Articles
Genetic Disorders: SCIDFrom public posts made by Dr. Heidi Smith, DVM
> What would be best scenario, of course, would be for every
> stallion owner to test their stallion and geld if he is positive. That
> would eliminate the bulk of the problem.
No, that is not what the geneticists say is the best scenario. If all positive horses were eliminated, we would lose roughly 10% of our gene pool, and would most likely shift it sufficiently to bring more lethals to the forefront (such as lavendar foal, cerebellar abiotrophy, and perhaps some not even yet known) for which we have no genetic tests.
The best scenario is to test, and then to treat SCID carrier status like any other fault, so that the frequency of the gene in the population is gradually reduced over time. In this way, the millions of other genes carried by horses that happen to carry this one faulty gene are not lost to the gene pool. The ability to test essentially disarms the disease--there is no longer ANY excuse to produce an affected foal. No carrier stallion should EVER be bred to a carrier mare, and all offspring of carriers should be tested to see if they are carriers or not. (Remember, HALF of them will be clear!)
Also, the best scenario involves honesty. The problem with the pressure from folks who would geld all the carriers is that it causes people to hide the carrier status of their stallions. If folks would listen to the experts in the field of population genetics and use testing as a management tool, as it SHOULD be used, there likely would be less folks unwilling to disclose the carrier status of their stallions.
I hope the QH folks realize that they don't need to flush all the Poco Bueno horses down the drain, either--in all likelihood, given the state of genetic technology today, they, too, will have a carrier test in the very near future. And if the quality of the individual carrier merits being bred on, I hope they will likewise have the sense to manage the disease and reduce the gene's frequency over time, instead of just jettisoning some of their best breeding stock.
It took hundreds of years for SCID to become entrenched in the Arabian population--it doesn't make sense to try to eliminate it overnight. Likewise, the information about the "Poco Bueno thing" states that as many as 5% of the pedigrees don't double Poco Bueno himself but rather go back to his ancestors, so clearly that mutation didn't arise with him, either, but rather further back. So they shouldn't expect to solve the problem in one generation, either.
But again--thanks to genetic testing, one need never produce a SCID affected foal even if one DOES judiciously breed a carrier individual (why is it always the great hue and cry to geld the stallions and ignore the mares, BTW?), and in the near future, the same should be true of HC in QHs.
Heidi
> As far as I am concerned - recessive or not I personally would not breed
> to horse (or breed a horse) that carried any genetic disorder.
Then I guess you don't plan on breeding horses? I doubt there is a horse alive that does not carry some sort of "genetic disorder" of one form or another.
The sad part about genetic testing is the backlash of folks who would toss every horse that carries a testable problem gene--the geneticists themselves don't recommend tossing them all out either, because that increases the frequency in the gene pool of the lethals (and the genes that cause clinical disease) for which there IS no test. Instead, geneticists advise using gene testing to manage diseases and gradually reduce their frequency, instead of simply using them as a black-and-white cull factor.
Heidi
> What is the history of CIDs in Arabs?
SCID has apparently been in Arabs since as far back as we know. It certainly isn't a "new" thing--the only thing new is the ability to do a genetic test to detect horses that are carriers. It was at one time thought that the carrier incidence might be as high as 25%, but from what I understand now, the geneticists feel that the incidence is much less.
> My breed is Morgans, not Arabs, but I have some dim
> recollection that not everybody in the Arab world shared
> Lif's outlook. In fact, I kind of thought the discovery of
> CIDs in Arabs created a huge storm of controversy and a
> knock-down, drag-out, blood all-over-the-place fight within
> the breed over whether CIDs carriers should be allowed to be
> registered at all.
Originally, when the condition was first identified and the mode of inheritance figured out, there WAS a big furor within the ranks of breeders, but for the most part, rationality and knowledge of modes of inheritance have taken over, and it is now better understood as a disease that can be avoided altogether with genetic testing.
> It was a lot like the Impressive Syndrome
> problem in Quarter Horses.
Not hardly! HYPP in Quarter Horses is not a recessive! Instead, it is a condition in which the genes can have an additive effect--i.e. the heterozygote is often affected with the problem, and a horse carrying two copies of the gene (the homozygote) is usually affected worse. On the contrary, the heterozygote SCID horse is COMPLETELY CLINICALLY NORMAL. Big difference! I personally would rather have a breeding animal that is conformationally correct, metabolically tough, athletic, kind, etc., and is a SCID carrier than I would want to trade any of the above traits for a clear SCID test. Don't get me wrong--I think it is wonderful to strive for less SCID carriers in the breed. But meanwhile, education is the key, and as long as breeders are aware of the problem and test for it, and make their breeding decisions accordingly, then there need never be another foal afflicted with SCIDS. Already they are declining in number with the availability of the test.
> Are Arabs required to be CIDs tested before they can be
> registered? With a positive or negative indication on the
> papers?
No.
> As somebody interested in heirloom mare lines in Morgans, for
> instance, one would hate to get in to the position where
> the only living descendants of a "clean" mare line now
> have the CIDs gene because the last daughter was crossed
> with a CIDs stallion who happened to fullfill the 50%
> chance to pass on the gene.
Unfortunately, it is the "heirlooms" of the Arabian world that seem to have the highest incidence, and lord knows that already too many of them have been lost to not being the "in" thing in the show ring, etc., etc. And thanks to the genetic test--if the "last daughter" of somebody happens to produce a carrier foal, that foal can go on and contribute, provided its mates are chosen wisely.
Heidi
Why would you breed a positive stallion to an untested mare? The whole point of having a genetic test is to be able to eliminate having an affected offspring. If you own the stallion, you have control over what he breeds. You can choose not to breed untested mares.
The point I was making is that per those who study population genetics, one should use gene tests to steer away from those very sorts of matings, not as a means of just chopping carriers out of the gene pool. When you do the latter, you also eliminate genetic material that you can never replace.
Heidi