by

Dr Ross McKenzie BVSc MVSc DVSc

Plant poisoning is nationally important to Australia. About a thousand species of plants here are known to be toxic to livestock and humans. Poisoning of livestock by the more important of these costs us about $100 million each year. Deaths of cattle from plant poisoning in Queensland cost over $10 million yearly.

Sixty percent of the toxic plants in Australia are native to this country and have their major economic effect on agricultural enterprises which produce livestock by grazing native pastures. These industries contribute significantly to export-generated income and thus to our prosperity as a nation. This makes the reduction of the effects of plant poisoning on livestock one of the serious concerns of the pastoral industries.

The toxic native species belong to about 70 of the over 200 native plant families in Australia. Families with more than 10 toxic species include the legumes (Fabaceae, Mimosaceae), the nightshades and tobaccos (Solanaceae), the spurges (Euphorbiaceae), the grasses (Poaceae), the cycads (Cycadaceae, Zamiaceae), the saltbushes (Chenopodiaceae), the riceflowers (Thymelaeaceae) and the buttercups (Ranunculaceae). Some genera are generally toxic (for example Macrozamia and Pimelea), but it is important to note that a number of important toxic species (for example Erythrophleum chlorostachys and Trema tomentosa) have few or no known toxic relatives.

What is a poisonous plant?

Poisonous plants broadly include members of the cyanobacteria (blue-green algae), fungi, ferns, cone-bearing and flowering plants. They are understood by most people to be those which cause disease in humans or domestic animals after being eaten. This is quite true from the medical and veterinary viewpoint, but not really so when the whole spectrum of animal-plant interaction is considered. Strictly, if a plant is to be defined as poisonous, the animals that are susceptible to the toxin or toxins in the plant should be defined as well. For example, the nectar of Lomatia silaifolia flowers contains cyanide and is reputed to kill flies, but is not known to poison livestock. Conversely, I have seen beetle larvae feeding on the fronds of Cycas armstrongii which would be fatal for cattle. Whether a plant is poisonous or not depends on the capacity of the animal eating it to cope with the chemicals it contains. The outcome of eating such plants may be further complicated if the concentration of their toxins varies with the stage of growth of the plants or the seasonal conditions, rendering the plant more hazardous at some times than at others.

What do poisonous plants look like?

There are no common characteristics of form, colouring, odour or taste which distinguish a poisonous plant from a non-poisonous plant. To avoid poisoning, we need to learn what the known poisonous plants look like, based on the knowledge generated by past experience and scientific studies of the subject. The books of Everist, McBarron and Dowling and McKenzie (see the reading list at the end of this article) can help with the recognition of poisonous plants in Australia.

Why are plants poisonous?

Current opinion appears to be that many of the plant chemicals toxic to humans and livestock are produced as part of the plant's defences against being eaten or to gain an advantage over competing plants. Many of these defences are directed against insects. This makes the poisoning of people and their animals truly "accidental" and "collateral damage" in the long wars between plant and plant and between plants and the insects which eat them.

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Eremophila maculata is known as the "Spotted emu bush" and occurs widely in semi arid areas of Australia. It is also widely cultivated, apparently without any toxicity problems.

The known plant toxins are part of a group of chemicals known as secondary compounds or metabolites because they are not essential to the basic bio-chemistry of the plants. They include such things as waste products, storage products and flower pigments as well as growth inhibitors and toxins against predators. The caboxyatractylosides in noogoora burr (Xanthium pungens) seeds probably function to inhibit the growth of other plants which may compete with young burrs. These toxins will severely damage the liver of animals which eat them. They are also found in the native Wedelia asperrima which is known to poison sheep in northern Queensland. The cyanogenic glycosides in many plants including native or spotted fuchsia (Eremophila maculata) and native birdsfoot trefoil (Lotus australis) liberate cyanide when the plant tissue is damaged and kill insects, snails or slugs feeding on them. The same process may kill domestic animals as well.

Are native animals poisoned?

Under normal circumstances, plant-eaters (including mammals and insects) have developed ways of avoiding being poisoned. These mechanisms are through modified behaviour and through chemical-based defences. Certain insects have developed to a point where plant toxins form part of their own defence, thus turning a hazard into an asset. The classic example of this is the association of milkweeds (Asclepias curassavica) with monarch butterflies whose larvae store the bitter and toxic cardiac glycosides from the plant. This deters birds from feeding on the caterpillars.

