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OAAM is a bone defect which causes neurological problems

Full name

Occipitoatlantoaxial malformation

Breeds known to be affected

Arabian and related breeds

Inheritance pattern

Autosomal recessive


OAAM is a developmental defect where the first cervical vertebra is malformed and resembles the base of the skull. The second cervical vertebra resembles the first. This compresses the spinal cord near the base of the skull, causing neurologic effects. Symptoms vary from abnormal head carriage, reluctance to move, neck twisting, progressive incoordination and weakness and inability to stand. Affected horses are usually euthanised.

There appears to be more than one mutation that causes OAAM, and there is a test available for only one of these.

Interpretation of results

n/n: Horse does not carry the mutation associated with OAAM.

n/OAAM: Horse has one copy of the mutation associated with OAAM.

OAAM/OAAM: Horse has two copies of the mutation associated with OAAM.


Bordbari et al. Deletion of 2.7 kb near HOXD3 in an Arabian horse with occipitoatlantoaxial malformation. Animal Genetics 2017; 48:287-294 doi:10.1111/age.12531

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