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Talking point SCID

by Inge Craik

Frequency of SCID Carriers within Arabian Horses

Studies from just over 20 years ago, Poppie and McGuire, 1977 and Studdert, 1978, placed the SCID carrier frequency at about 28%. New studies using the now available DNA test for this specific defect provide more accurate and more current frequency data on this disease gene.

As of January this year, with over 3,926 Arabian Horses tested by DNA (across the globe and including Australia), the percentage of carriers is at 15.1%, somewhat lower than in the study mentioned above. This would indicate that if breeding selections were random, one in forty odd matings would produce a possible SCID affected foal. One could argue that the horses tested are not a random sampling and therefore the carrier percentage would be biased. This could be because people testing are suspicious of SCID in their line or that people testing are confident of a clear horse and want a certificate. In either case, with the number of horses tested to date, the carrier frequency reported above is probably not to far from the actual frequency of the breed as a whole. A clearer picture will develop as more horses are tested and using the DNA test as a tool in breeding programs will certainly reduce the frequency of carriers.

The Probabilities of producing SCID affected foals that die

When testing stallions and mares which are used in a breeding program, and only breeding carriers to clear animals, carriers may be successfully used thereby maintaining their valuable attributes, without ever producing a SCID affected foal, which always dies. Thereby the stigma of owning a carrier horse has been removed. See Breeding Strategies below:

Breeding Pair Combinations CLEAR
100% Clear 50/50
50/50 Carrier/Clear 25/50/25 Clr./Carr./Affctd.

The early research some 20 years ago, indicated that the frequency of affected Arabian foals was around 3% Current statistics, by the number of horses DNA tested, show a frequency of 0.5% NOTE however, that the SCID Affected Percentage only refers to the horses that have been DNA tested before they died (they ALL died). Other affected foals have died before being tested. It is therefore not possible to get a clear picture of the true statistic of affected foals within the Arabian horse breed.

Breeder experiences in using the SCID DNA test

Yvonne Barlow, Natham Park Stud (QLD) Australia

“I am very happy to have a DNA test for SCID. It is very important for the Arabians in the future that we try to eliminate the chances of producing even one or two affected foals. If a stallion is at public stud, it is important to know his SCID status. Most breeders have a live foal guarantee and we should have the chance of producing all live foals. Testing works for our breeding program by having the knowledge of the SCID status of our horses and therefore having the power to never produce an affected foal and reducing the incidence of SCID within the breed. I have had the experience of loosing a foal to SCID. At that time it was unknown to me that the stallion I used was SCID positive. After this unfortunate incidence, I DNA tested my mare and my own stallion, as I never wanted to have the experience of loosing a foal to SCID ever again.”

“I made the DNA test results public, both the clear result for my stallion and the carrier result for my mare. This issue has been in the cupboard far to long and if we are to reduce the incidence of SCID within the Arabian breed, it is important for breeders to be upfront with their carrier results. I was greatly encouraged by the fact that some years ago Ron and Val Males came forward and publicly announced that their stallion was a carrier and I respected them as breeders. SCID is nothing to fear and there is nothing to fear in owning a carrier, as long as bred responsibly and a carrier is put to a clear and not to another carrier. If we have a SCID carrier horse we should state this honestly and openly and should I ever sell my carrier mare, I will be sure to inform the new owner of her SCID carrier status, so they can be informed and use her wisely within their breeding program.”

Val Males, Ralvon Stud (NSW) Australia

“Ron and I worked with the researchers for years in the hope of getting a SCID test developed. We are thrilled this test is now available. If we wanted to use a specific stallion we would ask if he was SCID tested. If not, and the owner willing, we would offer to pay for the test if this were necessary. We have done so in the past and found stallion owners to be very co-operative. It seems the cost of the test is the biggest deterrent. Most people think it is available in the US at a much lower rate then here.”

