Friday, January 25, 2008

Let's Talk Genetic Disorders in Horses

Reading through other blogs today, I came across this post
Down on the farm...Lethal White It tells the writer's heart breaking experience with a lethal white foal. It got me thinking with breeding season just around the corner here in the northern hemisphere, this is probably a good time to talk about genetic disorders in horses.

While there are many genetic disorders in the equine world, I'm going to target the lethals that I am aware of with one exception. I would be remiss to not include HyPP in the AQHA. While this gene causes death in only very extreme cases, it is truly insidious.

In the lethal category there is the lethal white gene in the overo and both SCID (formerly known as CID) and Lavender Foal Syndrome in the Arabian. With both of the Arabian genes it is possible for it to manifest in descendants of the Arabian horse so half Arabians are not exempt from scrutiny.

All four of these genetic diseases are caused by a recessive gene. What that means is that it takes a gene from both parents for the disease to manifest itself. A horse that has only one gene for the disease is considered to be a "carrier" of the disease. A carrier will not ever manifest symptoms. It only has the ability to "pass" the gene on to its offspring providing. Breeding two "carrier" horses has a 25% chance of producing a foal free of the gene, a 50% chance of producing a carrier and a 25% of producing an "affected" foal. "Affected" means the resulting foal has the disease.

Hyperkalemic Periodic Paralysis

HyPP has been traced back to the stallion, Impressive. It is a neurological disorder that can express itself across a full spectrum from no outward signs at all to death. Also those horses that do show symptoms can do so sporadically making it much more difficult for the uninformed horse owner to identify.

From the website Bringing Light to HyPP

HyPP affects the sodium channels of afflicted horses and overloads the system with high potassium levels that cause episodes which may include: mild muscle twitching that is undetectable to the human eye; noticeable muscle twitching; "crawling" skin, ranging from slight to very noticeable and usually from the back flank area forward; hind quarter paralysis; excessive yawning; and paralysis of the muscles surrounding the heart and/or lungs, causing death due to heart attack or suffocation.

Because symptoms are not always detected and horses are not being tested, carrier horses are unknowingly being bred to carrier horses. The numbers of affected horses are growing within the breed. In addition unsuspecting buyers are being sold horses who sporadically manifest symptoms. Neither scenario is good for the horse industry.

I would advise readers to please visit the above website for more information on this disease. There is a genetic test to identify carriers of this disease.

Severe Combined Immunodeficiency

SCID previously known as CID (Combined Immunodeficiency) causes the foal to be born with a severely compromised immune system that cannot be fixed. The foal must be euthanized or die an agonizing death. Lacking immune capacity, such foals will succumb before 5 months of age of massive infections.

SCID foals were identified as early as the 1960s in Australia. Records of their birth in the United States didn't happen until the late 1970s. Because there was no known way to determine where this curse came from, rumors spread like wildfire. Horses and breeder's reputations were destroyed by talk without evidence. Many didn't talk about it at all for fear that they might become the subject of such rumors.

The year that Legs was born, 1996, VetGen perfected the test to identify the carriers of SCID. Since the test made it possible to determine the carriers of the disease, it was time to get this thing out of the closet and into the open. Never again did any breeder need to lose a cherished foal from this disease.

The Paradigm Has Shifted was written after the test for this disorder was available It tells in layman's terms about the disease and responsible breeding practices now that horses can be tested to determine carriers. Having the test available has changed the history of the Arabian horse.

It is important to note that a carrier can be ANY horse with Arabian blood, it does not have to be a purebred. Many people believe (and I used to be one of them) that only purebreds can carry this gene. That is NOT the case. Even breeders of half Arabian horses when mating two horses that each carry Arabian blood should be testing their breeding stock. They can lose foals from SCID by breeding two carrier horses.

Lavender Foal Syndrome

The second lethal genetic disorder known to Arabian horses is Lavender Foal Syndrome . It is found in straight Egyptian breed horses mostly but can happen in horses with certain Egyptian bloodlines in the pedigree.

