Cerebellar Abiotrophy (CA) , A Serious Concern

Originally posted by SiriusThanks for your reply, but I'm still not happy. I think that testing now could actually lead to a worsening of the situation if people are overconfident about their results.

Why should people be over confident about their results Dom?
A N/N CLEAR horse can not have the defective CA allele.
A N/CA CARRIER horse does carry one copy of the defective CA allele.
A CA/CA AFFECTED horse/foal is obviously affected and its parents both will be CARRIERS. When euthanized its affected status is confirmed by histopathological diagnosis.

My problem is in your assertion that the markers accompany 100% of defective alleles. I would suggest that this is over confident. I am sure that it is a confident statement in that 100% of unambiguously diagnosed affected CA/CA so far tested have been reported to have the markers.

Please read post on Haplotype testing of well over one thousand samples.

I presume that UC Davis have been breeding and testing a herd of N/CA and CA/CA in-house rather than just testing samples from the national herd. However, it is vital to identify all of the carriers and only the carriers in this, and my worry is that those who test now will believe the results are definitive.

Your presumption is incorrect. UC Davis does not have a test herd. Nor is their testing limited to USA arabians only. They receive samples for testing from all countries and all bloodlines. Do not confuse the VGL lab at UC Davis with the VetGen Laboratory which handles SCID testing.

I am quite concerned that we now have people posting "clear" results with the implied confidence that the horses are N/N.

They are posting N/N "Clear" results with confidence because their horses DO NOT have the haplotype for the defective mutated CA allele. If it is not there Dom , it simply is not there.

Undoubtedly they have tested for all of the right reasons, and have done absolutely everything available to avoid breeding CA/CA foals, but there are three big questions for me:

1/ With rare bloodlines said to be carrying the CA allele, will owners of quality horses that have CA/N indications now terminate those lines? A great shame if so, as our experience of SCID, and of PRA, CNM, EIC and narcolepsy in labradors, shows that they can be completely eradicated in two generations at the worst;

There is NO reason for quality breeding stock that are N/CA, CARRIERS, to terminate breeding forward their bloodlines. N/CA stallions require outside mares to be tested; N/CA mares are used to breed to N/N Clear stallions. If two tested carriers are bred together , both mare and stallion owners are aware of the risks. Please study the posted Status List at the CA website (not the very outdated one here on this thread)and you will see tested clear foals resulting from 2 carriers. It can happen.

2/ Will those people retest their horses if/when the allele is identified and a direct test becomes available? This is my main concern, as I was about to spend around �1000 which I cannot afford to do and then redo.

Why should they? The present testng is a reliable tool for making breeding selections in order to avoid producing affected foals.
If you yourself are uncomfortable about the marker scanning test and do not fully understand about Haplotype testing, then dont spend your money.

3/ Have any clears thrown positive results for the markers, or is this possible? Indeed, is there a way of knowing the answer to this, as carriers are asymptomatic? If the test mislabels a rare-lined clear as a carrier, that could terminate what we are all trying to preserve. Only a very large test herd can answer this question, and then over several generations.

Your question about clears being able to throw positive results for markers tells me that there are still some fuzzy borders around your own knowledge of genetics. It is NOT possibe for a clear N/N horse to have carrier markers in the genome makeup. That is why they are called "Clear or Normal". Yes. Carriers are asymptomatic as you say. It is more correct to say that Carriers do not have or express any clinical signs. (In medical language only those who can explain what symptoms they have,can use the terminology of symptoms. All else are clinical signs.) As stated UC Davis no longer has a test herd and I dont know where you got that idea from. Back in 1985 the late Ann T. Bowling delt with a test herd of 40 head which consisted of carriers and affecteds collected from different bloodlnes and from across the United States. From that herd she established the recessive mode of inheritance. Post mortum necropsies proved the atrophy of the Purkinje cells, the definitive diagnosis of CA.

If the current test eventually proves not to be 99.99% reliable in picking up the defective allele in the wider international herd, the premature publication of "clear" results could actually have an enormously damaging Affect(sic).

The current test HAS BEEN proven to be reliable in the "International Herd" (again please read Haplotype Post and review the Public Status List.) To call it a "premature" publication of results may be reflective of your own approach, but it is obviously not reflective of the many well known and responcible breeders who have disclosed publically their "clears", "carriers" and "affecteds". In furthering your own research, I highly recommend that you look up the pedigrees at www.allbreed.com and/or the AHA Datasource. Links are provided onsite. This will give a better idea of the scope of bloodlines and the variation of countries involved.

If our stallions test N/N, then with our plans they will be breeding prolifically, and the publication of their results will probably increase the number of mares visiting them, helping to recoup the testing cost.

Hopefully that would be correct.

There are a number of cases of carrier labrador retrievers of magnificent working achievement who have had truely monsterous numbers of puppies, e.g. XXX who sired over 8000 and was certainly a carrier. Their owners all used the best knowledge available at the time to produce the best quality sire dogs, and the parallel is chilling.

In the arabian horse world a similar situation has occurred on several occassions: Muscat 1971-1996 , sire of significance had over 1000 get and 5524 grandget and his bloodlines are still sought out for their superlative action in spite of being an acknowledged carrier. Magnum Psyche, sire of 1197 and 1637 grandget(a large number for a living sire),is another prolifically used confirmed carrier. A good horse is a good horse and a top quality breeding sire is not going to be limited by a CA Carrier status.[

u]Let me add that I am a huge believer in testing[/u]. All of our herd are SCID tested, and the results are publicly available on our website especially the late Hanson's SCID carrier status. We have been successfully breeding out the large number of genetic diseases that afflict working labradors, and have the most tested (and most multiply completely N/N clear) working breeding pack in the UK. I went onto the Van Haeringen website specifically to find the test order form for CA, which is where I found the info I originally quoted.

Yes . I am aware of the Van Haeringen website as I was the one who first started to post it on the discussion groups. As the testing process orignated at UC Davis I suggest reading their website information and contacting Dr. Penedo or her assistant Leah Brault directly. This may assuage some of your doubts as to the accuracy of the present testing, if the Haplotype post has not.

We will test, publish, and publicise as soon as we are confident that the test available is definitive. In the meanime, we are certainly more aware of the possibility of CA and will be vigilant.

Dom
Seren Arabians

p.s. I want to be wrong on this one!