Generally, insects eat only one or a limited range of plant species and have developed specific chemical means of dealing with the toxic compounds in those plants. In contrast, mammalian plant-eaters generally use a wider range of plants and depend on taste, smell and learning to avoid toxic species. They have more broadly-based chemical detoxication systems in the chemical mechanisms of their livers and other organs and in the microbes in their stomachs and intestines.

A good example of the adaption of mammals to a plant toxin by developing an efficient chemical detoxication method in the liver is the interaction of some Western Australian animals with many fluoroacetate-containing plants (Gastrolobium species) in the south-western part of the state. Brush-tailed possums, bush rats and western grey kangaroos from this area are capable of safely eating these plants which are rapidly fatal for livestock, red kangaroos, eastern grey kangaroos and for brush-tailed possums and bush rats from eastern Australia. Gastrolobium grandiflorum and Acacia georginae, which grow in Queensland, also contain this toxin which is the toxic component of 1080 poison.

Because plant-eaters are well adapted to their natural environments, poisoning occurs only when those environments are seriously disturbed (for example by drought or human interference) and animals are forced to leave their accustomed ecological niches to feed on dangerous plants. Koalas are believed to have been poisoned by cyanide while feeding on fresh young regrowth of manna gums (Eucalyptus viminalis) after bushfires in Victoria.

Perhaps the ultimate in forcing animals out of their ecological niches was the introduction of European farm animals and humans to the totally new plants of Australia and other European colonies in the Americas and southern Africa.

A little Australian history

The first plant poisonings in eastern Australia recorded by Europeans were suffered by the people and pigs of Cook's expedition of 1770 while they were repairing HMS Endeavour at the mouth of the Endeavour River in northern Queensland. Joseph Bank's journal describes the plant and its effects. The seeds of Cycas media seem to be the most likely cause of the diarrhoea and vomiting among the men and death in some of the pigs. In the previous century, Dutch explorers of south-western Australia had similar experiences with the seeds of the cycad Macrozamia riedlei. The first human inhabitants of Australia 40,000 years ago may have had similar experiences initially, but they learned to extract the toxins from the seeds of Cycas and Macrozamia species. By about 4000 years ago, they were using the resulting food to support large gatherings of people for ceremonies in Arnhem Land and in the Carnarvon Ranges of central Queensland. The known chemical compounds in untreated cycad seeds are not actually toxic themselves but need modification by bacteria in the intestines to yield the toxin methylazoxymethanol (MAM) which is a potent cancer-producer and causes liver damage. The toxin which causes spinal cord damage in cattle which eat cycad leaves is not known.

Another food plant of the original Australians and described near Brisbane by Alan Cunningham on expedition in 1828 and 1829 was the Moreton Bay chestnut or black bean (Castanospermum australe). Its seeds, toxic when raw, were prepared by roasting or soaking in water.

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Pimelea ferruginea occurs naturally in south Western Australia and is widely cultivated. Although not normally associated with toxicity, most pimeleas may contain animal toxins.

Few European overland explorers of Australia suffered from plant poisoning, but some are recorded. During the 1864-65 journey of the Jardine brothers from Rockhampton to Cape York, 30 of their 42 horses and 50 of their 250 cattle died after eating a poisonous plant near the Wenlock River on Cape York peninsula. It seems likely that the plant involved was the Cooktown ironwood (Erythrophleum chlorostachys) which is common in tropical Australia and highly toxic to livestock. Ernest Giles reported the death of two of his camels and illness of others after eating Gyrostemon ramulosus (sandhill corkbark, camel poison) during his central Australian expeditions of 1875-76.

Once the pastoral invasion of Australia began in earnest, many thousands of domestic grazing animals were poisoned by native plants to which they were un-adapted. 

A posy of native poisonous plants

The following is a mixed bouquet of toxic native flowering plants (with some greenery!), not mentioned elsewhere in this article. Some of these may find their way into our gardens. Some certainly deserve to do so as very attractive specimens.

Pimelea species in eastern Australia have been recognised for some time as the cause of a serious disease of cattle which produces anaemia, diarrhoea and heart failure with spectacular fluid accumulations under the skin. The plants involved are usually Pimelea trichostachya, P. simplex or P. elongata, collectively known as flaxweeds. The toxins in these plants are particularly irritant and probably occur in most, if not all, Pimelea species. Two other toxic species in Queensland are rather attractive red-flowered P. haematostachya and P. decora.

