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 mode of inheritance of the genetic defect is an autosomal recessive trait. The DNA-PK enzyme defect results from a deletion mutation of the gene encoding the catalytic subunit. 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 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). This fatal disease was first described in the horse by McGuire and Poppie in 1973. Foals are normal at birth but soon develop fatal infections, particularly when circulating colostrum-derived antibody concentrations become low. adenovirus, coronavirus, Rhodococcus equi, Pneumocystis carinii, and/or Cryptosporidium parvum). 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.
Severe combined immunodeficiency (SCID) is a fatal condition of both B (humoral) and T (cellular) cell dysfunction.
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