Abstract
Jacob sheep (Ovis aries) are a pedigree breed known for their “polycerate” (multihorned) phenotype. We describe a four-horned Jacob lamb that exhibited progressive congenital hindlimb ataxia and paresis, and was euthanased four weeks post-partum. Necropsy revealed deformity and asymmetry of the occipital condyles, causing narrowing of the foramen magnum and spinal cord compression. Histopathology demonstrated axonal degeneration of the spinal cord. These findings are consistent with occipital condylar dysplasia. This condition has been infrequently reported in the literature as a suspected heritable disease of polycerate Jacob sheep in the USA, and is assumed to arise during selection for the polycerate trait. This is the first reported case in European-bred Jacob sheep. Occipital condylar dysplasia should be considered as a differential diagnosis in polycerate Jacob lambs showing ataxia. It is important to raise awareness of this disease due to its suspected heritability and link to the popular polycerate trait.

Key words: Occipital condylar dysplasia, Jacob sheep, congenital, ataxia, polycerate