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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
Occipitoatlantoaxial Malformation (OAAM) is an inherited developmental disorder found primarily in Arabian horses. The first cervical vertebra (the atlas) resembles the base of the skull, and the second (the axis) resembles the atlas, with the atlas typically fused to the occipital bone. This abnormal structure compresses the upper cervical spinal cord and damages the tissue, producing neurological signs.
Affected horses show abnormal head and neck carriage with the neck extended, reluctance to move the neck, or neck twisting, sometimes accompanied by a clicking sound. The disease may progress with age from mild incoordination and weakness of the limbs to the inability to stand. Depending on severity, affected foals may be stillborn, show signs at birth, or not show signs until a few weeks after birth. Diagnosis of the malformed atlas and axis is generally confirmed with radiographs, and affected foals are typically euthanized on welfare grounds.
This test detects OAAM1, one form of the disorder, caused by a 2.7 kb deletion near the homeobox D3 gene (HOXD3). HOXD3 governs normal development of the back of the skull, the atlas, and the axis. OAAM is presumed to be inherited as an autosomal recessive trait in Arabians, but more than one variant appears to be involved, and the genetic basis of other forms remains under investigation. A negative result for OAAM1 does not rule out other forms of the disorder.
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 the disorder. If two carriers are bred, each foal has a 25% chance of being affected and a 50% chance of being a carrier. DNA testing is recommended for horses in Arabian lineages that have produced OAAM-affected foals, and allows breeders to identify carriers and select mating pairs that avoid producing affected foals.
Possible results
| Genotype | Description |
|---|---|
| OAAM/OAAM | Affected: Horse has two copies of the OAAM1 variant and is expected to develop signs of the disorder. |
| N/OAAM | Carrier: Horse has one copy of the OAAM1 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 OAAM1 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
Deletion of 2.7 kb near HOXD3 in an Arabian horse with occipitoatlantoaxial malformation.
[PMID: 28111759]