C. botulinum, which causes the disease botulism, is worldwide in distribution; it is found in soil and occasionally in animal feces.

Types of C. botulinum are distinguished by the antigenic type of toxin they produce. Spores of the organism are highly resistant to heat, withstanding 100 °C for several hours. Heat resistance is diminished at acid pH or high salt concentration.

Toxins
During the growth of C. botulinum and during autolysis of the bacteria, toxin is liberated into the environment. Seven antigenic varieties of toxin (serotypes A–G) are known. Types A, B, E, and F are the principal causes of human illness. Types A and B have been associated with a variety of foods and type E predominantly with fish products. Type C produces limberneck in birds; type D causes botulism in mammals. Type G is not associated with disease. Botulinum toxins have three domains. Two of the domains facilitate binding to and entry of toxin into the nerve cell. The third domain is the toxin which is a 150 kDa protein that is cleaved into a heavy chain (H, 100 kDa) and a light chain (L, 50 kDa) that are linked by a disulfide bond. Botulinum toxin is absorbed from the gut, enters the blood circulation, and binds to receptors of presynaptic membranes of motor neurons of the peripheral nervous system and cranial nerves. The toxin does not cross the blood brain barrier or affect the central nervous system. Proteolysis—by the L chain of botulinum toxin—of the target SNARE proteins (soluble-N-ethyl maleimide-sensitive factor attachment protein) in the neurons inhibits the release of acetylcholine at the synapse, resulting in lack of muscle contraction and paralysis. The SNARE proteins are synaptobrevin (also known as vesicle-associated membrane protein or VAMP), SNAP 25, and syntaxin. The toxins of C. botulinum types A, C, and E cleave the 25,000 kDa SNAP 25. Type C also cleaves syntaxin. Types B, D, F, and G toxins cleave only synaptobrevin. C. botulinum toxins are among the most toxic substances known: The lethal dose for a human is probably about 1–2 µg/kg. The toxins are destroyed by heating for 20 minutes at 100 °C. Strains that produce toxins A, B, or F are associated with infant botulism. Additional details on toxin production and function are described in the review by Rossetto et al.

Pathogenesis
Resurgence of wound botulism caused by types A or B toxin has occurred recently in the United States, in the United Kingdom, and in Germany in association with skin-popping using contaminated “black tar” heroin. However, most cases of botulism represent an intoxication resulting from the ingestion of food in which C. botulinum has grown and produced toxin. The most common offenders are spiced, smoked, vacuum packed, or canned alkaline foods that are eaten without cooking. In such foods, spores of C. botulinum germinate; that is, under anaerobic conditions, vegetative forms grow and produce toxin.

In infant botulism, honey is the most frequent vehicle of infection. The pathogenesis differs from the way that adults acquire infection. The infant ingests the spores of C. botulinum, and the spores germinate within the intestinal tract. The vegetative cells produce toxin as they multiply; the neurotoxin then gets absorbed into the bloodstream. In rare instances, adults with gastrointestinal anatomical abnormalities or functional disorders may develop “infant botulism.”

Wound botulism is the result of tissue contamination with spores and is seen primarily in injection drug users. Very rarely, inhalational botulism occurs when toxin enters the respiratory tract.

The toxin acts by blocking release of acetylcholine at synapses and neuromuscular junctions (see earlier discussion). The result is flaccid paralysis. The electromyogram and edrophonium strength test results are typical.

Clinical Findings

Symptoms begin 18–24 hours after ingestion of the toxic food, with visual disturbances (incoordination of eye muscles, double vision), inability to swallow, and speech difficulty; signs of bulbar paralysis are progressive, and death occurs from respiratory paralysis or cardiac arrest. Gastrointestinal symptoms are not prominent. There is no fever. The patient remains fully conscious until shortly before death. The mortality rate is high. Patients who recover do not develop antitoxin in the blood.

In the United States, infant botulism is as common as or more common than the classic form of paralytic botulism associated with the ingestion of toxin-contaminated food. The infants in the first months of life develop poor feeding, weakness, and signs of paralysis (floppy baby). Infant botulism may be one of the causes of sudden infant death syndrome. C. botulinum and botulinum toxin are found in feces but not in serum.

Diagnostic Laboratory Tests

Clinicians who suspect a case of botulism should contact the appropriate public health authorities before submitting specimens to the laboratory. Detection of toxin and not the organism is required for definitive diagnosis. Toxin can often be demonstrated in serum, gastric secretions, or stool from the patient, and toxin may be found in leftover food. Clinical swabs or other specimens obtained from patients should be transported using anaerobe containers. Suspect foods should be left in their original containers. Mice injected intraperitoneally with such specimens from these patients die rapidly. The antigenic type of toxin is identified by neutralization with specific antitoxin in mice. This mouse bioassay is the test of choice for the confirmation of botulism. C. botulinum may be grown from food remains and tested for toxin production, but this is rarely done and is of questionable significance.

In infant botulism, C. botulinum and toxin can be demonstrated in bowel contents but not in serum. Other methods used to detect toxin include ELISAs and PCR, but the latter may detect organisms that carry the gene but do not express toxin.

Treatment

Supportive care, especially intensive care, is key in the management of patients with botulism. Adequate respiration must be maintained by mechanical ventilation if necessary and in severe cases may need to be maintained for up to 8 weeks. These measures have reduced the mortality rate from 65% to below 25%. Potent antitoxins to three types of botulinum toxins have been prepared in horses. Because the type responsible for an individual case is usually not known, trivalent (A, B, E) antitoxin must be promptly administered intravenously with customary precautions. Antitoxin does not reverse the paralysis, but if administered early, it can prevent its advancement. Although most infants with botulism recover with supportive care alone, treatment with human derived botulinum immune globulin (BIG) is recommended.

Epidemiology, Prevention, and Control

Because spores of C. botulinum are widely distributed in soil, they often contaminate vegetables, fruits, and other materials. A large restaurant-based outbreak was associated with sautéed onions. When such foods are canned or otherwise preserved, they either must be sufficiently heated to ensure destruction of spores or must be boiled for 20 minutes before consumption. Strict regulation of commercial canning has largely overcome the danger of widespread outbreaks, but commercially prepared foods have caused deaths. A chief risk factor for botulism lies in home-canned foods, particularly string beans, corn, peppers, olives, peas, and smoked fish or vacuum-packed fresh fish in plastic bags. Toxic foods may be spoiled and rancid, and cans may “swell,” or the appearance may be innocuous. The risk from home-canned foods can be reduced if the food is boiled for more than 20 minutes before consumption.

Botulinum toxin is considered to be a major potential agent for bioterrorism and biologic warfare.

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