“However, Inge explained that when the exchange rate is taken into account, this is not so. I have always thought that the benefit of knowing the SCID status far outweighs the cost, but cost does have to be taken into account in each individual situation and in some cases, may well be prohibitive. We have had many of our horses tested and know their status and are always pleased to discuss this. Whilst it is important to know the SCID status of stallions, we feel it is a great advantage to know the status of our mares. It is vital for people to understand that a clear stallion can be mated to an SCID carrier mare without any fear at all of producing an SCID affected foal. However, half of such offspring will be carriers and the other half clear. Thus, it is logical that breeders may wish to continue using an outstanding carrier stallion over clear mares and gradually have offspring tested. This would allow them to maintain progeny with the qualities they admire in their stallion and at the same time make an informed decision about any carriers produced. This would probably prove more expensive in the long run but the cost would be spread over a long period of time and may be more affordable.”

“Whichever way it is approached, the topic of SCID is still not as openly discussed, as it should be. As we are only human and hide our fears in various ways, this will not change quickly. Believe me, it was absolutely devastating for us both Emotionally and financially when The Puritan imp USA proved to be an SCID carrier. Taking the next frightening step to openly disclose this was almost as scary as not doing it and having somebody else do it for us! But it was worth the risk and allowed us the privilege of being able to hold our heads high and look the world in the face¹ knowing we had nothing to hide. On the plus side, many SCID carriers of the past have now been proven as influential sires and grandsires to talented and valuable endurance horses of today. Perhaps this talent would have been lost if those stallions had been immediately culled. We would certainly, if our stallion was still alive today with the test available, use him and in this case we would test the mares. We do know that many more foals have died than ever have been tested. This can now be avoided using the DNA test for SCID.”

Melanie Steel, Mihrabi Stud, (W.A.) Australia

“I have been involved with pure bred Arabians since 1986 when I purchased my first stallion that apparently was sired by a well known SCID carrier which I was made aware of by his owner. Of course at that time we did not have a test. Was I just lucky that he never produced an affected foal, or was he indeed clear of the disease; I guess I will never know whether my stallion was clear or a carrier? As people became aware that he was sired by a carrier, the mares stopped coming, as he was automatically assumed to be a carrier. Had the DNA test for SCID been available then, I could have had him tested this would have stopped the gossip and the uncertainty and had he proven to be a carrier I would have still purchased him for the wonderful attributes he had, but would have bred him to clear mares only. I am delighted that a DNA test for SCID is now available. I intend to test my future stallion and after that, one by one my mares. I think it is important to test your horses in particular the stallions as they produce the greater amount of progeny and you have a responsibility to the owners of outside mares.”

“It gives a breeder an informed choice. It would not stop me using a horse if he was a SCID carrier, but obviously you would not breed a carrier to a carrier. There are many good horses out there that are carriers and yes I would want to know because it is heart breaking when you loose a foal. There will always be those people who don't want to know about it, but jet are quick to point fingers. There are still many people who do not understand SCID and believe it is something horses can catch from each other. I think the more open and honest we can be about it, the better it will be for the benefit of the breed. If all horses that are carriers were gelded we would loose the often great attributes the can offer to the breed, but with the now available test they no longer have to be eliminated. As far as cost effectiveness goes, it is certainly cheaper to test your horses then to loose a valuable foal.”

The potential of combining SCID testing and Profiling for the Arabian Horse.

The current procedure consists of having one organization DNA profile a horse for the purpose of registration with one sample and another company perform the completely different DNA test for SCID status from a separate sample. It is often discussed that this seems unduly inefficient and cumbersome for the horse owner/breeder and perhaps overly expensive. A much better way would be to accept results for registration purposes also from the company providing the SCID tests. Test validations between the two organizations would assure the breed fancy that each laboratory was producing accurate and compatible results. Another possibility would be to have the profiling laboratory split the sample, at the option of the breeder/owner, and forward these samples on for SCID testing. Either of these techniques would be more convenient for the horse owner and through convenience and education, lead to more SCID testing and test utilization and ultimately to the significant reduction in the incidence of SCID in the breed