Phytotoxins – Cassava
Introduction
Plant toxins, or phytotoxins, are poisonous chemicals synthesized naturally by plants to defend themselves against threats like bacteria, fungi or predators. Phytotoxins can be ubiquitous, yet unnoticed by most people, such as hidden in food, drinks or decorative flowers. The danger is present in every aspect of life, becoming a concealed threat to our health. A well-known example of this is cassava – a staple food that is abundant in Asia, South America and Africa. This plant contains cyanide compounds in high concentration and has caused several outbreaks around the globe.
Cassava
Cassava is a woody shrub of the spurge family, Euphorbiaceae, native to South America, from Brazil, Paraguay and parts of the Andes. It is extensively cultivated in many regions worldwide for its edible starchy root tuber. The most predominant form of consumption is boiled, but recently, there has been an increase in the quantity of processed tapioca, which is used in food and beverage. Being the 3rd largest source of carbohydrates globally, cassava has been an important staple food, feeding more than 500 million people.
Toxin in Cassava
Raw peels, leaves and roots of cassava are lethal to ingest due to the accumulation of linamarin and lotaustralin, which are toxic cyanogenic glycosides and are decomposed by the cassava enzyme linamarase; thus, releasing poisonous hydrogen cyanide. The presence of these compounds are remarkably abundant in cassava grown in drought. Bitter cassava is found to be more poisonous than sweet ones, as the quantity of toxin can be up to 1000 mg per kilogram, in comparison to 25 mg per kilogram in the latter.
Hydrogen cyanide
Hydrogen cyanide, previously known as prussic acid, is a highly flammable and volatile liquid with the formula HCN. Famously known for its adverse reputation, however, hydrogen cyanide is used widely in the chemical industry, ranging from polymers to pharmaceutical products. Cyanide is a potent cytochrome c oxidase (COX, a.k.a. Complex IV) inhibitor, which leads to cell asphyxiation. It interferes with the ability of cells to take or use oxygen via oxidative phosphorylation.
Mechanism
Cyanide is a non-competitive inhibitor of the heme a3-CuB binuclear center of COX, and as it binds to the COX, electrons can’t be transferred to O2. The process not only disrupts the mitochondrial electron transport chain but also affects the pumping of protons out of the mitochondrial matrix. Cyanide not only interferes with the process of aerobic respiration but also ATP synthesis.
Symptoms of cyanide poisoning
Acute exposure
When inhaled in large amounts, cyanide causes coma with seizures, apnea, and cardiac arrest,and death will just be a matter of seconds. Exposure with lower dose results in loss of consciousness, dizziness, headaches, vertigo and difficulty in breathing. Eventually, the state of the patient progresses towards a deep coma, ending with a cardiac arrest. A dose as little as 1.5 mg/kg body weight is enough to kill a human.
Chronic exposure
Over a long period of exposure, cyanide accumulate in blood, causing an increase in its concentration, which later leads to weakness accompanied with several symptoms such as permanent paralysis, nervous lesions, hypothyroidism, and miscarriages.
Real-life cases
Outbreak in Uganda, 2017
On September 5, 2017, after attending a funeral, 33 people underwent symptoms such as vomiting, diarrhea and abdominal pain. An investigation was carried out and traced back the reason to the consumption of a dish containing cassava, which included mixing its powder with hot liquid. This was due to the improper processing of cassava powder leading to the presence of cyanide there. 98 cases were identified in the outbreak, with 2 fatal cases.
Outbreak in Tanzania, 1988
A massive outbreak due to acute intoxication of cyanide occurred in southern Tanzania in 1988. A number of people felt nauseous and vomited after consuming insufficiently cropped and processed cassava roots, as they were just a method to survive the drought. The roots were harvested and grinded directly into powder, without soaking or boiling in advance, leading to the fact that the toxins still remained.
Prevention & First aid
If someone is exposed to cyanide, we should immediately move them to an area with fresh air to prevent further exposure to the gas. Next, any body parts that have been in contact with cyanide should be rinsed with water to decontaminate and to protect everyone else. Perform CPR if needed and provide antidotes like Hydroxocobalamin or Sodium Thiosulfate as they bind to cyanide, forming a harmless compound that can be excreted by the body. Make sure to call for emergency as fast as possible as only a second can determine whether it is fatal or not.
In order to minimize the risk of cyanide poisoning, all products derived from cassava have to be processed following a standard. All peels should be removed and the whole roots must be soaked for several hours to reduce the concentration of cyanide. After that, cassava must be cooked thoroughly or fermented for 2-3 days before consumption.
Sources
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