Inherited non-syndromic polydactyly in a Berber and Arabian-Berber horse family.

Background: Supernumerary digits, or polydactyly, have been described in various species including humans, wild and domestic animals. In horses, it represents the most common congenital limb malformation, which has only been described in isolated cases or nuclear families. Molecular aetiology has not been reported.

Objectives: To characterise the phenotype of a non-syndromic pre-axial polydactyly in a horse family and to decipher the inheritance pattern.

Study design: Retrospective study.

Methods: Forty-three members of the family including a previously reported polydactyl case were recruited. Available clinical and radiographical findings from the initial case and its family members were summarised and karyotypic examinations of the horses were performed.

Results: On clinical examination, eight horses (including the previously reported case) had one or two supernumerary digits on their forelimbs and one additional case was diagnosed using radiography. Additional digits were located on the medial side of the forelimbs in all nine polydactyl horses. Radiography highlighted variable expression of the defect, which was either unilateral or bilateral. Variations were observed in the number of supernumerary phalanges, the level of development of a rudimentary metacarpal bone, the individualisation of a supernumerary digit and the existence of a rudimentary hoof. All nine affected horses were related to a single stallion. Pedigree analysis revealed that the most likely inheritance pattern was autosomal dominant with incomplete penetrance and variable expressivity. A more complex mode could not be ruled out.

Main limitations: Restricted recruitment of the family members due to confidentiality constraints and to international dispersal of the relatives, quality of radiographs.

Conclusions: We describe an equine preaxial polydactyly in a Berber and Arabian-Berber family most likely with autosomal dominant inheritance with incomplete penetrance. This is the first description of an inherited non-syndromic polydactyly in horses.