SCID
SCID - Severe Combined Immunodeficiency in Arabian Horses

What is SCID?

SCID is an autosomal recessive disease, 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. Both male and female foals can be affected.

Disease Statistics

VetGen has been collecting statistics on the Arabian Horse SCID test. As of November 2007, having tested 7,994 Arabian Horses, the percentages from VetGen's DNA tests are as follows:
SCID Test Results
 Number of Arabians Percentage
CLEAR 6,607 82,7%
CARRIER 1,360 17,0%
AFFECTED 27 0,3%
TOTAL 7,994


NOTE: The SCID Affected Percentage only refers to the horses that have been tested by VetGen before they died (and they ALL died). Other affected foals died before they were tested.

How can SCID be avoided?

Matings between two clear animals as well as matings between a clear and a carrier animal 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 animal 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. However, this is no longer necessary and not in the best interest of the breed. Carrier animals that have all the desirable traits for which the breed is known can now be mated to other tested animals who are clear and then never produce an affected foal. Similarly, their offspring can be tested and appropriate matings set up in the next generations without the breed ever suffering the loss of another foal to SCID. In this manner, the breed still continues to benefit from all of the outstanding traits that a carrier animal may possess. Thus, the economic value of the animal should not be affected by being clear or carrier.

SCID Genetic Testing

There is now a definitive test for the Severe Combined Immunodeficiency - SCID gene in Arabian and Arab cross bred horses. This test unequivocally determines if an animal is affected, a carrier or clear of the mutant gene. The major and most important implication is that now there is no guesswork in avoiding SCID offspring.

Testing is easy and highly accurate, and can be performed at any point in time in the life of the animal with a simple oral swab or a blood sample. The cost of testing is a small fraction of the value invested in the animal. There is no reason to gamble on fate. All breeding animals need to be tested to avoid major losses and heartache to the humans and to prevent morbidity in the animals.

Interpretating SCID Test Results

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

CLEAR
This finding indicates that the gene is not present in your Horse. Therefore, when used for breeding, a Clear animal will not pass on the disease gene.

CARRIER
This finding indicates that one copy of the disease gene is present in your Horse, but that it will not exhibit disease symptoms. Carriers will not have medical problems as a result. Horses with Carrier status can be enjoyed without the fear of developing medical problems but will pass on the disease gene 50% of the time.

AFFECTED
This finding indicates that two copies of the disease gene are present in the Horse. Unfortunately, the Horse will be medically affected by the disease. Appropriate treatment should be pursued by consulting a veterinarian.

Helpful Equine Breeding Chart

The chart provided below outlines the implications of various breeding pair combinations. Remember, it is always best to breed "Clear to Clear". If followed by all breeders, these strategies will ensure a significant reduction in the frequency of the targeted disease gene in future generations of horses. However, to maintain a large enough pool of good breeding stock, it may be necessary for some breeders to breed "Clear" to "Carriers" (see below).
Breeding Combination CLEAR Stallion CARRIER Stallion
CLEAR Mare 100% CLEAR 50% CLEAR
50% CARRIER
CARRIER Mare 50% CLEAR
50% CARRIER		 25% CLEAR
50% CARRIER
25% AFFECTED

Breeding CLEAR to CLEAR = Ideal Breeding Combination
Breeding CLEAR to CARRIER = Safe Breeding Combination
Breeding CARRIER to CARRIER = High Risk Breeding Combination

Ideal Breeding Combination - Foals will not have the disease gene.

Safe Breeding Combination - No Affected foals will be produced. However, some or all foals will be Carriers. Accordingly, it is recommended that Carrier Horses which are desirable for breeding be bred with Clear Horses in the future, which will produce 50% carrier and 50% clear animals, to further reduce the disease gene frequency. These offspring should be tested by VetGen's test for this defective gene, and if possible, only the clear animals in this generation should be used.

High Risk Breeding Combination - Some foals are likely to be Carriers and some foals are likely to be Affected. Even though it is possible that there will be some clear foals when breeding "Carrier to Carrier", in general, breeding this pair is not recommended.

Thanks to VetGen for providing the information.
 
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