After attending a weekend of talks at Fairhill Nursery last year where I caught up with Nan & Hugh Nicholson (they wrote that excellent series of rainforest books) Nan mentioned that in certain species of Macadamia the kernels contained traces of cyanogenic glycosides. This was the first I had heard about toxicity in Macadamias and thought I would go looking for other sources of information to do an article for our newsletter. Our previous Guest Speakers Merv & Elwyn Hegarty also knew about it and sent me some detail on the subject. The final piece of information came while I was attending a lecture by Dr. Ross McKenzie on "Cyanide, Strychnine Bush and other poisonous hazards in the Queensland flora". He sent me an article that appeared in Australian Vet Practitioner 30(1) March 2000 and to summarize:
Plants in the genus Macadamia (family Proteacae) are endemic to eastern Australia (seven species) and Sulawesi in Indonesia (one specie) (Gross 1995) Macadamia tetraphylla, M integrifolia and hybrids of these species – variously known as the Macadamia Nut tree, Qld. Nut tree or Bopple Nut tree are cultivated in Australia, Hawaii and elsewhere for the edible kernels. Some kernels of these species contain traces of cyanogenic glycosides while the kernels of M. ternifolia and M. whelanii contain significant amounts of cyanogenic glycosides, are bitter and not currently used as human food (Aboriginees would remove the toxins before eating them (Gross 1995).
In the clinical study, thirteen adult dogs of both sexes, various ages and at least five different breeds developed sudden onset of various combinations of either paralysis, pain in the hindquarters, listlessness, limping 6-24 hours after eating several kernels etc. with one Rottweiler being given 2 tablespoons of macadamia butter. All dogs recovered within 24 hours.
The close association between ingestion of kernels and the onset of the syndrome in dogs and the consistent signs displayed among cases strongly support the hypothesis that toxicity from a chemical or chemicals in the kernels was responsible for the syndrome. Such chemicals may well be part of the plant’s defence system protecting its genetic material from attack by insects or microbes. The only known toxins in Macadamia spp. are cyanogenic glycosides. In one case where roasted kernels were fed, it indicates that heat does not denature the toxin or toxins.
Given the widespread consumption of Macadamia kernels by humans and the lack of reports of subsequent poisoning, this syndrome appears to be a further illustration of the differing reactions of mammalian species to plant toxins. Dogs and humans for example differ significantly in their likelihood of being poisoned by theobromine in chocolate (from Theobroma cacao) and their susceptibility to N-propyl disulphide and thiosulphates in onions (Allium cepa). Dog owners should be warned not to regard their animals as "small humans". As M. ternifolia is native in the area where I live, I rang our local Veterinary Clinic which has a large country practice (the Vet is an ex-President of the Australian Veterinary Association) and they said they have never come across this syndrome in their practice. I am a little surprised by this as our "Chuck" (actually – Charles Henry Robert Backhouse - the Bull Terrier) who actually belonged to the neighbour but lived at our place during the day could crack an unshelled nut with his teeth – loved them.