Konzo is an irreversible disease that appears suddenly and causes spastic paralysis of both legs. Konzo affects mainly women and children, afflicting thousands in the remote rural areas of Bandundu Province in the Democratic Republic of Congo. It has also been reported from remote rural areas in Central African Republic, Mozambique and Tanzania.
We mentioned this disease in our book Amaranth to Zai Holes: ideas for growing food under difficult conditions, pages 265 to 268. “Epidemic spastic paraparesis occurs mainly among women and children. It permanently cripples the victim ‘from one day to the next’ by damaging parts of the spinal cord that transmit signals for movement. Muscles are not flaccid, as in polio, so the legs usually support affected persons sufficiently to let them stand, especially if supported by a stick. Walking is often uncontrolled jerks.”
This laboratory study measured the content in processed cassava of both cyanide and cyanogens. Cyanogens are compounds in cassava that can produce cyanide. The particular processed cassava product that they studied is called cossettes, but similar results might be obtained with other processing methods. Cossettes are produced by soaking or immersing fresh bitter (high cyanide content) cassava roots, whole or peeled, in a stream or standing water for at least three days to allow them to ferment until they become soft. The fermented roots are then taken out, peeled, and sun dried on mats, racks, roofs, etc. Depending on the weather, this takes two to five days. This form of cassava product is preferred because it can be stored for a long period and traded over greater distances. If for some reason the roots are processed for a shorter period, the remaining cyanogen content will be much higher than normal.
It is generally assumed that cyanogen content of 10 mg or less per kilogram of cassava is safe. All cossette samples had less than 10 mg/kg (the highest from Cameroon was 9.37 mg and the lowest from RondPoint was 1.45 mg). So why is there sometimes a problem?
Konzo is normally brought about by a combination of exposure to cyanide and malnutrition from a low-protein diet. The risk of developing the disease is even greater when that low amount of protein is deficient in sulfur-containing amino acids (amino acids come from protein after it is digested). The cassava in the study was very low in protein and especially low in sulfur-containing amino acids. It is this combination of a nutritional problem and toxicity that causes the problem.
In the Amaranth to Zai Holes article about how/when cyanide in food becomes a problem (see above) we wrote the following about what happens to cyanide after it is eaten.
“The body is protected from cyanide in two steps. The blood contains a substance that can, within minutes, bind up to 10 mg of cyanide. This is then taken to the liver and detoxified in a process that takes a few hours.
“If more than 10 mg of cyanide is consumed, but not enough to be fatal, it is converted to a far less toxic substance called thiocyanate. The thiocyanate is eventually excreted in the urine. This detoxification process requires the element sulfur, which is obtained from protein in the diet. In protein deficient diets the detoxification process ceases to operate. So lack of protein in the diet accentuates the toxicity of cassava. “It should be noted that considerable amounts of fish are consumed in areas of the Amazon, the Congo basin and southern India where cassava has been established as the dominating staple for centuries.” If other food is not available, an adult will consume daily about ... 0.5 kg dry (1.5 kg wet) weight of cassava.”
Fish contain abundant protein and sulfur-containing amino acids, so fish-eaters would be less likely to become ill from eating food with over 10 mg of cyanide/kg than would people who eat mainly cassava roots and other foods low in protein. Konzo is not found in the cities, perhaps because urban diets typically contain more protein. It is also possible that the cyanogens in cassava react to form cyanide (which evaporates; see next article for more detail) as the cassava is transported to a city and stored, but that it is eaten much sooner in rural areas.
Other diseases caused by cyanide toxicity develop only after exposure to moderate amounts over a period of time. A physician visiting ECHO mentioned his puzzlement over the prevalence of goiter in a particular Pacific island, even though there was iodine in the diet. This too is likely caused by cyanide. Quoting our book again, “The thiocyanate produced when cyanide is detoxified (see above) interferes with uptake of iodine by the thyroid gland. Fortunately this interference can happen only when iodine intake is already low, below 200 micrograms per day. Populations in northern Zaire with very low iodine in the diet and who regularly ate inadequately processed cassava suffered from severe endemic goiter and a high prevalence of cretinism [a condition resulting in growth retardation, developmental delay and other abnormalities]. When iodine supplements were used the goiter problem decreased considerably even though the cassava was still not adequately processed due to adverse conditions.” If you suspect this may be occurring where you work, do a search on the web for cassava and goiter and you will be led to several articles on that topic.
Abstracted by Price, M.L. 2005. Understanding Why/When Cassava Can Cause the Disease Konzo. ECHO Development Notes no. 89