Estimation of the prevalence of severe combined immunodeficiency disease in UK Arab horses as determined by a DNA-based test.

This research focuses on estimating the prevalence of Severe Combined Immunodeficiency Disease (SCID), a genetic defect in Arabian horses, using a DNA-based test. The defect, which causes enhanced susceptibility to infection and claims afflicted foal’s lives irrespective of veterinary care, has largely been diagnosed post-mortem due to ambiguity of symptoms similar to normal infections.

Background

• Severe Combined Immunodeficiency Disease (SCID), is an inherited defect found particularly in Arab or part-bred Arab horses. The disease reduces the number and function of B and T lymphocytes, leading to a weakened immune response against infectious diseases.
• Foals suffering from SCID display heightened susceptibility to infection. The first signs of the disease appear between two days and eight weeks of age. Symptoms including an increased temperature, respiratory complications, and diarrhoea closely resemble those of several other infections seen in newborn foals, making clinical diagnosis challenging.
• The disease invariably culminates in death within the first six months of the foal’s life, regardless of the level of veterinary care provided. Apart from the distressing impact on the animals and their caretakers, the disease also results in significant financial burdens link to veterinary expenses and loss of foals.

Inheritance and Breeding Concerns

• SCID is caused by a simple recessive mutation, traced to a common progenitor horse, which was active in the earlier part of this century. The heterozygous carriers of this defective gene are phenotypically normal but pass on the gene to half of their offspring. An estimated 25% of carrier-to-carrier matings result in affected foals.
• Until now, the birth of affected foals has been the only method of identifying carrier horses, causing anxiety among breeders due to unanticipated foal deaths that could indicate the potential contamination of their breeding stock with this inherited disease.

DNA-based Test for SCID

• The research introduces a recently developed DNA test for SCID, which enables the screening of Arab horses used for breeding. The application of the DNA test signifies an evolution in dealing with SCID, from post-mortem diagnosis to preventive screening.
• The test emerged from the identification of the lesion responsible for SCID – a five base pair deletion which results in a frameshift in the gene for DNA-dependent kinase, catalytic subunit DNA-PKcs. The protein involved in this process is significant in V(D)J recombination, a step in the formation of immunity-boosting antibody and T cell receptor genes.