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Foodborne Botulism Associated With Home-Canned Bamboo Shoots -- Thailand, 1998
On April 13, 1998, the Field Epidemiology Training Program in the Thailand Ministry of Public Health (TMPH) was informed of six persons with sudden onset of cranial nerve palsies suggestive of botulism who were admitted to a provincial hospital in northern Thailand. To determine the cause of the cluster, TMPH initiated an investigation on April 14. This report summarizes the results of the investigation, which indicate that the outbreak was caused by foodborne botulism from home-canned bamboo shoots.
Of the six patients, five resided in one village (village A), and the other patient resided in another village (village B). A case was defined as at least three symptoms (ptosis, dysphagia, dysarthria, dysphonia, dry mouth, symmetrical paralysis, diarrhea, or vomiting) that developed in a resident of village A or B during April 8-17. TMPH reviewed medical records and interviewed patients in the provincial hospital; seven additional cases were identified. Twelve (92%) case-patients resided in village A; nine (69%) were hospitalized. The median age was 44 years (range: 38-68 years), and nine were women. In the 13 case-patients, symptoms included dysphagia (85%), dry mouth (62%), vomiting (54%), dysphonia (54%), diarrhea (38%), symmetrical paralysis (31%), dysarthria (31%), and ptosis (23%). Four required mechanical ventilation. Two (15%) patients died; both were women, ages 46 and 68 years. Electromyography of two ill persons showed an incremental response to rapid repetitive stimulation consistent with botulism (1).
TMPH interviewed 11 case-patients and the family members of the two who died. All 13 ill persons had eaten home-canned bamboo shoots. No other common food was identified. Sixty-six healthy controls were selected among residents of village A and B who were preparing foods on April 16 for the burial services of the two decedents. All controls were women; 38 (58%) resided in village B. Four (6%) of the controls had eaten home-canned bamboo shoots (odds ratio [OR] undetermined; p less than 0.001). Cooking the food containing the bamboo shoots was protective; one (7.7%) of the 13 case-patients cooked bamboo shoots compared with three of four controls who had eaten bamboo shoots (OR=0.03; 95% confidence interval=0.0-0.95). The time between eating bamboo shoots and onset of illness was 6 hours to 6 days (median: 2 days).
All 13 case-patients ate bamboo shoots from one 20-L (5.3-gallon) can. The bamboo shoots had been canned and sold by a village B resident, who also was a case-patient. The vendor picked the shoots, then cleaned and processed them by boiling them in a 20-L galvanized iron container for approximately 1 hour. While the bamboo shoots were boiling, the vendor sealed the container with lead. The canned bamboo shoots were stored at ambient temperatures for 3-6 months until they were sold.
Cultures of stool samples from two case-patients were negative for Clostridium botulinum at Siriraj Hospital. Cultures from six specimens of the implicated home-canned bamboo shoots sent to the U.S. Army Medical Research Institute for Infectious Diseases in Fort Detrick, Maryland, were negative for C. botulinum. One of six bamboo shoot specimens was positive for botulinum toxin type A by enzyme-linked immunosorbent assay and mouse antitoxin bioassay (1). The pH of two bamboo shoot specimens was measured at the Regional Medical Sciences Center and was 5.3 and 5.7.
As a result of this investigation, TMPH recommended increasing control of home-canned food production in all provinces and strengthening surveillance for foodborne botulism. The provincial government prohibited sale of the remaining 650 cans (13,000 L [3421 gallons]) of home-canned bamboo shoots in affected villages. Provincial authorities conducted an education campaign advising the population to buy only government-approved food and to heat home-canned bamboo shoots before eating. The national food safety committee in Thailand also instructed all 75 provincial authorities to enforce high temperature processing of home-canned foods.
