Introduction

Metabolic bone disease (MBD) occurs in a variety of wild and domesticated mammals and can be caused by dietary deficiencies in vitamin D or calcium, dietary calcium:phosphorus imbalances, and inadequate exposure to ultraviolet (UV) light. To maintain ambient temperature and humidity within their preferred ranges, zoo-based koalas are usually housed indoors when the climatic conditions are not suitable for them to be housed outdoors. We hypothesized that koala joeys raised indoors are susceptible to MBD due to a lack of exposure to natural ultraviolet light.

Materials and Methods

Radiographs of 31 juvenile koalas from 4 zoos obtained from 1982 to 2011 were evaluated for signs of MBD. Radiographs were assessed for loss of normal corticomedullary differentiation, reduction in mineralization of skeleton, deformity or failure of normal formation of epiphyses, increased width of physes, deformity in shape of long bones with or without pathological fractures, and abnormal joint formation. Koalas were classified as 1) definite MBD, 2) likely or resolving MBD or 3) unaffected. Serum biochemical profiles for 15 joeys were also reviewed.

Results

Seven joeys had MBD, eight joeys had likely or resolving MBD and sixteen joeys did not have MBD. Serum calcium levels were within reference intervals irrespective of metabolic bone disease status. Two normal koalas and two with MBD had elevated serum phosphate levels and one normal koala had increased ALP activity.

Discussion

MBD has not been previously diagnosed in koalas. All the cases of definite or likely MBD were indoor-housed joeys and therefore most likely due to vitamin D deficiency resulting from lack of exposure to UV light. In other mammalian species, hypocalcaemia, hypophosphataemia and increased ALP activity may be found in animals with MBD, but these were not seen in these koalas. Further work would include measurement of serum vitamin D to aid diagnosis and to monitor the supplementation of artificial UV light and vitamin D. Current recommendations are to house koalas outdoors whenever possible, even if it can only be for short periods of the day during the warmer months that corresponds with the breeding season.