Certain ferns contain important toxins. Bracken (Pteridium esculentum, Pteridium revolutum) and mulga or rock fern (Cheilanthes sieberi) contain ptaquiloside which damages the bone marrow of cattle when taken in large amounts and produces bladder cancer in cattle when eaten in small amounts over several years. These ferns and nardoo (Marsilea drummondii) contain on enzyme which destroy vitamin B1, leading to brain damage in sheep and horses. You may recall that Bourke and Wills were fed nardoo cakes made from the ground sporocarps by their aboriginal rescuers near Cooper's Creek. It has been thought that their subsequent deaths may have been related to the toxin in nardoo, but this seems very unlikely. Bowenia serrulata, known as Byfield fern but in fact a cycad, has poisoned cattle in the same way that other cycads in Australia have.

Two rainforest trees whose fruits have poisoned children are white cedar (Melia azedarach var. australasica) and finger cherry (Rhodomyrtus macrocarpa). Finger cherry fruit were associated with cases of permanent blindness in northern Queensland in the first half of this century. The stage at which the fruit are dangerous is unclear. Prudence suggests that they should not be eaten at all, but the local aboriginal people have used them as a food source. See Everist (1981) for more details. The details of poisoning of children by white cedar fruits are even more vague, but pigs are occasionally poisoned and have damage to their brains and intestines as the main effects.

The seeds of Abrus precatorius are highly toxic, but the seed coat is very hard and seeds swallowed intact pass through the gut without any ill effect. Immature seeds or those with a cracked seed coat will release toxin. One seed contains enough toxin to kill an adult human. There is severe damage to the stomach and intestines and other body systems.

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The bulging trunk of the bottle tree, Brachychiton rupestris, is unmistakable.

Anthocercis viscosa (sticky tail flower) from the Albany region of Western Australia is reputed to be toxic to sheep, but evidence is scanty. Anthocercis littorea (yellow tail flower) is also suspected of poisoning. The fruits of this species have poisoned children in Western Australia.

Bottle trees (Brachychiton rupestris) have been known to poison cattle when felled and the pith of the trunk has been made available for drought feeding. The nitrate accumulated in this material has proved toxic.

The rattlepods (Crotalaria species) are a group of plants which contain pyrrolizidine alkaloids. These toxins accumulate in the liver and produce long-term damage which is often fatal. Horses and cattle are more susceptible to poisoning than sheep, but sheep have been poisoned by bluebush pea (Crotalaria eremea ssp. eremea) in western Queensland. Poisoned horses develop a condition called "walk-about disease" or "Kimberley horse disease" in which they become unaware of their surroundings and wander blindly. A major cause of this poisoning is Crotalaria crispata, a small plant common in the Kimberley region of Western Australia. Other Crotalaria species cause the disease in the remainder of tropical Australia. Recently, cattle in central Queensland have been poisoned by another plant containing these toxins, the variable groundsel or fireweed (Senecio lautus). Cattle with this long-term liver disease gradually lose weight and die.

In central and northern Queensland an unusual disease of horses is caused by two other species of Crotalaria, C. aridicola (Chillagoe horse poison) and C. medicaginea (trefoil rattlepod). Horses may develop a taste for these plants which damage the oesophagus (gullet) producing ulceration severe enough to stop the horse swallowing food.

Butterfly flag (Diplarrena moraea) from Tasmania and south-eastern mainland Australia and Morgan flower (Morgania floribunda) from drier parts of mainland Australia probably contain cardiac glycosides, the toxins in foxgloves (Digitalis purpurea) and cane toads (Bufo marinus). They have poisoned sheep and cattle, producing severe diarrhoea.

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Hoya australis is a rainforest climber from eastern Australia

Waxflower (Hoya australis) growing in the softwood scrubs of Queensland has been used as a drought fodder for cattle. Too much will damage spinal cord function and cattle will collapse and may die.

Nervous system damage of a more severe kind in cattle enabled the rediscovery of a rare rainforest tree, Idiospermum australiense, in northern Queensland. Its seeds were found in the stomach contents of cattle which had died rapidly after feeding beneath the trees in 1971. The toxin responsible is unknown, but may be related to strychnine.

Nervous derangement is also a feature of Weir vine (Ipomoea sp. aff. calobra) poisoning of sheep in the Maranoa district of Queensland and caustic vine (Sarcostemma brevipedicellatum - formerly S. australe) poisoning of sheep in inland Queensland. Sheep are also the victims of muscle degeneration produced by eating the mature seed heads of the plains plover daisy or flat billy buttons (Ixiolaena brevicompta).

Selenium poisoning of horses leading to hoof deformation and loss of hair from the mane and tail has been associated with eating mapoon or ada-a (Morinda reticulata) on Cape York peninsula or Neptunia amplexicaulis in the Flinders River region of northern Queensland. Cattle and sheep may also be poisoned.

