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Large Botulism Outbreak in Thailand

By Eric Toner, M.D., April 21, 2006

The April 14, 2006 edition of the MMWR reports an outbreak of food borne botulism in rural Thailand, with 163 people affected, 42 of whom required mechanical ventilation. The victims shared a common meal during a religious festival, and apparently consumed home-canned bamboos shoots that were contaminated with Clostridium botulinum. The bacterium was cultured, and type A botulinum toxin was identified by multiplex PCR [1].

Of 354 villagers who attended the festival, 200 reported having eaten some of the food, but it is not clear if all these people ate the bamboo shoots; 163 people developed symptoms of food poisoning, and 141 were hospitalized.

While 3 patients experienced onset of symptoms the same day as the festival, the vast majority of patients became symptomatic over the subsequent 2 days, and 5 did not have symptoms until 3 or 4 days after the festival. Initial symptoms were consistent with gastroenteritis: 116 (77%) experienced abdominal pain, 76 (50%) nausea, 53 (35%) vomiting, and ~42 (28%) diarrhea. Neurological symptoms were also reported, as follows:

Dry Mouth

76 (50 %)


52 (38 %)


26 (17 %)


16 (11 %)

Weakness of extremities

14 (9 %)

Need for mechanical ventilation

42 (30 %)

Further details on clinical course and progression of neurological symptoms were not reported. Several other details are also not clear, including whether there were patients with milder degrees of respiratory depression who did not require endotracheal intubation, or how many patients required mechanical ventilation for hypoventilation vs. for airway protection in the setting of bulbar dysfunction.

Botulinum antitoxin is the treatment of choice for botulism; however, to be most effective it must be administered early in the course of the disease, generally within 24 hours of symptom onset. Since no antitoxin was available in the country, it took approximately 4 days before the first patients could be treated. The number of patients who received antitoxin is not reported.

As of April 10, 26 days after the poisoning, 25 patients remained hospitalized, and 9 (36%) remained on respirators; no patients had died.


This outbreak is of interest to the clinical biosecurity community because botulinum neurotoxin is a Category A biological agent with the potential for use as a bioterrorist weapon. Botulinum toxin, which is the most poisonous substance known, was used unsuccessfully in a series of at least 3 bioterrorist attacks by the Aum Shinriko cult in Japan in between 1990 and 1995. In addition, botulinum toxin was developed and manufactured as an offensive military weapon by several countries [2].

While gastrointestinal symptoms are very common in cases of food borne botulism, they are probably caused by bacterial metabolites rather than the botulinum toxin itself. These byproducts would not be expected to be present if the purified toxin were used as a weapon, even if ingested. The term “botulism” refers specifically to the acute, symmetric, descending, flaccid paralysis that begins in the bulbar muscles. Subsequent progression to involve the respiratory muscles and the limbs is common. While the rapidity of progression and severity of symptoms is related to the dose of toxin absorbed, it is not unusual for the onset of neurological symptoms to take several days [2].

Because botulinum toxin binds irreversibly to the neuromuscular junction where it blocks acetylcholine release, recovery requires the growth of new connections between the motor axons and the muscles, a process that can take months. Botulinum antitoxin, if administered early, binds the toxin before it can attach itself to the motor neurons. Thus, time to treatment is essential. Antitoxin cannot reverse neurological symptoms, it can only stop their progression [2].

There are 7 known types of botulinum toxin, designated A through G. The vast majority of U.S. food borne cases are caused by types A, B, and E. In the U.S., a bivalent antitoxin containing antibodies to toxin types A and B is available, as is a separate investigational monovalent E antitoxin. In other countries, a trivalent (A, B, E) antitoxin is still produced. The U.S. government also has a limited supply of a heptavalent antitoxin (A-G) for use in a bioterrorism attack with botulinum toxin [2]. Botulism can be caused by C. botulinum colonization of necrotic wounds or the gut, but this is rare.

Botulism is rare in industrialized countries, with an average of only 24 cases per year in the U.S. Large outbreaks of botulism are even rarer—the largest outbreak in the U.S. in the last century involved just 59 cases [2]. The discovery of a single case of botulism without concomitant GI symptoms or obvious wound infection should raise concerns about the possibility of bioterrorism and should be reported to public health authorities immediately. Until proven otherwise, a cluster of such cases should be considered evidence of an attack.


  1. Botulism from Home-Canned Bamboo Shoots --- Nan Province, Thailand, March 2006. MMWR;55;389-392. Available at: Accessed April 14, 2006.

  2. Arnon S, Schechter R, Inglesby TV, et al. Botulinum toxin as a biological weapon: Medical and Public Health Management. JAMA 2001;285:1059-1070. Accessed April 20, 2006.