Occipitoatlantoaxial Malformation (OAAM)
Occipitoatlantoaxial malformation (OAAM) is an autosomal recessive developmental skeletal defect which causes compression of the upper cervical cord. This malformation of the occipital bone of the skull results in a neurologic disorder caused when the first two cervical vertebrae (the atlas and axis), fuse to the base of the skull. This structural change creates pressure on the upper portion of the cervical spinal cord, damaging the tissue.
The disease may progress with age from mild incoordination and weakness of the limbs to the inability to stand. Depending on the severity of the disorder affected foals may be stillborn, show signs at birth or, in some cases, not show signs until a few weeks after birth.
Diagnosis of the malformed atlas and axis is generally confirmed with radiographs. Affected foals are generally euthanized.
In order for a foal to be affected, it must be homozygous (have two copies) for the mutated gene, inheriting a single copy from each parent. Horses that have one copy of the mutated gene are not affected but are considered carriers and, if bred, have a 50% chance of passing on the mutated copy to its offspring.
Additional, yet unidentified, mutations may exist in Arabian foals affected with OAAM.
Testing for OAAM1 is valid for Arabian Horse and Arabian mix breeds.
Affected OAAM1 Arabian filly: Video
Additional Tests For Arabian Horses:
Animal Genetics offers DNA testing for the 2.7 kb deletion near HOXD3 gene. The genetic test verifies the presence of the OAAM1 mutation and presents results as one of the following:
|OAAM/OAAM||Affected||The horse carries two copies of the variant and is homozygous for the OAAM mutation. The horse is affected with Occipitoatlantoaxial Malformation.|
|n/OAAM||Carrier||Both the normal and mutant alleles were detected. Horse tested heterozygous for OAAM and is a carrier of the OAAM mutation.|
|n/n||Clear||Horse tested negative for OAAM and does not carry the OAAM gene mutation. The horse will not pass on the defective gene to its offspring.|