gene whose simplified name is TRPM1. Simply, decreased expression of TRPM1 in the eye and the skin alters signaling from the rod-shaped photoreceptors in the retina as well as the function of certain pigment cells in Apps, thus causing both CSNB and leopard complex.

I may not have ever noticed the degree of Daltrey's impairment were it not for having to play Cafeteria Lady each evening. I have seen first hand that he operates by feel and memory. For example, if he raises his head to chew a mouthful, and he either takes a step or two, or I slide the feed pan, he has to feel around to find it again. It doesn't seem to bother him. I have not found him to be anxious or apprehensive at all. And (knock wood), he has not injured himself.

Gives a whole new meaning to Mike's coining of the term daylight pony for Baby D.

Disclaimer: I am not a veterinary professional. This information is presented as entertainment only. I attempted to obtain permission from the authors and publishers to use this information, but after more than 60 days, my efforts have not been acknowledged. Information came from the following sites: www.appaloosaproject.com, www.horsetalk.co/nz, www.genetics.org and www.thehorse.com. Regardless, most of the actual data about CSNB is the result of a 2005-2006 study by Lynne S. Sandmeyer, DVM, DVSc, DACVO; Carrie B. Breaux, DVM; Sheila Archer, BSc Hons; and Bruce H. Grahn DVM, ACBP, DACVO. The information about TRPM1 is from a 2008 study, which also included Dr. Sandmeyer, as well as Rebecca R. Bellone, Samantha A. Brooks, Barbara A. Murphy, George Forsyth, Sheila Archer (of the Appaloosa Project), Ernest Bailey, and Bruce Grahn.

24 comments:

fernvalley01 said...

Very interesting . But truly ,as he was born with it I suspect it will not be a huge issue for him

Mrs Mom said...

Excellent excellent information packed post Leah. I'm with Ms Fernvalley too- since the smashingly sexy Baby D the Daylight Pony has had it since birth, he'll cope just fine! ;)

Crew is looking mighty fine out there this fall! It's all kinds of fun watching Baby D grow up in the blog!!!

Pinzgauer said...

You phrased that really really well Leah! And having a horse that is both hearing impaired as well as night blind will, be, um... interesting.

Never in a million years did I think I would have all of those genes express at once(it's like what, a 1.8% chance?), and I know he'll do great with you, and that you'll understand his limitations, what little there really are of them. Most of your management practices fit with his "special" needs anyway.

Super nice post. And you know how I love all that sciency stuff.

Leah Fry said...

Glad you enjoyed, Fern & MM. I've been working on this post off and on for 2 months.

Heather, coming from you that's quite a compliment. I worked hard on this one, researching, digesting, and translating into semi non-geek speak. Thanks to college zoology and your posts, I was able to make sense of it all. Maybe I'm turning into a science geek.

I'll tackle his hearing impairment next, but it might take a while to research.

Johara said...

I have an Lp/Lp Appy mare with it, and my 2yr old gelding son of hers has it as well. They cope very well, except for the one night an unhappy momma cow came rampaging through the neighborhood. My mare body-slammed my big Dodge truck, but at least she hit it instead of the flimsy storage shed it was parked next to. >.<
(The truck is out of commission, but the mare was fine - just scared!)

Laughing Orca Ranch said...

Fascinating information. I had no idea. Does this mean you won't be doing much night riding on D?

I would have guessed that D's patterning fit the picture of the second horse from the left and not the third, because his neck and front legs are covered with so much white. And his his tail is so lught colored, too.
He sure is a horse of a different color, that's for sure.

~Lisa

Dunappy said...

My old mare was 20 years old when I put her down due to arthritis. She was night blind and we didn't have to many problems. Just remember the following things.
1. Keep the feed tub in the same spot.
2. Turn him out in strange places early in the morning so he can explore and find all the spots during the day.
And finally if you want to ride at night, Check this out,
http://www.dog-e-lites.com/horse-e-lites/nylon-classic-collection/67.htm

http://www.saddlelights.com./Home_Page.html

I'm going to get some just because I don't have time to ride during the day here all winter and It will help me get in more riding time.

the7msn said...

Wow...I was able to read through your post on genetics and my head didn't explode like it usually does. Interesting stuff, and it only adds to Daltrey's unique, charming self.

Nuzzling Muzzles said...

You blinded me with science. Interesting post. I haven't known a horse with this condition.

Hopeful said...

wow, you really know your stuff! very interesting and i had no idea. best of all, it sounds like he will be find and it's just something good for you guys to be aware of. thanks for sharing your information!

Leah Fry said...

Sylvia, I just did lots of research and took my time figuring it out, that's all. Trust me, I'm not that smart.

Rising Rainbow said...

I had no idea such a thing existed in horses. Thanks for the informative post.

Fantastyk Voyager said...

Thanks for such an informative post. It's too bad little Daltry's got it but he seems to be managing quite well. Knowing about it is good because you can work through it easier.

Sage Gray said...

Your post was sent to me by Kristin a friend.Thanks for all the great information. My 12 year old mare just started showing enough signs for me to realize something was wrong this summer. I got Star as a yearling from a guy who saved her and some others from the killers at an aution. At that time we didn't know she was Appy as she had no spots just a white star on her forhead. Over the years the she has got more and more spots in her sorrel coat mostly on her face, neck, sholders, chest, and belly. None on her back or butt. She always had a strange personallity that never led me to trust her as a saddle horse, or even out of her pen in a starnge area. Now I know why. I didn't understand all of what you posted but will reread and study it and maybe I will. I am working hard to get her to respond more to voice comands as the vet thinks she only has about 25 percent of her sight left. She seems to hear good, and knows her way around her regular pen and our larger turnout pen. I post at http://tumbleweedcrossing.blogspot.com under the name of Sage but my friend knows my real name of Barbara.

morningbrayfarm said...

How absolutely interesting - I didn't know all of this. I'm glad it's not progressive and agree with the other posters that Baby D should end up doing just fine. You're an awesome mamma! :)

Chelsi said...

Thanks for all the info! When I read "blindess" and "blue eyes" I have to admit I was tempted to pretend I hadnt:) I have noticed that LP doesnt like shadows, it is like she cant see in to them. When I have the vet out for shots in the spring I am going to have her looked at. Excellent post. Thanks;)

Leah Fry said...

Chelsi, I was wondering the same thing when I looked at Princess. It's amazing how well they adjust, though.

Dunappy said...

Sage, If your horse has difficulty seeing during the Day, then your horse does not have CSNB. There is something else going on with your horses. There are multiple eye related issues that can cause DAY time blindness. Things like
ERU (Equine Recurrant Uevitis)
Cataracts,
Glaucoma and several other issues.

CSNB is only when a horse cannot see in low light.

Leah Fry said...

Dunappy, I told her the same thing over at her blog. Either way, I hope she figures it out.

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