Plant-associated Toxins. Some Animal Health Issues
Dr Steven Colegate
CSIRO Livestock Industries
Plant Toxins Research Group
Australian Animal Health Laboratory
Private Bag 24
Geelong, Victoria 3220
In: Plant toxins and their effects on animal welfare. Paper presented at the Animal Welfare Centre Scientists' Meeting, 17th May 2002, DPI Werribee, Mickleham Road Attwood, Victoria.
For whatever reasons, and protection against herbivory seems to be preeminent, plants biosynthesise chemicals that are not directly related to the nutritional development of the plant. Among these plant secondary metabolites can be found some of the most toxic chemicals known.
Animals that evolve with these plants can adapt, developing either an awareness and consequent avoidance of the plant or some mechanism for detoxifying the secondary metabolites. Other animals, especially domesticated animals used in farming, are vulnerable to the toxins with differing degrees of susceptibility and sometimes with species-specific clinical effects.
Our farmed animals are not only subject to toxic plant secondary metabolites, but also to plant-associated toxins produced by fungi and bacteria that are growing on or within a plant species.
These plant-associated toxins are varied in chemical structure and consequent bioactivity and can target specific organs and tissues or can exert a multi-system insult with consequent variation in clinical signs and effects.
The native Stypandra spp. (nodding blue lily, blindgrass) can cause a nervous disorder characterised by severe pelvic limb weakness and can result in death. Permanent blindness occurs in animals that survive acute intoxication. The toxic phenolic compound, stypandrol, causes demyelination of nerve axons, swelling and atrophy of the optic nerve and adversely affects the photoreceptor layer in the retina.
The introduced plants, Heliotropium europaeum (common heliotrope, potato weed) and Echium plantagineum (Salvation Jane, Paterson's curse), both produce toxic pyrrolizidine alkaloids that primarily target the liver of grazing animals leading to liver damage or a fatal haemolytic crisis due to the sudden release of stored copper in the liver.
The introduced pasture grass, Lolium rigidum (Annual ryegrass) can become infected with a corynetoxin-producing bacterium. Ingestion of these very potently toxic, cumulative tunicaminyluracil glycolipids affects all animal species resulting in tetanic convulsions prior to death. Survivors retain reproductive abnormalities.
The native sedges, Schoenus rigens and S. asperocarpus have resulted in sheep deaths including examples of 2000 head dying overnight during one episode. The sedges become a problem for graziers when new land is cleared or at the break of a dry season. The isoprenoid guanidine compound, galegine, causes vascular leakage and a consequent heavy oedema resulting in respiratory distress and death.
There is evidence to suggest that the introduced pasture grass Phalaris spp. (canary grasses) can adversely affect sheep, cattle and horses in different ways. In addition to two clearly defined nervous system disorders attributed to indole alkaloids, sheep can also be exposed to a "sudden death" toxin usually at the break of season. The equine neurotoxins associated with P. coerulescens (blue canary grass) do not seem to affect sheep or cattle.
Recently identified steroidal glycosides have been attributed to the adverse gastrointestinal and cardiac effects of sheep that ingest native Stemodia kingii on pastoral properties in the NW of Western Australia.
Photosensitisation, resulting in skin lesions and oedema, can be due to ingestion of primary photosensitisers such as found in Hypericum perforatum (St John's Wort) or secondary photosensitisers where liver damage leads to elevated levels of the photoactive chlorophyll degradation product, phylloerthyrin. The Acute Bovine Liver Disease affecting beef and dairy cattle in South Australia, SW Victoria, Tasmania and NSW has an unknown aetiology but is associated with the presence of the native grass Cynosurus echinatus (Rough dogs tail). It is unknown whether a plant toxin per se or a plant-associated mycotoxin is responsible for the hepatogenous mortalities or photosensitisation.
These are but a few of the plant-associated toxins that can adversely affect animal health, welfare and productivity in Australia.
The Animal Welfare Science Centre was established by the University of Melbourne, Monash University and the Department of Primary Industries, Victoria. The Centre focuses and coordinates the research and academic resources of the three collaborating organisations, providing the animal industries, animal users, the farming community, Government, the general community and the academic community with an internationally competitive research, teaching and training resource in animal welfare.