Severe Combined Immunodeficiency (SCID)

Severe combined immunodeficiency (SCID) is a recessive genetic defect of the immune system that occurs in Arabian and part-Arab horses. The mutation responsible for SCID is a five base pair deletion in the DNA-dependent protein kinase gene resulting in a truncation of the protein. The genetic defect results in lack of activity of the enzyme DNA-dependent protein kinase (DNA-PK) which is critical for development of immune system diversity and its ability to protect against infections. SCID foals have increased susceptibility to infection by viral, bacterial, fungal and protozoan organisms. SCID foals are normal at birth but soon present signs such as elevated temperature, respiratory stress, and diarrhea at an early age, typically between 2-8 weeks of age. SCID is lethal and cannot be cured; affected foals do not survive past the first six months of life. Both male and female foals are equally affected. The prevalence of the SCID mutation is estimated to be between 1-8% in Arabian horses, varying by country.

Testing for this SCID variant can help breeders and owners to identify carrier horses and to select mating pairs accordingly to avoid producing affected foals. Matings between two SCID carriers have a 25% percent chance of producing SCID foals. The test also provides information to clinicians to differentiate between SCID and other immune system diseases arising from non-genetic circumstances such as inadequate levels of colostrum antibodies caused by failure of passive transfer.