Familial congenital occipitoatlantoaxial malformation (OAAM) in the Arabian horse.

The researchers have defined a new congenital disease in Arabian horses known as occipitalization of the atlas with atlantalization of the axis, or OAAM. This disease, characterised by malformations in the occiput, atlas, and axis, was identified through clinical, radiologic, and morphologic studies of affected horses, and shows variances in how it presents clinically. This study provides evidence that it is an inherited condition appearing solely in Arabian horses.

Objective of the Research

• The research aims to define a congenital disease in Arabian horses referred to as occipitalization of the atlas with atlantalization of the axis, or OAAM affirm its familial character. This disease was identified through studies of horses showing congenital malformations in certain parts of the skull and spine.

Methods of the Research

• Sixteen horses with obvious malformations in the occiput (the back part of the skull), the atlas (the first cervical vertebra), and the axis (the second cervical vertebra) were studied clinically, radiologically, and morphologically.
• Three previously reported cases were also incorporated into the study.
• The consistent morphologic features across these cases were identified as distinct signs of OAAM.
• The researchers also studied the pedigrees of the affected horses to confirm the familial nature of this malformation.

Findings of the Research

• The research identified consistent morphologic features among the affected horses. These features indicate congenital fusion of the atlas and the occiput, underdevelopment of the atlas and the axillary spine, deformation of the axis, and modifications in the atlantoaxial joint (connection between the first and second cervical vertebrae). Atlantoaxial subluxation, or displacement, was also commonly observed.
• The disease showed variability in its clinical presentation, but the symptoms could be broadly classified into four groups. Neurologic symptoms ranged from symmetrical ataxia (loss of coordination), hypometria (shortened stride length), tetraparesis (weakness in all four limbs), to total paralysis.
• These clinical symptoms, which were often progressive, were a result of a localised and progressive compression of the proximal part of the cervical spinal cord.
• The pedigree studies verified that this OAAM malformation occurs within families and is exclusive to the Arabian horse breed.