Severe combined immunodeficiency (SCID) is an inherited disease of Arabian foals. The disease was first reported around 50 years ago. It is inherited as an autosomal recessive disease. If a mare has one copy of the diseased gene (heterozygous) is mated to a stallion that also one copy of the gene (heterozygous), then there is a 25% possibility that the resultant foal will inherit two copies of the affected gene (homozygous) and develop SCID. It was estimated, albeit more that 20 years ago that 8.4% of Arabians were carriers, and based on this number it would be expected that 0.18% (1 of 567 matings) would result in a SCID foal based on random breeding. The number of carriers is likely much reduced based on genetic profiling of Arabian horses. For example in South Africa the carrier prevalence fell from 6.4% in 2004 to 3.4% in 2009.
Affected foals are severely immunocompromised because they lack critical defensive lymphocytes, both T- and B-lymphocytes. Although affected foals will be normal at birth they lack the ability to generate antigen-specific immunity. This will ultimately lead to overwhelming infection and death, although there will be a positive early response to antibiotics. Early protection is gained through ingestion of colostrum from the mare, but this will wain shortly after birth. Foals rarely survive past 6 months of age.
The diagnosis of SCID can be suspected through an absence of IgM pre-suckle, a persistent low lymphocyte count, and hypoplasia of lymphoid tissue. SCID can be confirmed through generic testing. The specific mutation in SCID is in the enzyme DNA-protein kinase catalytic subunit. This enzyme is critical for development of lymphocytes.
The testing for carriers is through submission of 20-30 hairs with roots to an accredited laboratory.
Photo credit: Charlotte Venema and Annie Splatt @ Unsplash
Tags: Foals