Severe Combined Immunodeficiency (SCID) in Arabian Foals

  Severe combined immunodeficiency (SCID) is an important genetic condition that results in the death of a significant number of Arabian foals.  The condition is inherited as an autosomal recessive trait and was first identified in 1973.  In 1997 it was estimated that 2.5% of Arabian foals died from this condition.

  The pathogenesis of SCID is based on a mutation in the allele encoding for a DNA-dependent protein kinase (DNA-PK) that is needed for lymphocyte V(D)J recombination.  When both alleles are mutated, the condition develops.  V(D)J recombination is essential for expression of antigen receptors on B and T lymphocytes.  Without these receptors, B and T lymphocytes do not differentiate and lymphoid tissue fails to develop so severe immunodeficiency results.

  SCID foals appear normal at birth and, if they receive colostrum (with maternal antibodies), signs related to their immuno-deficiency develop later, when serum antibody levels wane.  Clinical signs are related to the development of opportunistic infections, usually of the respiratory tract.  These include adenovirus, Pneumocystiscarinii, and various bacteria such as Streptococcus.  SCID foals inevitably die before five months of age.

  Immunologic characteristics of SCID foals include an absence of B and T lymphocytes, and a lack of serum immunoglobulins.  Natural killer cells function normally.  On gross examination, severe hypoplasia of the thymus and lymphoid tissue is the distinguishing feature.

  The diagnosis of SCID is presumptive when the following antemortem criteria are present: (1) lymphopenia with less than 1000 lymphocytes per ml, (2) lack of serum IgM, and (3) hypoplasia of lymphoid tissue.  Postmortem findings of lymphoid tissue hypoplasia and unusual opportunistic infections support this presumptive diagnosis.  Definitive antemortem diagnosis is based on a DNA molecular technique, polymerase chain reaction (PCR) (Vet-Gen

Laboratories, Ann Arbor, Michigan) that can identify the mutant allele of the DNA-PK gene.  This test utilizes DNA extracted from leukocytes (from whole blood) or other cells (e.g., obtained from swabs of buccal mucosa).  It is considered to be 99.9% accurate and also detects heterozygous carriers of the SCID trait.  Identification of carriers is important so that their mating can be prevented.

  -By Barbara Atkinson, Class of 2000

  -Edited by Evan Janovitz, DVM, PhD