Reported by: P Wongwatcharapaiboon, MD, L Thaikruea, MD, K Ungchusak, MD, Field Epidemiology Training Program, S Wattanasri, MD, Div of Epidemiology, Ministry of Public Health; P Sriprasert, MD, S Nanthavas, T Visajsuk, Nan Provincial Health Office, Nan; S Chaiupala, MD, K Tuntisririwit, MD, S Leksririvili, Nan Hospital, Nan; A Thanawong, Thawangpha Hospital, Nan Province, Nan, Thailand. Regional Medical Sciences Center, National Institute of Health, Food and Drug Administration Committee, Dept of Agriculture, Toxicological Section, Siriraj Hospital, Bangkok, Thailand. Armed Forces Research Institute of Medical Science, Bangkok, Thailand. US Army Medical Research Institute for Infectious Diseases, US Dept of Defense, Fort Detrick, Maryland. Div of International Health, Epidemiology Program Office; Foodborne and Diarrheal Diseases Br, Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, CDC.
Botulism is caused by a neurotoxin produced from the anaerobic, spore-forming bacterium C. botulinum and, in humans, is usually caused by toxin types A, B, or E. Botulism is characterized by symmetric, descending, flaccid paralysis of motor and autonomic nerves, usually beginning with the cranial nerves. Blurred vision, dysphagia, and dysarthria are common initial complaints. Foodborne botulism is caused by eating preformed toxin produced in food. The most frequent source is home-canned foods in which spores that survive an inadequate cooking and canning process germinate, reproduce, and produce toxin in the anaerobic environment of the canned food (1).
The findings in this report indicate that this outbreak was caused by botulism type A and implicated home-canned bamboo shoots as the common source. This is the first laboratory confirmed outbreak of botulism in Thailand.
Inadequate cooking of the bamboo shoots, the anaerobic condition in the can, and lack of an acidifier allowed C. botulinum spores to germinate and produce toxin in this food. Because C. botulinum spores are ubiquitous and commonly present in soil, these bamboo shoots probably contained spores (2). Boiling the shoots for an hour was not enough to kill the spores because they are highly resistant to heat. To safely prepare foods intended for canning or long-term storage, the U.S. Department of Agriculture recommends that all low-acid foods (i.e., foods with pH greater than 4.6, including red meat, seafood, poultry, milk, and fresh vegetables) be sterilized at temperatures of 240 F (116 C) to 250 F (121 C) in pressure canners operated at 0.68 to 0.97 atm (10-15 lb/in2). At these temperatures, the time needed to destroy bacteria in low-acid canned food ranges from 20 to 100 minutes (3). Spores that survive the cooking process generally will not grow in an acidic environment (pH less than 4.6) (2); however, the pH of the bamboo shoots was not low enough to prevent growth and toxin production. The toxin is heat-labile and can be destroyed by heating to 176 F (80 C) for 30 minutes, or 212 F (100 C) for 10 minutes (2).
Sale of home-canned food is a means of supplementing income in Thailand. The Department of Agriculture in Thailand requires that all canned low-acid foods be sterilized at temperatures of 250 F (121 C), and the Food and Drug Administration in Thailand requires that the canning process be approved and the cans be labeled. The label should include a date indicating when the food should be discarded and the place of manufacture. Lack of compliance with these recommendations and rules may have contributed to the outbreak.
A diagnosis of botulism can be confirmed by detecting toxin in serum or stool samples from patients or in implicated foods or by culturing the organism from patients' stools. Toxin detection using the mouse bioassay is performed only in selected laboratories and was not available in Thailand. The capacity to perform toxin detection in Thailand is being developed in collaboration with the Thai government and CDC.
The standard treatment for severe botulism is supportive therapy with mechanical ventilation. Trivalent botulinum antitoxin can reduce mortality if administered early; however, for the outbreak in Thailand, supplies were not available locally (1). The high case-fatality rate in this outbreak suggests that antitoxin should be made available in Thailand. In the United States, CDC releases antitoxin through an emergency distribution system. CDC has an agreement with the Pan American Health Organization to supply botulism antitoxin to other countries in the Western Hemisphere (3,4). A regional coordinated botulism antitoxin release system could facilitate availability of antitoxin in Thailand and other neighboring countries.
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