Epidemiology and Microbiology of Botulism
C. botulinum bacteria and spores are widely distributed in nature because they are indigenous to soils and waters. They occur in both cultivated and forest soils, bottom sediment of streams, lakes, and coastal waters, in the intestinal tracts of fish and mammals, and in the gills and viscera of crabs and other shellfish. Human botulism is caused by types A, B, E, and, rarely, type F. Patients diagnosed with botulism demonstrated a predominance of toxin type A west of the Rocky Mountains and type B east of the Rocky Mountains, while type E is more prevalent in the Pacific Northwest, Alaska, and the Great Lakes area. Type E also occurs in northern latitudes and in Japan. National botulism surveillance information is available from the Centers for Disease Control and Prevention (CDC).
Unlike Clostridium perfringens, which requires the ingestion of large numbers of viable cells to cause symptoms, the symptoms of botulism are caused by the ingestion of highly toxic, soluble exotoxins produced by C. botulinum growing undetected in contaminated foods. These rod-shaped bacteria grow best under anaerobic (or low oxygen), low-salt, and low-acid conditions. Bacterial growth is inhibited by refrigeration below 4°C, heating above 121°C, and high-water activity or acidity. Although the toxin is destroyed by heating to 85°C for at least five minutes, the spores formed by the bacteria are not inactivated unless the food is heated under high pressure to 121°C for at least twenty minutes.
Modern canned goods are heated under steam pressure at temperatures of 240-250°F (116-121°C), which are sufficient to destroy microorganisms. Processing conditions are chosen and designed to be the minimum needed to ensure that the foods are made “commercially sterile,” while still retaining the greatest flavor and nutrition. All canning processes must first be approved by the U.S. Food and Drug Administration (FDA). Once the cans are sealed and heat-processed, the resulting canned food must maintain its high eating quality for more than two years and be safe to eat as long as the can is not damaged in any way. Historically, commercially canned food has a near-perfect track record, having caused only four outbreaks in over forty years. The last outbreak occurred in 1974 and involved beef stew.