Severe combined immunodeficiency (SCID)
SCID is a fatal immunodeficiency of Arabian foals that results in foals succumbing to infection once maternally immunity acquired from colostrums fades at around 4-7 months of age. The genetic defect was identified in 1997 and confirmed that SCID was a recessive trait (two copies of the mutation are necessary to produce disease). Affected foals lack the ability to generate some of the white blood cells (lymphocytes) involved in the immune system.
A genetic test is available at VetGen in Ann Arbor, Michigan. Approximately 8% of Arabians are heterozygous for the mutation and with random matings (breedings), about 1 in 526 foals would be expected to have SCID. The Arabian Horse Registry recommends that all breeding stock be tested and interbreeding of carriers avoided.