Toxic substances
The castor plant produces several toxic substances, particularly ricin, a lethal and cytotoxic lectin notorious for its potential use as a biological weapon. Other undesirable substances include the alkaloid ricinine and allergen CB-1A (Maroyi, 2007).
Ricin
The endosperm of castor seeds contains about 1-5% of ricin. Ricin is found only in the endosperm and is no present in the oil as it not oil-soluble (Johnson et al., 2005; Lord et al., 1994). Ricin is a glycoprotein lectin composed of 2 chains, A and B, linked by a disulphide bond (Audi et al., 2005; Akande et al., 2016). The B chain is a lectin and binds to galactose-containing glycoproteins and glycolipids expressed on the surface of cells, facilitating the entry of ricin into the cytosol. The A chain inhibits protein synthesis by irreversibly inactivating ribosomes which prevents chain elongation of polypeptides and leads to cell death (Audi et al., 2005). Ricin structure and its mode of action have been extensively reviewed (Lord et al., 1994). The toxicity of ricin mainly consists in the inhibition of protein synthesis, but other mechanisms like apoptosis pathways, direct cell membrane damage, alteration of membrane structure and function, and inflammatory mediators are also described (Al-Tamimi et al., 2008). Symptoms of ricin poisoning begin within hours after exposure by ingestion or inhalation (Haritha et al., 2019). They may include stomach irritation, vomiting, bloody diarrhea, abdominal pain, increased heart rate, low blood pressure, profuse sweating, collapse, convulsions, and death within a few days (Salihu et al., 2014a). Symptoms and their intensity depends on the animal species. In horses, signs include: sweating, trembling, incoordination, vigorous heart action that shakes the whole body but pulse weak, muscle spams, erection of penis or clitoris, abdominal colic and shallow respiration. In cattle, signs similar to those of horses have been observed as well as elevated temperature, and diarrhoea stained with blood. In swine, vomiting occurs (which often saves their life of animal), and the skin of ears, flanks and hams becomes cyanotic. In poultry, depression is observed; feathers are ruffled, the wings are drooping, comb and wattles are greyish. In hens, egg production is stopped and moulting occurs. Birds that do not quickly die lose weight (Weiss, 1971).
Ricinine
Ricinine is an alkaloid found mainly in the leaves but it exists in all parts of the plant, including the seed pericarp (Severino et al., 2006). Foliage ricinine contents of 1.3% DM (Severino et al., 2006) and 0.13-0.26% fresh weight (Burgess et al., 1988) have been reported. Ricinine content is higher in young tissues, damaged or stressed plants (Azevedo et al., 2007; Tavora, 1987; Moshkin, 1986). While less toxic than ricin, ricinine is still detrimental to animal health (Albuquerque et al., 2014). In cattle and rabbits, cases of ricinine intoxication from leaves or fruit pericarps have been reported to have neurological effects. Symptoms included lack of equilibrium, ataxia, head deviation, muscular tremors, sialorrhea (excessive salivation), and eructation. These signs could be followed by sudden recovery or death. No lesions were found in the central nervous system (Ferraz et al., 1999; Tokarnia et al., 1975). The ricinine LD50 for rabbits is about 3 g/kg LW in a single oral dose (Al-Khafaji et al., 2017). In mice, animals receiving the higher doses of ricinine became exophtalmic, had tremors and clonic seizures, and died in a few minutes of tremors and clonic seizures and died a few minutes after receiving the extract (Ferraz et al., 1999). Ricinine is toxic to some insect species and castor plants can be used as insects traps in horticultural crops and gardens (Burgess et al., 1988).
Allergen CB-1A
Some complexes of protein and polysaccharides have been referred to as allergenic. However, the main concern is with people handling castor bean products and animals were not reported to suffer from allergies (Candido et al., 2008; Anandan et al., 2005).
Castor plant poisoning
Castor plant poisoning occurs when animals ingest broken seeds, chew the seeds or eat high quantities of forage. Unbroken seeds may pass through the digestive tract without releasing ricin. Most of the cases reported in the literature are due to accidental situations where animals consume entire plants containing both foliage and seeds. Growers of ornemental castor plants are advised to remove flower clusters from the plant as they appear so that no seeds are produced, reducing the risk of accidental poisoning (Salihu et al., 2014a). It is recommended to limit the access of animals to areas where castor plants are grown, particularly during periods of forage scarcity, or to cut and eliminate the shrub, preventing any intake of leaves and seeds (Aslani et al., 2007).
Cases of cattle poisoning were reported in Brazil, when animals consumed leaves but also fruits with seeds due to severe scarcity of pasture. The main reason of the death of the animals is probably due to seed consumption (observed into the rumen and omasum) (Albuquerque et al., 2014). Cases of poisoning have also been described with sheep consuming leaves and seeds (Aslani et al., 2007; Bianchi et al., 2018). In California, sheep wandering into an unharvested field of castor plants were poisoned and some died (Weiss, 1971). Poisoning was also reported with goats consuming only leaves from recently cut branches (Brito et al., 2019). The lethal doses of fresh leaves for adult cattle was reported to be 20 g/kg (fresh) body weight (Albuquerque et al., 2014; Armien et al., 1996). In cattle, dry leaves consumed after a 19 weeks storage period lost half of their toxicity (Tokarnia et al., 1975). For sheep the lethal doses of castor plant foliage would be 30 g/kg (fresh) BW and for goats it could be higher than 40 g/kg BW but this needs confirmation (Brito et al., 2019).Rabbits and sheep fed on small dried castor seedlings mixed with alfalfa hay showed no toxicity or difference in growth between treated and control animals (Weiss, 1971). It has been reported that frequent consumption of castor seeds could alleviate castor plant poisoning in cattle but this effect was not observed when cattle ate castor plant foliage (Afonso et al., 2001).
