Equine Severe Combined Immunodeficiency

Severe Combined Immunodeficiency (SCID) is a disease with a genetic background in Arabian horses and crossbreeds with Arabians. The disease has been known and documented since the 1970’s and was early suggested to have an autosomal recessive mode of inheritance. Foals that are affected with SCID lack production of both B and T lymphocytes and consequently die from secondary diseases within a short period of time. The most common cause of death is infections in the respiratory tract caused by adenovirus. Maintaining foals affected with SCID has been proven difficult, but their lives can be prolonged with injections of hyperimmune plasma. A successful bone marrow transplant has also been performed. A DNA-test for detection of carriers of the defective allele was developed during the 1990’s. The test made it possible to detect whether a horse was a carrier or a non-carrier of SCID, without the use of test mating. Previous estimations of the frequency of the defective allele were based on clinical diagnosis on foals suspected to be affected with SCID. These studies were later found to have overestimated the frequency of carriers. The frequency of SCID carriers in Sweden is low and carriers are not allowed in breeding. The present legislation is however not acted upon since horses are only tested for SCID if suspected to be carriers. The question whether heterozygotes should be allowed in breeding or not should be decided while considering the population size and the frequency of carriers. The total population of Arabian horses in Sweden is large enough to exclude carriers from breeding.