A foal born with this syndrome tends to have an odd coloration as an indicator. The actual problem is within the brain, the neuro centers don't work properly causing the foals to have rigid arched backs and necks with heads up high. The foals are unable to stand. The only way this disorder can be positively identified is through brain tissue from affected foals.

This disorder is very rare and relatively new. Little is known about it other than it is lethal. There are studies being conducted but it is imperative that samples from suspected affected foals be submitted for use in these studies. Most importantly required is brain tissue. Contact information and instructions for submission of samples are here This disease can be stopped on if those affected by it today will participate these studies.

Some of you may remember that I witnessed the birth of a Lavender foal in the 2007 foaling season. I wrote about that experience Lavender Foal Syndrome - An Arabian Horse Breeders Nightmare I also wrote a follow up regarding breeding issues Breeding Carriers of Lavender Foal Syndrome in Arabian Horses

All of these genetic disorders are devastating to the owners and breeders who experience them. Through education, testing and co-operation with studies, it is possible to restrict and eventually eliminate this heart-ache. Please check out the included links for more complete information. Have a happy, informed and successful breeding season.

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  1. arched backs and necks , with extended limbs, generally indicates that the higher thinking parts of the brain are not functional. Very interesting post. I think as breeders it is important to know what your horse carries so you can avoid diseases like this. The avoidance of the HYPP thing is really terrible.

  2. Good post.....very enlightening.

  3. I remember, when I came to your blog for the first time, the first blog I read on your blog was about a lavender foal.. so sad. It's good to know, that people are working on to prevent these kind of things.

    I would just thing, if you breed a res horse, or red roan, to blue roan, you'd get a purple horse, because when you paint, and add blue and red, you get purple. And if you breed red roan to blue roan, you'd get a lavender color foal.. but really it's just the parents being a carrier of the disease.

    Anyway, I really like your blog a lot, and I've learned quite a bit from reading your blog. So never stop blogging :)

  4. Thanks for the information, MiKael. It's so heart-breaking to lose a foal.

  5. Interesting post. I read through it and discovered the part about high potassium levels and realized that I was just in the doctor's office yesterday because of high potassium levels. I didn't know it until the doctor told me that they use high potassium level shots to put dogs to sleep. He also told me there are problems with how blood is drawn that may effect the results. Anyway, I got to start drinking more water.

    I wish, the diseases showed or described the results or how the horse looks if it has that disease. I guess that would not be possible.

    An interesting post.

  6. You bring up some good points to ponder MiKael. Not being a breeder myself, or even interested in breeding (though I DO dearly love foals) I have never really give much thought to genetic disorders- aside from HyPP and other QH associated disorders (ie: HERDA). I had heard my life long and dearest friend discuss the Lethal White gene, as she breeds Paints, but had absolutely NO clue about what may affect the Arabian breed. (Since the only time these days I deal with Arabs is under them on a rare occasion...LOL) (Oh, that just did not sound right- I think I need more coffee...)

    At any rate- thank you for passing along valuable information, so that people can make a more informaed choice in their breeding efforts.

    As always, keep up the excellent work here! Cant wait to read more! ;)

  7. beckz, yes the damage is not visible to the naked eye or the vets dealing with the breeders would be able to do a necropsy and make the diagnosis. And how this goes hand in hand with the double dilute that affects the color will be interesting to learn.

    callie, thank you, this is scary stuff.

    kim, unfortunately the genetics of color are not as simple as color on a palate.

    I hope to never quite blogging, but then never is a pretty long time. lol

    victoria, yes it is heartbreaking to lose a foal. This lethal white was especially traumatic because she kept the mare alive to save the foal in the first place. So very sad.

    abraham, potasium levels are nothing to mess around with. You take care of yourself.

