Evidence of occipitoatlantoaxial malformations independent of the HOXD3 mutation in an Arabian horse.

Overview

• This study reports on a case of occipitoatlantoaxial malformations (OAAM) in an Arabian horse that occurred independently of the known HOXD3 gene mutation usually linked to OAAM type 1.
• The research highlights variability in the manifestation of OAAM and suggests the need for further genetic exploration beyond the currently identified mutation.

Background on OAAM

• OAAM refers to developmental disorders involving malformations at the occiput (back part of the skull), atlas (first cervical vertebra), and axis (second cervical vertebra).
• Type 1 OAAM is known to be a familial (hereditary) condition primarily affecting Arabian horses.
• The diagnosis of OAAM type 1 is usually confirmed via genetic testing of a mutation in the HOXD3 gene region.

Case Description

• The subject was a 3-year-old Arabian gelding presenting with physical abnormalities in the neck area.
• Clinical findings included:
  • Proximal cervical deformation located on the right side of the neck
  • Severe stiffness of the neck when moved in the sagittal plane (front-to-back bending)
  • No apparent pain associated with neck movement
• Neurological examination showed generalized proprioceptive ataxia, which means impaired coordination and body positioning awareness.
• Radiographic imaging confirmed severe malformations of the cranio-cervical vertebrae, consistent with OAAM.
• However, genetic testing for the known HOXD3 mutation came back negative, indicating the malformation was independent of the typical mutation.

Clinical Outcome and Implications

• Because of the poor prognosis given the severity of neurological signs (grade 3/5 proprioceptive ataxia) and structural malformations, euthanasia was elected for the horse.
• This case illustrates that not all OAAM cases are linked to the HOXD3 mutation, suggesting:
  • Phenotypic variability exists even within familial OAAM type 1 presentations.
  • Occipitoatlantoaxial malformations might develop or be detected in adult horses, rather than only in young ones.
  • Further genetic research is necessary to identify other mutations or genetic factors contributing to OAAM.

Significance and Future Directions

• This case expands understanding of OAAM variability, indicating that horses can present with similar malformations without the known genetic mutation.
• It points to genetic heterogeneity in OAAM, meaning multiple genes or mutations could be responsible.
• Improved genetic screening tools will be needed to detect these other potential mutations to better diagnose and manage OAAM in horses.
• The findings emphasize the importance of clinical and radiographic examination alongside genetic testing for comprehensive diagnosis.