Introduction to Poisonous Plants

Introduction to Poisonous Plants

Plants contain a large number of biologically active chemicals. Some of these have been found to be extremely useful for treating various human and animal diseases. However, some plant constituents produce adverse health effects following exposure. The onset of these adverse effects can be quite sudden or take some time to develop. Fortunately, there are relatively few plants that, when ingested, cause acute life-threatening illnesses.

The diversity of chemical substances in plants is quite amazing. In many instances, the role that a particular chemical plays in the normal ecology of the plant is unknown. In many cases, the presence of certain chemicals in plants is believed to confer some degree of protection from plant predators such as insects and ruminants.

There are a number of broad categories of toxicologically significant plant constituents. These include alkaloids (basic substances with nitrogen bound in a ring structure), amino acids, peptides and proteins, glycosides (chemical groups such as cyanide linked to sugars), acids (oxalic acid), terpenes (substances that contain the branched 5-carbon skeleton of isoprene), phenolics and tanlnins, and essential oils (various steam-volatile, primarily lipophilic plant metabolites stored in special plant organs and percieved by man through the stimulation of the sense of smell)

A number of factors can contribute to an animal being poisoned by plants. Fundamentally, there is the requirement that a sensitive species of animal ingest, or otherwise be exposed to, a toxic plant at an appropriate time. There are many examples of species differences with regard to sensitivity to the toxic effects of plants. In addition, it is possible for species to adapt to a potentially toxic plant if exposure is allowed to occur over a period of time. For example, ruminants adapted to oxalate-containing plants such as Halogeton glomeratus can tolerate concentrations that are lethal to non-adapted animals .

Ingestion of a potentially toxic plant is the number one route of poisoning in animals. It is important to emphasize that many, but certainly not all, toxic plants are not very palatable. Therefore, if given the choice, animals will avoid ingesting them even though they may be prevalent in the environment of the animal. In these situations, animals will often eat these plants only when other suitable feedstuffs are not available or when the animal is not able to selectively avoid the plants. The later situation may occur when toxic plants or plant parts such as seeds are inadvertently incorporated into hays, silages, or other foodstuffs

The timing of ingestion may be critical. The concentrations of toxic constituents in plants can vary from year to year, throughout the growing season of the plant, or as a result of environmental factors such as drought. As one example, the accumulation of potentially toxic concentrations of nitrate in forages most often occurs during periods of drought that prevent the normal growth of the plants (Pfister, 1984).

The diagnosis of plant poisonings can be difficult. Ingestion of many plants produce non-specific clinical signs that must be differentiated from other disease conditions. In addition, death due to toxic plant ingestion often does not result in characteristic post-mortem lesions. Relatively few tests are available to detect plant toxins in either ante-mortem or post-mortem samples. In many cases, the best way to support a diagnosis of plant poisoning is to confirm the presence of a toxic plant in the animal's environment (this will require positive identification of the suspect plant), to confirm that the plant has been ingested (noting that the candidate plants have been chewed and/or finding plant fragments in vomitus or gastrointestinal tract samples), and to correlate clinical findings, where possible, with those know to be associated with the suspect plant.

Unfortunately, there are few antidotal therapies for treating plant poisonings. The best approach for treating intoxicated animals often involves routine decontamination procedures such as induction of emesis (in appropriate species) and the administration of activated charcoal and a cathartic to hasten elimination of the plant from the gastrointestinal tract. In addition, symptomatic and supportive care need to be provided. Obviously, continued exposure to the suspect plant should be stopped. For a few plant poisonings, specific antidotes may be indicated; the treatment of cyanide or nitrate intoxicated animals are examples (see Prunus and Nitrate-Containing Plants for specific treatment protocols).

Information on poisonous plants can be found in a number of books. In addition, veterinary toxicologists at veterinary schools and/or veterinary diagnostic laboratories can provide information and identification services.

When submitted plants for identification it is important to collect specimens of the entire plant, including the roots. Wet newspaper should be wrapped around the roots of the specimen and the specimen placed in a plastic bag (it is acceptable to bend the plant along its stem so that it will fit in the plastic bag). The specimen should then be kept chilled until it arrives at the laboratory. Alternatively, plants can be dried and pressed, although this will take more time for processing.