or sections of small rivers that have been dammed to give relatively still water. The fish are stupefied and, after a short while, float to the surface of the water. They are easily picked up by hand and then eaten without any untoward effects on the health of the consumers. Women have traditionally carried out this activity in Africa [39–41].
The main active compounds are saponins, rotenoids, and diterpene esters. They represent the most important and most common constituents in ichthyotoxic plants and are of great interest to scientists [39, 40]. These biologically active compounds have potential as insecticides and many are widely used in traditional medicine and in preparation of arrow poisons [39]. For example, Cissus quadrangularis is used in Nigeria both as a fish poison and as an arrow poison to kill small birds. Many piscicidal plants from Africa are predominantly from the Leguminosae family, followed by the Euphorbiaceae family [39, 40]. Ichthyotoxic compounds have several other biological activities, such as the anticancer and potent antiviral activities of compounds isolated from Dryopteris fragrans [42] and the antibacterial activity of rotenoids against the ulcer‐causing Helicobacter pylori [43].
The most commonly used piscicidal plants in Africa are Tephrosia vogelii, Mundulea sericea, Euphorbia tirucalli, Gnidia kraussiana, Adenia lobata, Balanites aegyptiaca, Swartzia madagascariensis, Neoratanenia mitis, Tetrapleura tetraptera, and Strychnos aculeata [40]. C. quadrangularis is often used in conjunction with other local plant poisons such as B. aegyptiaca and T. vogelii. The mixture of the three toxic plants is more toxic than any one of the poisons used alone [39].
1.6 Poisonous Plants as Food
Oftentimes, there is no clear line between food plants and toxic and medicinal plants [44]. In many cases, one plant may have particular parts that are poisonous, whereas the other parts are edible or medicinal. A case in point is Abrus precatorius, the seeds of which are highly toxic; however, the leaves are edible and are used in traditional medicine [45]. The seeds of Malus spp. (apple) are also poisonous and contain the toxin amygdalin, which is a cyanogenic glycoside [5]. In some cases, the toxic parts have to be detoxified first through various elaborate and careful processes to render them edible. Although several plant species that are potentially toxic are consumed as food and also used as medicine, this section will focus on a few examples of plant species that are widely used as food and are also known to be toxic.
One of the most prominent examples of such toxic and edible plants is the cycad. Cycads have been widely researched as poisonous food plants. Cycads are indigenous to the tropics and subtropics, where they have been used both as a staple and as emergency food and medicine for various ailments [46, 47]. Natives of particular areas have long been aware of their toxicity, which especially manifests as gastrointestinal and neurological effects. In cattle, for instance, continued ingestion leads to irreversible paralysis of the extremities [44].
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
