An outbreak of rickets in Corriedale sheep: Evidence for a genetic aetiology

Abstract

CASE HISTORY: A skeletal disease characterised by lameness, limb deformities and reduced growth rate occurred over two successive years in lambs born on a commercial sheep farm in Marlborough. A genetic aetiology was considered likely following exclusion of other known causes of rickets and because of the progressive nature of the disease, even after affected animals were transferred to another property.

CLINICAL FINDINGS: Affected lambs appeared normal at birth but developed clinical signs during the first 2 months of life. The most severely affected animals either died or were euthanised within the first year of life, but some survived to breeding age. Serum biochemistry revealed hypocalcaemia, hypophosphataemia and increased concentrations of 1,25 dihydroxyvitamin D. The mean serum 25 hydroxyvitamin D concentration was similar to that of control lambs.

PATHOLOGICAL FINDINGS: Gross lesions included enlarged costochondral junctions, bilateral irregularity of articular surfaces on humeral heads due to collapse of subchondral bone, thickened cortices in long bones and irregular thickening of physeal cartilages. Microscopically, tongues of hypertrophic chondrocytes extended from physes into metaphyseal regions; metaphyseal trabeculae were thick, disorganised and often lined by wide osteoid seams. Osteoclastic activity was excessive both in cortical and trabecular bone.

DIAGNOSIS: Inherited rickets in Corriedale sheep.

CLINICAL RELEVANCE AND CONCLUSIONS: This disease is likely to be present in several Corriedale sheep flocks in New Zealand and may have been misdiagnosed as arthritis or other diseases causing lameness and/or poor growth. A defect in end-organ responsiveness to 1,25 dihydroxyvitamin D is the likely mechanism. This disease of sheep may be a useful model for studying vitamin D metabolism and the treatment of inherited forms of rickets in human beings.

KEY WORDS:

• Bone
• rickets
• vitamin D
• genetic disease
• sheep
• Corriedale

Acknowledgements

The authors wish to acknowledge the enthusiasm and observational skills of the farmer who suspected that he had a new disease on his property, and for ensuring that it came to the attention of his veterinarian. It is only through the cooperation of such individuals that new diseases become available for scientific investigation. We are also grateful to Pat Davey and Evelyn Lupton for their skilled technical assistance; and to Sandra Tillack, Sullivan Nicholaides Pathology, for performing the vitamin D analyses. The investigation was financially supported in part by the Massey University Research Fund.