Bacteriology at UW-Madison
Foodborne botulism in the United States 1990-2000. CDC
Botulinum toxin may be transported within nerves in a manner analogous to tetanospasmin (tetanus toxin), and can thereby gain access to the CNS. However, symptomatic CNS involvement is rare.
Botulinum toxin is synthesized as a single polypeptide chain with a molecular weight around 150 kDa. In this form the toxin has a relatively low potency. The toxin is nicked by a bacterial protease (or possibly by gastric proteases) to produce two chains: a light chain (the A fragment) with a molecular weight of 50 kDa; and a heavy chain (the B fragment), with a mw of 100kDa. As with tetanospasmin, the chains remain connected by a disulfide bond. The A fragment of the nicked toxin, on a molecular weight basis, becomes the most potent toxin found in nature.
Structure of the botulinum toxin
The heavy chain of the toxin mediates binding to presynaptic receptors. The nature of these receptors is uncertain; different toxin types seem to utilize slightly different receptors. The binding region of the toxin molecule is located near the carboxy terminus of the heavy chain. The amino terminus of the heavy chain is thought to form a channel through the membrane of the neuron allowing the light chain to enter. The toxin (A fragment) enters the cell by receptor mediated endocytosis. Once inside a neuron, the toxin types probably differ in mechanisms by which they inhibit acetylcholine release, but a mechanism similar to or identical to tetanospasmin has been reported (i.e., proteolytic cleavage of synaptobrevin II). The affected cells fail to release a neurotransmitter, thus producing paralysis of the motor system. Once damaged, the synapse is rendered permanently useless. The recovery of function requires sprouting of a new presynaptic axon and the subsequent formation of a new synapse.
As stated above, the mechanism by which acetylcholine release is prevented is not known. However, recent evidence suggests that both botulinum toxin as well as tetanus toxin are zinc-dependent endopeptidases that cleave specific proteins that are involved in excretion of neurotransmitters. Both toxins cleave a set of proteins called synaptobrevins. Synaptobrevins are a set of proteins found in synaptic vesicle of neurons, the vesicles responsible for release of neurotransmitters. Presumably, proteolytic cleavage of synaptobrevin II would interfere with vesicle function and release of neurotransmitters.
The toxins that cause botulism are each specifically neutralized by its antitoxin. Botulinum toxins can be toxoided and make good antigens for inducing protective antibody. As with tetanus, immunity to botulism does not develop, even with severe disease, because the amount of toxin necessary to induce an immune response is toxic. Repeated occurrence of botulism has been reported.
Once the botulinum toxin has bound to nerve endings, its activity is unaffected by antitoxin. Any circulating ("unfixed") toxin can be neutralized by intravenous injection of antitoxin. Individuals known to have ingested food with botulism should be treated immediately with antiserum.
A multivalent toxoid evokes good protective antibody response but its use is unjustified due to the infrequency of the disease. An experimental vaccine exists for laboratory workers.
To the Editor:
A historical incident illustrates a number of features of botulinum toxin not discussed in the review of bioweaponry by Dr Arnon and colleagues.
During World War II, the US Office of Strategic Services (OSS) developed a plan for Chinese prostitutes to assassinate high-ranking Japanese officers with whom they sometimes consorted in occupied Chinese cities. Concealing traditional weapons on the women at the appropriate time would obviously be difficult. Therefore, under the direction of Stanley Lovell, the OSS prepared gelatin capsules "less than the size of the head of a common pin" containing a lethal dose of botulinum toxin. Wetted, a capsule could be stuck behind the ear or in scalp hair, later to be detached and slipped into the officer's food or drink. The OSS recognized that the normal background of botulism cases would deflect suspicion from the women.
The capsules were shipped to Chunking, China. The Navy detachment there, taking nothing for granted, tested the capsules on stray donkeys. The donkeys lived. Lovell was informed that the capsules were faulty, and the project was abandoned. Much later, Lovell learned of the donkey test with, one imagines, some consternation, since "donkeys are one of the few living creatures immune to botulism."
This incident has been retold in other publications. No source for the donkey-resistance information is ever given. More recent experience shows that botulism can occur in mules and donkeys (R. H. Whitlock, DVM, PhD, written communication, April 27, 2001).
Nevertheless, this incident
raises 2 points: (1) botulism need not occur in epidemics when it is
as a bioweapon, and (2) botulism in animals may be a sign of biowarfare.
CDC - Botulism: Disease Information
CDC - Disease Listing, Botulism, General Information
CDC - Facts About Botulism
CDC - Technical Information About Botulism
CDC - Video: "The History of Bioterrorism: Botulism"
CDC - What treatment is available for botulism?
IL Public health - Botulism Fact Sheet
JAMA - Botulinum Toxin as a Biological Weapon: Medical and Public Health Management
Infectious Diseases Society of America: Botulism Resource List