Severe Combined Immunodeficiency (SCID) in Arabian Horses

SCID (Severe Combined Immunodeficiency Disease) is a serious inherited disorder of the immune system that has been described in Arabian horses. Affected foals are born apparently healthy, but due to a severe impairment of B- and T-lymphocyte function, they are unable to mount an effective immune response. As a result, they develop recurrent severe infections from approximately 10 days of age, such as bronchopneumonia and diarrhea. The organism is unable to respond adequately even to common infectious agents, the disease has a very severe course, and it often results in death.

SCID in Arabian horses is caused by the c.9478_9482del mutation in the PRKDC gene. The PRKDC gene encodes the catalytic subunit of DNA-dependent protein kinase, which is essential for the repair of DNA double-strand breaks and for the proper development of B- and T-lymphocytes.

The mutation is inherited in an autosomal recessive manner. This means that the disease develops only in individuals that inherit the mutated gene from both parents. Carriers of the mutated gene are clinically healthy, but they transmit the mutation to their offspring. In a mating between two heterozygous individuals, theoretically 25% of the offspring will be completely healthy, 50% will be carriers, and 25% will inherit the mutated gene from both parents and will therefore be affected by the disease.

Genetic testing makes it possible to reliably identify mutation carriers and prevent the birth of affected foals through appropriate breeding planning.

Reference:

Shin, E.K., Perryman, L.E., Meek, K.: A kinase-negative mutation of DNA-PK(CS) in equine SCID results in defective coding and signal joint formation. J Immunol 158:3565-9, 1997. Pubmed reference: 9103416