Control of Konzo and Kits to Determine Cassava Cyanide and Urinary Thiocyanate
TAN is a condition that has been known for more than a century and occurs in many countries of Africa, the West Indies and India (see Madhusudanan et al., CCDN News No 11, p3). It is a chronic condition of gradual or insidious onset, which is commonest amongst poor people in their 50's and 60's and is rare in children under 10. Patients are usually poor and consume a monotonous cassava diet. Symptoms include loss of sensation in hands, a sensation of something smeared on the soles of the feet which cannot be removed by trying to wipe it off, loss of vision, ataxia of gait, deafness, weakness and thinning of the legs. The disease is persistent in Nigeria and its incidence can be up to 18-26 persons per 1000 in villages where the diet is nearly all cassava. By comparison in yam-eating villages there is no incidence of TAN. It was considered that TAN was due solely to chronic cyanide intoxication but studies by Rosling and coworkers have shown that the situation may be more complex. By contrast konzo is due to acute cyanide exposure and levels of blood cyanide in konzo patients were found to be about 20 times as high as those in TAN patients. The onset of TAN is usually slow over months or years, whereas the onset of konzo is abrupt and its course is non-progressive. The mean age of people affected by TAN is greater than 40 years whereas konzo is primarily a disease of children and young women.
Cyanide is very poisonous because it binds to an enzyme called cytochrome oxidase and stops its action in respiration, which is the key energy conversion process in the body. The lethal dose of cyanide for an adult depends on body weight and is between 30 and 210 mg of hydrogen cyanide. Sometimes these limits are exceeded by persons eating a cassava meal and deaths occur due to cyanide poisoning. Smaller (non-fatal) amounts of cyanide cause acute intoxication with symptoms of dizziness, headache, nausea, stomach pains, vomiting and diarrhea.
Low dietary intake of iodine causes insufficient production of iodine-containing hormones in the thyroid gland and this produces goitre (enlargement of the thyroid gland) and cretinism (shortness of stature and severe mental impairment). Iodine deficiency disorders including goitre and cretinism are a major global health problem which affects about 500 million people. Ingested cyanide from cassava consumption is converted in the body to thiocyanate which is removed in the urine. The thiocyanate is a goitrogen, because it inhibits the uptake of iodine by the thyroid gland. Cyanide intake from cassava consumption thereby aggravates goitre and cretinism, but only if the iodine supply is below the recommended daily intake. Thus the effect of cassava in exacerbating goitre and cretinism may be corrected by increasing the intake of iodine.
Boivin et al., Pediatrics 2013, 131, e1231 have shown that children with konzo have impaired motor proficiency skills and impaired neurocognition compared with control children from a non-konzo area. This may extend the human burden of konzo with important public health implications.