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 http://risingrainbow.blogspot.com/2007/06/lavender-foal-syndrome-arabian-horse.html 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.



Visit Blog Village and vote daily for this blog Here They are now measuring the rankings by votes out, so if you find my blog on the site, please click that link too to improve my rankings. TY


Arabian horse arabian horses horses horse lavender foal syndromeHyPPSCIDSevere Combined ImmunodefiencyLethal White SyndromeSCIDCID