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Re: RC: CIDs



>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.  It was a lot like the Impressive Syndrome
>problem in Quarter Horses.
>

	Most of the controversy was due to the lack of a test (other than a dead
foal)--one was at a disadvantage before the recently developed carrier
test, and as the attitude towards carriers was (and still is, to some
extent) that is was a moral flaw as opposed to a genetic one, even folks
who *knew* the carrier status of their stock as a result of production of a
SCID foal would hide the fact.


>Are Arabs required to be CIDs tested before they can be
>registered?  With a positive or negative indication on the
>papers?


	No. The Registry has a longstanding position of only being in charge of
the accurate recording of parentage. Nothing more. On a certain level, I
agree with them.

	If someone is planning to breed, they should test their own stock and ask
for results from the stallion or mare owner. That's part of responsible
breeding. 
	I consider it a different kettle of fish from HYPP, as carriers are
themselves completely unaffected by the mutation.


>
>The problem is that going "laissez-faire" with CIDs, even
>with testing, still means that the recessive spreads
>much more widely through the population and there become
>fewer and fewer horses that you can match together. 


	Not necessarily.  As Michael Bowling has pointed out, there are a number
of recessive lethals in Arabian genetics. Selecting *against* the one we
can test for will result in an increase in the untestables. Better the
devil you know...
	With the test available, one could safely breed the carrier to a
non-carrier, and there's a 50/50 chance the offspring won't carry the gene
at all. I wouldn't see that as spreading the gene more widely. 


 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.


	In which case, one might simply breed the carrier mare to a suitable
stallion who *wasn't* a carrier, test the offspring, and end up with
progeny which were "clean".

>
>A human worst-case example with similar inheritance
>properties and similar ultimately fatal results is
>sicle cell anemia.


	No, because heterozygotes are perfectly normal (not the case with SCA) and
homozygotes don't live long enough to reproduce. Besides which, given the
existence of the test, there is no reason at all to ever produce another
homozygous foal. So, with the current technology, no foal need ever die of
SCIDS. One can't apply the sort of  "planned mating" to humans without
getting into serious ethical issues that don't apply to livestock. 

>
>What is the history of CIDs in Arabs?  Did it start out
>in one horse or one breeder's herd like Impressive Syndrome,
>or was it always kind of endemic and spread evenly
>throughout the population? 

	It is still a matter of speculation, though there are closed breeding
groups which have not had any SCID foals.

>How widespread is the presence
>of the gene now? 

Previous estimates were in the vicinity of 25%, but that figure is way
high, according to informed opinion.


>Is its incidence being tracked?

		Not in any formal way, though the labs which perform the test are most
likely keeping statistics on percent positives.

				--CMNewell, DVM 



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