1. Koalas almost exclusively eat Eucalyptus leaves. Therefore koalas are ‘specialist herbivores’ 1 or ‘specialist folivores’. 2, 3 Other specialist marsupial eucalypt folivores include the common ringtail possum (Pseudocheirus peregrinus) and greater glider (Petauroides volans). In contrast, the common brushtail possum (Trichosurus vulpecula) is a generalist Eucalyptus spp. folivore as they also eat fruits, seeds and flowers. 4, 5
  1. Eucalyptus leaves are very high in plant secondary metabolites (PSMs). Plant secondary metabolites (PSMs) are a part of plants’ defenses to avoid ingestion by herbivores or insects, or attack by microbial disease. 1 Some of the PSMs identified in Eucalypt spp. foliage include lignin which is firm and crunchy, condensed tannins which bind firmly to ingested protein inhibiting their absorption by the cells lining the intestine,6 hydrolysable tannins, essential oils (or terpenoids), polyphenolic compounds and formylated phloroglucinol compounds which are can kill cells. 2, 6, 7 Consequently the koala’s diet has one of the highest concentrations of PSMs of all mammals. A small proportion of the dietary concentrations of PSMs, especially terpenoids, ingested by specialist folivores on a daily basis would be toxic in people.8
  1. Because of this diet of Eucalyptus leaves, when some medicines are given by mouth to koalas, the medicine may well bind to the ingesta in both the stomach and small intestine and therefore not reach the walls of the gastrointestinal tract (GIT) and be absorbed via the gut into the circulation. Koalas are also most impossible to ‘fast’ as even sick koalas always almost have food in their stomachs.  So the take home message that administering some medicines by mouth will limit their absorption into the blood.
  1. Koalas have developed very efficient pathways in their livers to detoxify the PSMs. The pathway that oxidises the terpenoid PSMs to water soluble substances (which can be excreted via the kidneys) is extremely efficient. So those medicines that also undergo ‘oxidation’ in the koala’s liver are metabolized very quickly in the liver to water soluble substances which are excreted in the urine. One of the most popular veterinary drugs used to relieve pain and inflammation in animals is meloxicam. Meloxicam is one of those medicines that undergo metabolic oxidation. Benjamin Kimble reports that the half-life of meloxicam (the time for the blood concentration to halve) is approximately 1.19 hours (range 0.71–1.62 h) 9 compared to 13 h in people 10 and 24 h in dogs 11 suggesting that a higher dose rate and/or more frequent dosing of meloxicam may be required to maintain therapeutic blood concentrations in koalas.

Many more interesting facts regarding medicines in koalas coming shortly.

References

  1. Freeland WJ, Janzen DH. Strategies in herbivory by mammals: The role of plant secondary compounds. The American Naturalist 1974;108:269-289.
  2. Moore BD, Foley WJ. Tree use by koalas in a chemically complex landscape. Nature 2005;435:488-490.
  3. Burgdorf-Moisuk A, Stalis IH, Pye GW. Disseminated coccidoidomycosis in a koala (Phascolarctos cinereus). Journal of Zoo and Wildlife Medicine 2012;43:197-199.
  4. Tyndale-Biscoe H. Life in the trees: koala, greater glider and possum Life of marsupials CSIRO Publishing Collingwood, VIC, 2005:250 – 258
  5. McLean SR, Foley WF. Metabolism of Eucalyptus terpenes by herbivorous marsupials. Drug Metab Rev 1997;29:213-218.
  6. Hume ID. Hindgut fermenters – the arboreal folivores. Marsupial Nutrition. Cambridge University Press, Cambridge, 1999:149-204.
  7. Eschler BM, Pass DM, Willis R, Foley WJ. Distribution of foliar formylated phloroglucinol derivatives amongst Eucalyptus species. Biochem Syst Ecol 2000;28:813-824.
  8. Pass GJ, McLean S, Stupans I, Davies N. Microsomal metabolism of the terpene 1,8-cineole in the common brushtail possum (Trichosurus vulpecula), koala (Phascolarctos cinereus), rat and human. Xenobiotica 2001;31:205-221.
  9. Kimble B, Black LA, Li KM et al. Pharmacokinetics of meloxicam in koalas (Phascolartos cinereus) after intravenous, subcutaneous and oral administration. J Vet Pharmacol Ther 2013;36:486-493.
  10. Turck D, Roth W, Busch U. A review of the clinical pharmacokinetics of meloxicam. Rheumatology (Oxford) 1996;35(suppl 1):13-16.
  11. Busch U, Schmid J, Heinzel G et al. Pharmacokinetics of meloxicam in animals and the relevance to humans. Drug Metabolism and Disposition 1998;26:576-584.