Severe combined immunodeficiency (SCID) is a fatal condition of both B (humoral) and T (cellular) cell dysfunction. This immunodeficiency may occur in Arabian foals (or breeds carrying Arab bloodlines), and manifests clinically by susceptibility to viral, bacterial, fungal, and protozoal organisms (e.g. adenovirus, coronavirus, Rhodococcus equi, Pneumocystis carinii, and/or Cryptosporidium parvum). Foals are normal at birth but soon develop fatal infections, particularly when circulating colostrum-derived antibody concentrations become low.

This fatal disease was first described in the horse by McGuire and Poppie in 1973. The poor B and T cell development results in lymphopenia (less than 1,000 cells/uL), marked serum IgM and IgA deficiency, and hypoplasia of lymphoid tissues (thymus, lymph node, spleen, mucosa-associated). The disease is caused by the lack of activity of the enzyme DNA-dependent protein kinase (DNA-PK), which is required for gene rearrangement of the antigen-receptor on B and T lymphocytes. The DNA-PK enzyme defect results from a deletion mutation of the gene encoding the catalytic subunit. The mode of inheritance of the genetic defect is an autosomal recessive trait. The definitive diagnosis of carriers and affected foals can be done by a DNA test (VetGen, Veterinary Genetic Services, Michigan, USA) of whole blood or cheek swab samples. The test should be performed in all Arabian and Arabian-crossbred horses used in reproduction. The appropriate planning of breeding of carriers prevents the outcome of affected foals and decreases the incidence of the mutant gene in the population.