    I think that the lavendar foal syndrome is the only one that you can tell just looking at the foal that anything is wrong. The others look normal.

    mrs mom, I did chuckle at being under an Arabian. lol

  8. Thank you for the great post! I had no idea that there are lethal issues for the Arabian breed.

    As for HYPP, I definately knew about that one, and have been very careful with my AQHA mare (she does not go back to impressive).

  9. Some new posts on my blog.. :)

  10. Unfortunately, no pictures for my blog, but today, I got some awesome riding boots, they look like and sound like the ones you got for last Christmas, and a helmet, and we saddled Dandy, and we lunged Dandy (which was the best thing to do for a Arabian who has been stalled while his owwie has been healing) and he totally was just ripping it up in the arena and speeding, and then he tripped a little, and that made him fart, LOL... and then 1 of the 2 ladies that lunged him, rode him before me, and he did well, so then I got to get on him and she held the lunge line on him, and then I got to free ride while she directed me to ride, and I learned how to tuck my butt in, and hold the reins and ride around the arena, and we did sooo well, soo soo soo well for our first ride together!! and then he was going to lay down to roll, so I got my feet out of the stirrups and I was off and out of the saddle soo fast. And it just went so well. And when he was first lunging, I was remembering what you wrote about Rhythm when you lunged him, and how he lunged after being stalled for a week, and I was like oh boy, I kinda know what to expect from Dandy since he hasn't been worked for along time, and hasn't been able to run around like this for awhile. And they said Dandy had pep in his step, and like he was a ball of fire running around. So this is going to be fun now, and now when I come to see Dandy, I can have him lunged before riding, even though I'll be riding him a few times a week now. And Dandy was so happy to be worked and ridden today. And I am happy too. And I am so excited to be riding him now. Did you know, Dandy is 20 yrs young this year?? he sure doesn't act old at all. In fact, he probably wont act old for another 10 years :). But that's typical Arabian :) He act's like a 10 yr old. So, do you think, that I could show Dandy? could I possibly show this year or next if Dandy gets tune ups, and me get training? I know he'd need hair clipping, grooming, bathing, and hair laying on the right side of his neck, and to use the mane bands to help them stick on the right. What else should I learn?? can Dandy show, even though he has a circled W on his right rump? is there any disqualifications, that could disqualify Dandy from showing? and do you think that if I was the best and Dandy was the best, that we would show good? do you think the judge(s) would like us? do you think that me and Dandy would ever show well? is there required saddles for showing? do I need a show saddle? and if so, what kind of saddle, is the show saddle? and which shows should we be showing in, if me and Dandy wanted to go to shows to show for fun, and ribbons. But even if I was to never win a single ribbon, I would still have fun if me and Dandy could show. So do you think that Dandy and me could ever show? let me know! Thanks.

  11. I'm sorry the e-mail you sent me was returned :( I don't know why that would happen, but instead of sending the e-mail again I thought I would just come here and give you my e-mail address (I don't remember off the top of my head if it is on my blog, although I think it probably is). Anyway, the address is Astaryth at I don't put the @ sign in there in comments because I don't want it picked up by one of those automated bots LOL!

  12. Good information for everyone. Thanks.

  13. Straight Egyptian Arabs are one of the great joys of life--it's sad to hear that they have their own unique genetic defect!

  14. TCavanaugh, I've heard it said that the Arabian horse has the most closed gene pool of all horses. That would say that genetic difficulties like these are built in and breeding should be carefully planned.

    kim, glad to see you're posting again.

    Cathy ~ Tadpoles and Teacups, thanks for visiting my blog.

    kim, I think you wrote a book here. You aren't getting excited about the prospect of showing, are you?

    Astaryth, yes those spiders can be a pain. lol

    Grey Horse Matters, thanks I feel like I left so much out.

    PrayerMom, yes it is sad for them, because it makes that gene pool even smaller.

  15. Thank you for the great post! I have a similar blog, where I am talking about Genetic studbook winners of the Olympic Games (2012 London).