Combined immunodeficiency in foals in Arabian breeding: evaluation of mode of inheritance and estimation of prevalence of affected foals and carrier mares and stallions.

Combined immunodeficiency (CID), a defect in both B- and T-lymphocytes, was found to occur in 2.3% of 257 foals of Arabian breeding. All affected foals died by 5 months of age. The belief that CID is transmitted as an autosomal recessive genetic defect was supported by results from matings of dams and sires that had previously produced affected foals. Based on a prevalence of 2.3%, the proportion of carriers of the CID trait among the adult population surveyed was estimated to be 25.7%. Recent descriptions of other immunologic defects in foals emphasized the need for careful differential diagnosis. Disorders that could be confused with CID include failure in passive transfer of immunoglobulins from dam to foal, via colostrum; transient hypogammaglobulinemia; agammaglobulinemia (associated with B-lymphocyte deficiency and normal T-lymphocytes), and selective IgM deficiency.