Ellangowan poison bush (Myoporum deserti) is one of a number of plants in Australia, native and exotic, which cause severe liver damage to cattle and sheep. Stock travelling on foot are most at risk. This is also the circumstance which leads to poisoning by the native tobaccos (Nicotiana species). They contain nicotine alkaloids and produce nervous system malfunction.

Irritation of the intestines leading to diarrhoea is a common consequence of plant poisoning and is reported in cattle after eating woolly waterlily or frogsmouth (Philydrum lanuginosum), native leek (Bulbine bulbosa) and buttercups of various sorts (Ranunculus species). Diarrhoea is a major sign of poisoning of cattle by Thargomindah nightshade (Solanum sturtianum). While all Solanum species are often regarded as potentially toxic, in reality only a small number of them have been demonstrated to be poisonous under field conditions.

Congenital defects can be caused by eating poisonous plants. Wild parsnips (Trachymene ochracea, T. cyanantha and T.glaucifolia) have been associated with "bentleg" of lambs after pregnant ewes grazed on these plants in western Queensland and north-western NSW.

Blindness of sheep and goats has followed their eating of nodding blue lily or blind grass (Stypandra glauca - known as S.imbricata and S.grandiflora in Western Australia). Degeneration of the nerves connecting the eyes to the brain occurs. The toxin has been identified and is also present in smaller amounts in the blue flax lily (Dianella revoluta), but this plant has not been recorded as having caused poisoning.    Nervous system malfunction is seen in poisoning of livestock by some Darling peas (Swainsona species) and of cattle by some grasstrees (Xanthorrhoea species). Swainsona galagifolia is a common source of poisoning in eastern Australia, sheep being the most common victims. In Western Australia, Swainsona canescens is a common toxic species. Sheep are thought to develop a liking for these plants and may seek them out in the presence of other non-toxic feed. Cattle poisoned by grasstrees develop a disease called "wamps", a name derived form the sound made when an affected animal falls heavily to the ground. The grasstree most likely to be involved are X fulva (swamp grasstree) and X.johnsonii (northern forest grasstree).

Should we grow native poisonous plants in our gardens? 

Why do we grow plants in our gardens? Some of the more important reasons include the pleasure we get from the appearance or fragrance of plants, their use as food and their capacity to improve the physical conditions in our homes by shading, boosting humidity, moderating wind and providing privacy. Unless the toxic plants which we may grow are likely to be eaten in dangerous amounts by susceptible individuals (usually young children or domestic animals), no hazard exists and we can enjoy their benefits in the same way as for any other garden plant. As a general rule, it would be wise to check the toxic properties of all your garden plants in the appropriate reference books. Prevention is certainly far more certain than cure for plants poisoning.

Some further reading

1.     Anonymous, 1983, Pretty, but dangerous, Choice Magazine Volume 24, Number 4, pp.3-7.

2.     Covacevich J., Davie, P. and Pearn, J. (editors), 1987, Toxic Plants and Animals: A Guide for Australia, Queensland Museum, Brisbane.

3.     Dowling, R.M. and McKenzie, R.A., 1993, Poisonous Plants: A Field Guide, Queensland Department of Primary Industries, Brisbane.

4.     Everist, S.L., 1978, Botanical affinities of Australian poisonous plants. In "Effects of Poisonous Plants on Livestock", edited by R.F. Keeler, K.R.van Kampen and L.F. James, Academic Press, New York, pp.93-100.

5.     Everist S.L., 1981, Poisonous Plants of Australia, Revised edition, Angus and Robertson, Sydney.

6.     McBarron, E.J., 1983, Poisonous Plants: Handbook for Farmers and Graziers. Inkata Press, Melbourne.

7.     McCaughey, H., 1980, Is it Poisonous? Poisoning Prevention and First Aid. A Lifesaving Manual, Angus and Robertson, Sydney.

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This article was the basis of the Bill Tulloch Memorial Lecture presented to the Queensland Region of the Society for Growing Australian Plants on 8th November 1993 and is reprinted here with permission of the author and we offer him our sincere thanks.     Dr Ross McKenzie B.V.Sc., M.V.Sc., D.V.Sc, is a Senior Principal Veterinary Pathologist with the Animal Research Institute of the Queensland Department of Primary Industries at Yeerongpilly, Brisbane.     He is curator of the ARI Poisonous Plants Files and Chairman of the Queensland Poisonous Plants Committee.