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Threatening processes

Skeletal fluorosis in marsupials: a comparison of bone lesions in six species from an Australian industrial site

Death, C, Coulson, G, Kierdorf, U, Kierdorf, H, Ploeg, R, Firestone, SM, Dohoo, I & Hufschmid, J 2017, Science of the Total Environment, vol. 584-585, no. 1, pp. 1198-1211.

Koalas and other marsupials living in high-fluoride environments are at risk of developing skeletal fluorosis. The severity of skeletal lesions in an affected animal is positively associated with bone fluoride levels. Of the six species studied here, all exhibited either localised or generalised periosteal hyperostosis but the distribution of lesions varied according to the animal’s mastication and gait.

  Researchers examined the skeletons of deceased animals representing six marsupial species (three macropodids, two possums, and the koala) obtained from high- and low-fluoride environments. In all species examined, chronic exposure to excess fluoride levels was found to result in the formation of new bone on the outer surface of existing bone, or periosteal hyperostosis. The severity of lesions increased with bone fluoride concentration. The mean bone fluoride concentration of animals with normal skeletons was 1100 + 260 µg F-/g dry bone. In contrast, animals with hyperostosis classed as either mild or severe had mean bone fluoride levels of 4300 + 1200 µg F-/g and 6300 + 1200 µg F-/g dry bone, respectively. Biomechanical differences between species caused lesions to occur at different anatomical sites. In quadrupedal marsupials such as possums and koalas, skeletal lesions were typically distributed evenly between the forelimbs and hind limbs. Macropodids, which are bipedal, were more likely to be affected by periosteal hyperostosis in the hind limbs only. The prevalence of skeletal fluorosis of the mandible, or lower jaw, also differed among species. In the koala, mandibular hyperostosis was observed at the insertion points of the masseter and digastric muscles which enable the koala’s characteristic side-to-side chewing movements. This type of mandibular growth was not seen in any other species.

  High-fluoride environments result from both natural and anthropogenic forces. Fluoride is attracted by bone and teeth. Where developing bones are exposed to excessive fluoride levels, new bone with altered biomechanical properties forms and increases the bone mass at the affected site. The variation in skeletal lesion distribution between bipedal and quadrupedal marsupials can be explained by the mechanical forces acting upon the skeleton as a result of each species’ gait. The compressive load borne by the hind limbs is much greater for a kangaroo or wallaby than for a possum or koala, and hyperostotic lesions become more severe where this force is greater. Similarly, the significant lateral forces exerted by the jaw of the koala relative to that of other marsupials enable it to sustain its low-nutrient, high-fibre diet, but also make it more prone to mandibular hyperostosis.

  This study demonstrates that different marsupial species show varied responses to environmental fluoride exposure. Relationships between biomechanics and the distribution of skeletal lesions observed here make it possible to predict the effects of excessive fluoride exposure in species for which actual responses have yet to be described. As skeletal fluorosis has been reported to cause lameness in some animals, the effects of fluoride exposure as a potential threat to koalas should be monitored.

 

Summarised by Joanna Horsfall

 

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