Clostridium botulinum is a spore-forming, gram-positive anaerobic bacillus found in the soil and in sediments of streams, lakes, and coastal waters throughout the world. Botulinum toxin is one of the clostridial neurotoxins, and its extreme neurotoxicity— it is one of the most poisonous substances known— makes it one of the first agents to be considered as a biological weapon.1 A very small amount of the toxin could cause widespread mortality: the LD50—the dose that is lethal to 50% of the exposed population—is 1 ng/kg.1
The neurotoxin produced by C botulinum has been encountered in the canning or preservation of foods, although this occurs less commonly now with modern canning technology.2 E. van Ermengem first described the origin of C botulinum in 1897 while investigating a foodborne outbreak in Ellezelles, Belgium.3 The word botulism comes from the Latin botulus, which means sausage. Home-fermented sausages caused many of the early cases of botulism.3 An average of 110 cases of botulism are reported annually in the United States, with approximately 70% related to infant botulism and about 25% related to foodborne botulism.4 In fact, a recent CDC Health Advisory e-mailed to clinicians in early September reported an outbreak of botulism in Georgia caused by botulinum toxin type A in bottled carrot juice. Three people were affected.
The potential terrorist use of botulinum toxin has led the CDC to classify it as a Category A biological agent, owing to its ability to cause public panic and its potential impact on public health. These categories are designated by the CDC for biological agents to stratify their risks to public health.5
Botulinum toxin: The weapon
World War II The initial efforts to weaponize botulinum toxin began during the early part of World War II, when intelligence reports indicated that Germany was attempting to develop the toxin for use against invasion forces. Great Britain was known to have begun developing an extensive biological warfare program during the 1940s. In fact, it was reported that British forces might have used the toxin against Reinhard Heydrich, the head of the Gestapo and Security Service in Germany.1
The United States explored the use of a weaponized botulinum toxin shortly after World War II.1 Following reports that Germany had weaponized botulinum toxin, the United States developed more than 1 million doses of botulinum toxoid vaccine, which was intended to protect the allied troops in the event of an attack. Biological weapons research and development was subsequently discontinued by President Richard M. Nixon's executive order in 1969-1970. The 1972 Biological and Toxin Weapons Convention aligned 103 countries in the discontinuation of offensive biological and chemical weapons programs.6
Iraq After the 1990-1991 Persian Gulf War, Iraq reported the production of 19,000 L of concentrated botulinum toxin, of which approximately 10,000 L were loaded into military weapons, including missiles and bombs. The location of this stockpile—and whether it still exists—are subjects of controversy today.
Japan The most recent known attempt to use botulinum toxin came in 1995 from the Japanese cult Aum Shinrikyõ, who had attempted to disperse the toxin in an aerosolized form against United States military installations. Their failure was attributed to faulty microbiological technique, deficient aerosol-generating equipment, and internal sabotage. It is reported that they had obtained their C botulinum from soil they collected in northern Japan.6
Pathogenesis of botulism
There are seven distinct serotypes of botulinum neurotoxins labeled A-G. Toxins A, B, E—and, rarely, F— have been linked to human infection.6 Pharmacologically, the toxins share similar shape, structure, and mechanism of action. The toxin enters the neuronal cell and prevents the release of the neurotransmitter acetylcholine from presynaptic terminals of the motor neurons in skeletal muscle. This important neurotransmitter is responsible for neuromuscular communication throughout the body. Without acetylcholine, muscular contractions cease, leading to the symmetric descending paralysis pathognomonic of botulism.7,8