Severe Combined Immuno-Deficiency (SCID)


Severe combined immuno-deficiency, more commonly known by the initialism SCID, is a lethal genetic problem found in some horses with Arabian heritage.   It is transmitted to a foal by its parents.  Foals born with SCID lack an adequate immune system and do not live beyond the age of five months.  These affected foals have received two SCID genes, one from each of its "carrier" parents.  The disorder is not sex-linked; it affects both sexes equally.

Carrier animals have one gene for SCID but are totally unaffected by the disease.  If two carrier animals are mated, statistically one would be perfectly normal (25%), two would show no signs of the disease but would be carriers (50%), and one would be affected and die of the disease (25%).

If one carrier is mated with one non-carrier, statistically one out of two would show no signs of the disease but would be a carrier (50%), and one out of two would be SCID clear and not be a carrier (50%).

If two non-carriers are mated, all offspring should be SCID clear.

Despite the efforts of breeders, SCID first purportedly surfaced in a Pintabian horse foaled in 2003.  If this information is accurate, other Pintabian horses in the pedigree are undoubtedly carriers because of the autosomal recessive characteristics of SCID. 

The Pintabian Horse Registry, Inc. (PHRI)...

1)  recommends SCID testing prior to breeder.

2)  recommends that two known SCID carriers not be mated together.  Most breeders consider this cross unethical.

3)  recommends disclosure of a known SCID carrier be made by the owner to a potential purchaser or breeder prior to selling or breeding.

4)  recommends disclosure to the owner of of a sire of a foal found to be affected.

5)  will, for a fee of $10.00 each, include the results of any genetic testing on the PHRI Certificate of Registration, upon receipt of a hard copy of the test results.

Some breeders feel it is worth the risk to retain carrier horses while others do not.  A "carrier test" is available to the public which makes it simple to eradicate SCID from a particular herd.