Cassava (Manihot esculenta) is an edible root that resists drought, disease and pests. It is a major source of carbohydrates in the tropics, the second most widely grown and eaten food in Uganda after bananas, and a staple for about 57% of Ugandans. But cassava contains cyanogenic glycosides, which can cause fatal cyanide poisoning unless the root is detoxified by soaking, drying and scraping before it is eaten. Acute outbreaks from cassava are rarely described. This report, published in CDC's Morbidity and Mortality Weekly Report, describes one.
A funeral meal
On September 5, 2017, a funeral was held in Kasese District in western Uganda. Afterward, 33 people with diarrhea, vomiting and abdominal pain were admitted to Bwera Hospital. On September 8, the district health team notified the Uganda Ministry of Health of a suspected food poisoning outbreak.
The investigation
Fellows of the Uganda Public Health Fellowship Program and their supervisors reviewed records at Bwera Hospital, which serves the three affected subcounties, and searched for cases with community leaders. A probable case was sudden vomiting or diarrhea between September 1 and 9, plus at least one symptom such as muscle pain, a fast heart rate, rapid breathing, headache, dizziness, lethargy, convulsions or fainting.
- 98 probable cases, including 2 deaths (a case-fatality rate of 2%).
- Ages 11 months to 75 years, median 10. Rates were similar for males and females and across the subcounties, but lower among people aged 19–44 (5.5 per 10,000) than among those 18 and younger (15.1) or 45 and older (12.1).
- Symptoms: vomiting (95%), diarrhea (87%), malaise (60%), dizziness (48%), rapid breathing (27%), fainting (16%) and fast heart rate (10%); only 6% had fever. The pattern pointed to cyanide poisoning, and the lack of fever made an infection unlikely.
- Treatment: the recommended treatment for acute cyanide poisoning is hydroxocobalamin (injectable vitamin B12), but patients at health facilities received intravenous antibiotics and oral rehydration salts.
Illness began a few hours after the funeral and continued through September 8. Among people at the funeral, cases peaked a few hours after the evening meal there. Among people who had not attended, three smaller, successively shrinking peaks followed the evening meals of September 6, 7 and 8.
Tracing the flour
A case-control study compared 88 case-patients with 176 neighbors of similar age who had eaten cassava without falling ill. Case-patients were much more likely to have attended the funeral (odds ratio 40) and to have bought flour from retailers supplied by one wholesaler, "wholesaler A." Everyone at the funeral had eaten cassava bought from those retailers, and among those who had not attended, all of the case-patients but 79.2% of the controls had bought from them.
Local farmers grow their own "sweet" cassava, but residents buy flour from retailers, especially for large gatherings. The retailers' wholesalers usually bought from mills in Kasese town, about 31 miles (50 km) away. Wholesaler A, the main supplier during the outbreak, had bought the implicated batch in a town on the Uganda–Tanzania border about 174 miles (280 km) away, and it was traced further back to Tanzania. The batch was cheaper than others, and investigators suspected it came from wild cultivars. Funeral guests backed that up, describing the dish as pure white — typical of wild-cultivar flour — rather than the creamy color of flour from sweet cassava.
Five samples from the batch, tested by Uganda's Government Analytical Laboratory, were pure white and contained cyanogenic glycosides equivalent to an average of 88 ppm of cyanide (85–90 ppm), more than eight times the recommended safe level of 10 ppm. Police confiscated all the flour from the retailers the families had used, and health officials taught communities about cyanide poisoning and proper processing.
Why cassava can be dangerous
About 600 million people in the tropics, half of them in Africa, rely on cassava as their main food. Eating a large amount can cause acute, often fatal cyanide poisoning, especially where diets depend heavily on cassava, and repeated smaller exposures can cause lasting paralytic diseases such as tropical ataxic neuropathy and konzo, which brings sudden, permanent spastic paralysis. In sub-Saharan Africa, particularly Uganda, Tanzania and the Democratic Republic of the Congo, thousands of people may have been poisoned, though reliable data are lacking.
Symptoms typically begin 4 to 6 hours after a meal, as digestion releases the cyanide.
- Wild cultivars yield more, resist pests and keep longer in the ground, but they are bitter and cheaper, and can contain up to 2,000 ppm of cyanogenic glycosides by dry weight — 200 times the World Health Organization's safe level. They are not recommended for people to eat, yet some farmers still grow them.
- Sweet cultivars contain far less (up to 100 ppm) but still need processing: peel the roots, soak them in water for 4 to 6 days, sun-dry or roast them, scrape off the outer layer and grind the rest into flour. Soaking or drying too briefly leaves the glycosides high, and during droughts traders sometimes cut corners.
Prevention
A rapid, semiquantitative color test, free to workers in developing countries and usable by people with little training, can quickly show how much cyanide cassava flour could release. Wholesalers and food inspectors can use it to check commercial flour routinely. The authors call for warning farmers and consumers in cassava-dependent areas about poisoning from wild or poorly processed cassava, and for strict adherence to established processing methods.
Sources
- Phoebe H. Alitubeera, Patricia Eyu, Benon Kwesiga, Alex R. Ario and Bao-Ping Zhu, "Outbreak of Cyanide Poisoning Caused by Consumption of Cassava Flour — Kasese District, Uganda, September 2017," Morbidity and Mortality Weekly Report, CDC: https://www.cdc.gov/mmwr/volumes/68/wr/mm6813a3.htm
- The report's figures are not reproduced.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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