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Avoiding SCID in Arabian Horses

by Inge Craik

There is a DNA test for Severe Combined Immunodeficiency "SCID" in Arabian Horses.

SCID was first reported in Arabian foals in 1973 by McGuire and Poppie, Australia. In 1980, Perryman and Torbeck, in the U.S., showed that SCID in Arabian horses was inherited as an autosomal recessive condition, which means that one copy of the disease gene is inherited from a carrier stallion and another from a carrier mare. The foal which inherits two copies of the disease gene is affected with a lethal inability to fight infections, and dies within the first few months of life.

Matings between two clear horses as well as matings between a clear and a carrier horse will NEVER produce an affected animal. By definition, carriers of genes for autosomal recessive disorders are completely free of clinical signs of the disease. That is, carriers do not have any negative consequences to their health or performance. If two carriers are mated, there is a 25% chance that the foal will be clear, 50% chance that it will be a carrier and 25% chance that it will be affected, a chance not worth taking.

Prior to the advent of molecular genetic testing for autosomal recessive disorders, the only way an Arabian Horse was identified as carrier was when he or she produced an affected offspring. The traditional recommendation in veterinary medicine would be gelding of these animals to prevent other affected offspring being produced. This is no longer necessary and not in the best interest of the breed. Carrier horses that have desirable traits can now be mated to tested horses that are clear and never produce an affected foal. Their offspring can be tested and appropriate matings set up in the next generations without the breed suffering the loss of another foal to SCID. The breed continues to benefit from all of the outstanding traits a carrier animal may possess. The economic value of the Arabian Horse should not be affected by being clear or carrier.

Testing for SCID is easy and highly accurate, and can be performed at any point in time in the life of the animal and can be conducted by using simple cheek swabs. Sample collection using cheek swabs is non invasive, convenient and easy to use by the owner or veterinarian (Cheek Swabs have become the universal standard for human DNA screening, primarily for the purpose of parentage testing and the legal system). Tissues are collected from the inside of a horse's cheek. These tissues contain the DNA which is then extracted from the swap samples.

There are three possible test results: Clear, Carrier, and Affected. Below is a description of what each result means to the breeder.

Clear

The defective gene is not present in the Horse and when bred will not pass on the SCID disease.

Carrier

One copy of the disease gene is present. Carriers will not have medical problems, but will pass on the disease gene 50% of the time.

Affected

Two copies of the disease gene are present and the Horse will be medically affected by the disease.
It is recommended that Carrier Horses which are desirable for breeding be bred with Clear Horses, which will produce 50% carrier and 50% clear animals, to further reduce the disease gene frequency. These offspring should be tested for this defective gene, and if possible, only the clear animals in this generation should be used.

Although possible that there will be some clear foals when breeding "Carrier to Carrier", breeding this pair of Horses is not recommended. Some resulting foals are likely to be Carrier and some foals are likely to be Affected.

The cost of testing is a small fraction of the value invested in the horse. All breeding animals need to be tested to avoid major losses to the breed and heartache to the humans and to the breed.

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