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Breeds: Andalusian/Arabian, Anglo-Arabian, AraAppaloosa, Arabian, Arabian Cross, Arabian/Dutch Warmblood, Arabian/Friesian Cross, Arabian/Paint, Arabian/Pinto, Arabian/Quarter Horse, Arabian/Saddlebred, Arabian/Thoroughbred, Arabian/Warmblood, Egyptian Arabian, German Riding Pony, Half-Arabian, Hispano-Árab, Karabakh Horse, Lusitano, Mixed Breed, Part Bred Arabian, Shagya Arabian, Swedish Riding Pony, Unspecified
Description
Severe Combined Immunodeficiency Disease (SCID) is an inherited disease seen in pure and part-bred Arabian horses. Affected foals are born without a functioning immune system and are highly susceptible to viral, bacterial, fungal, and protozoal infection. Foals appear normal at birth but begin to show signs between two days and eight weeks of age.
Clinical diagnosis is not straightforward, because the signs resemble those of ordinary newborn infections: raised temperature, respiratory complications, and diarrhea. Affected foals cannot mount an immune response and do not recover, and they die within the first six months of life regardless of the level of veterinary care. SCID is therefore an extremely distressing condition for the animal and its owners, and frequently results in financial loss through lost foals and veterinary expenses.
SCID is caused by a five base pair deletion in the DNA-dependent protein kinase catalytic subunit gene, PRKDC (formerly DNA-PKcs), which truncates the protein. The enzyme it encodes is critical to the development of immune system diversity, and without it affected foals cannot produce functional B and T lymphocytes.
The disorder is inherited as an autosomal recessive trait, so a foal must inherit two copies of the variant to be affected, one from each parent. Carriers have one copy and show no signs of disease. If two carriers are bred, each foal has a 25% chance of being affected and a 50% chance of being a carrier. Because affected foals do not survive to breeding age, every affected foal comes from a carrier-to-carrier mating.
A number of studies have attempted to estimate the frequency of SCID carriers in the Arabian population. Most sources put the proportion of Arabian foals that die of SCID at 2 to 3%. Under random mating that would imply roughly 28 to 35% of Arabian horses are carriers, but in practice breeding is highly selective, so the true carrier frequency is somewhat unclear.
The Arabian Horse Association Code of Ethics requires that known carrier status for SCID be disclosed for any horse capable of reproducing that is offered for breeding, sale, or lease. Owners of a mare that produces an affected foal are required to notify the stallion owner immediately. Breeders should confirm current requirements directly with the relevant registry.
Animal Genetics offers DNA testing for the variant responsible for SCID. Testing is an important tool in breeding decisions, allowing carriers to be identified so that two carriers are not bred together.
Possible results
| Genotype | Description |
|---|---|
| SCID/SCID | Affected: Horse has two copies of the SCID variant and is expected to develop signs of the disorder. |
| n/SCID | Carrier: Horse has one copy of the SCID variant and is not expected to show signs of the disorder. Each offspring has a 50% chance of inheriting the variant. |
| n/n | Clear: Horse does not carry the SCID variant and cannot pass it on to offspring. |
Breeds Affected
Andalusian/Arabian, Anglo-Arabian, AraAppaloosa, Arabian, Arabian Cross, Arabian/Dutch Warmblood, Arabian/Friesian Cross, Arabian/Paint, Arabian/Pinto, Arabian/Quarter Horse, Arabian/Saddlebred, Arabian/Thoroughbred, Arabian/Warmblood, Egyptian Arabian, Half-Arabian, Hispano-Árab, Karabakh Horse, Lusitano, Part Bred Arabian, Shagya Arabian, Swedish Riding Pony
Reference
[PMID: 9373363]