The nutritional value of cycads principally lies in an edible starch extracted from the roots, stems, and nuts [44]. Cycads produce flour with a high nutritional value [48]. Several precautions are taken when preparing it as a food. A high‐quality food starch is extracted from the fibrous pulp of cycads through alternate processes of cutting, drying, and soaking [44]. In some parts of Uganda, for example, the hard seed of the cycad Encephalartos hildebrandtii can be boiled and ground into flour in times of famine. The starchy center of the stem is also edible [49].
There have been repeated accounts of poisoning from cycad ingestion during periods of famine. This has been attributed to inadequate preparation of cycad products, possibly because of a lack of knowledge of the toxicity of the plants or because of their unpredictable variations in toxicity [44].
Compounds from cycads are carcinogenic in various laboratory animals [50]. Hirono et al. [51] showed a high death rate from liver cirrhosis in the Miyako Islands of Japan that may be correlated to the consumption of cycads during periods of crop loss. Cycad flour contains the neurotoxin beta‐methylamino‐L‐alanine (BMAA) [52] as well as other neurotoxins, as reviewed by Rivadeneyra‐Domínguez and Rodríguez‐Landa [48].
Duncan et al. [52] showed that 87% of the total BMAA content of Cycas circinalis seeds collected on Guam island was removed during traditional processing. They concluded that processed cycad flour as prepared on Guam contains extremely low levels of BMAA (0.005% by weight), making it unlikely to cause the delayed and widespread neurofibrillary degeneration of nerve cells observed in amyotrophic lateral sclerosis and the parkinsonism–dementia complex of Guam [52].
Another widely used toxic plant species is cassava (Manihot spp.), which is consumed after detoxification. Cassava contains potentially toxic levels of cyanogenic glucosides, made up of linamarin (95% of total cyanogen content) and lotaustralin (5%) [53]. Sun‐drying and crushing cassava roots to make flour removes 96–99% of total cyanogens [54]. Cyanide intake from a cassava‐dominated diet has been put forward as a contributing factor in two forms of nutritional neuropathies in Africa: tropical ataxic neuropathy and epidemic spastic paraparesis. Therefore, proper processing of the cassava root is required to detoxify it for safe consumption [5, 55]. It is thus important to beware of the potential toxicity of various plant species while trying to meet the food security needs of vulnerable populations [48].
1.7 Poisonous Plants as Biopesticides
Botanical pesticides have a wide range of biological activities such as repellents, insecticides, fungicides, bactericides, molluscicides, nematicides, and rodenticides [21, 56]. Some of the plant species used as fishing poisons also have proven insecticidal properties include Derris sp. (containing rotenone) and Nicotiana sp. (containing nicotine) [57].
Rotenones are extremely toxic isoflavones from the roots or rhizomes of several tropical legumes. They act by suppressing the appetite of insects, leading to death within hours or a few days. There are more than 67 species of legumes that synthesize a broad spectrum of non‐systemic insecticides [58]. The roots of many species of Derris and Lonchocarpus (family Leguminosae) have insecticidal properties, which are mainly attributed to the presence of rotenone (3–10%), although other insecticidal compounds are usually present. Other genera with rotenoid‐producing species are Millettia, Neorautanenia, and Tephrosia [21]. Strychnine from Strychnos spp. has also been historically used as a pesticide [21]. Such compounds of botanical origin can be highly effective with low levels of toxicity toward non‐target organisms and multiple mechanisms of action [59, 60]. However, poor stability and other technological issues limit the large‐scale application of natural compounds for pest control [21, 61].
1.8 Toxic Psychoactive Plants for Recreational and Religious Purposes
All cultures around the world have some kind of drug culture that relies on psychoactive compounds for medicinal, recreational, or ritual purposes [62]. Psychoactive substances are compounds that have the ability to change consciousness, mood, and thoughts [63]. Psychoactive plant species contain compounds that work as hallucinogenics, sedatives, or stimulants [64, 65].
Alrashedy and Molina [64] conducted a phylogenetic analysis of 126 traditionally used psychoactive plants that indicates multiple ethnobotanical origins. The plant species documented were also used for several medicinal purposes. Rätsch [66] presented a detailed account of psychoactive plants. Some of the well‐known psychoactive plant species with medicinal, recreational, and other purposes include Cannabis spp. (marijuana), which has hallucinogenic, stimulant, antianxiety, antidepressant, sedative, analgesic, and aphrodisiac properties; Atropa belladonna (belladonna), which has hallucinogenic, stimulant, sedative, and aphrodisiac properties; and Papaver somniferum (opium poppy), Datura spp., and Mandragora spp. (mandrake), which all have hallucinogenic, sedative, analgesic, and aphrodisiac properties. In addition, Catha edulis (khat) has stimulant, antidepressant, and aphrodisiac properties.
Some plants containing psychoactive substances should be classified as harmful drugs since chronic administration has been linked to addiction and cognitive impairment [65]. Not much is known about the toxicity of many of the psychoactive plant species, mainly because of the limited number of studies conducted on their toxicity. A case in point is Datura stramonium, in which all parts are toxic. The plant contains a mixture of anticholinergic alkaloids such as atropine, hyoscyamine, and scopolamine, which are mainly responsible for its neurotoxic and hallucinogenic effects [67]. Datura has a narrow therapeutic window, implying a small difference between the active and lethal dose. It has been widely documented as a cause of accidental poisoning, particularly in contaminated food [68]. Another example is C. edulis, the consumption of which has been associated with several cases of acute liver failure and autoimmune hepatitis [69].
1.9 Poisonous Plants in Warfare and Bioterrorism
Bioterrorism refers to the use of biological agents to inflict disease and/or death on humans, animals, or plants by a political or religious group or cult to achieve a political or ideological objective [70, 71]. Such agents include some bacteria such as Bacillus anthracis (anthrax), viruses such as variola virus (smallpox), rickettsiae, fungi, or biological toxins such as ricin. For the agent to be used successfully, it must first be “weaponized,” or produced in sufficient quantities in relatively stable and easily disseminated forms [70]. Therefore, human populations, crops, and livestock are considered possible bioterrorist targets [70]. Concerns over the use of biological as well as chemical weapons have increased recently. In fact, bioterrorist incidents have increased markedly since 1985 [71], with attempted uses of ricin by various groups, especially in the USA [72]. According to Balali‐Mood et al. [73] there was a 10‐fold increase in the number of published articles following the terrorist attacks in the USA on 11 September 2001.
Ricin is a toxalbumin derived from the seeds of the poisonous plant Ricinus communis (castor bean; family Euphorbiaceae). Ricin acts by inhibiting protein synthesis and is one of the most toxic biological agents known. Ricin is classified as a category B bioterrorism agent and a schedule 1 chemical warfare agent according to the US Centers for Disease Control and Prevention [74]. It can be extracted from castor beans and purified. It is stable under ambient conditions and is readily accessible and relatively easy to extract. The fact that ricin has been used previously in high‐profile assassination cases – such as that of Georgi Markov, a leading communist dissident exiled in London, in 1978 [75, 76] and similar assassination attempts with ricin elsewhere [71] – has contributed to its publicity. On a larger scale, Iraq developed a weapons of mass destruction program between 1985 and 1991, in which approximately 10 L of concentrated ricin solution was produced for field testing [77]. Schep et al. [78] have, however, argued that although ricin is deadly it is not suitable as an agent of bioterrorism for a large population since a substantial mass of powder needs to be extracted, formulated, and produced in the right particle size to target the relevant parts of the lung to be fatal. Additionally,