Botulism is caused by a toxin produced in food by the bacteria, Clostridium botulinum, which is found in soil all over the world. The soil in the western United States is high in type A C. botulinum , which is the most dangerous type to humans. Alaska and Washington have higher rates of botulism than other states.

This bacteria has the ability to form spores which are very resistant to heat, chemicals and physical stress. When the spores grow, they produce the deadly neurotoxin that causes botulism.

The frequency of botulism cases is rare, but each year several outbreaks occur.

Many of the outbreaks of foodborne botulism in the United States have been caused by improperly home-canned foods. Fish, green beans, corn, beets, spinach, asparagus, and chili peppers are the most common foods implicated in botulism cases. Recently a case involved home-prepared "pickled" eggs.

Several conditions are necessary for a botulism outbreak. First of all, the botulinum organism must be in the food. Secondly, the acidity level must be low. Clostridium botulinum cannot grow when the acid level is high. When the pH is 4.6 or less, it means the conditions are not right for the growth of Clostridium botulinum . Most types of Clostridium botulinum grow best at warm temperatures; however, growth at temperatures as low as 38 ° F and as high as 118 ° F has been observed. The organisms cannot grow if air or free oxygen is available. Oxygen-free (anaerobic) conditions occur when food is canned.

Botulism can be prevented in home-canned foods if home canners properly process foods.

A pressure canner is essential for canning all low-acid foods. That's because Clostridium botulinum cells can form an extremely heat-resistant spore. The spore may survive boiling at 212 ° F. A temperature of 240 ° F, which can be achieved only with a pressure canner, is required to be certain that all spores have been destroyed. Home canners should follow recommended procedures and times to process low-acid foods and should make altitude adjustments when necessary.

For an extra guarantee of safety, home-canned vegetables, meat and fish may be heated before eating. Boiling destroys the botulism toxin. Bring foods to a rolling boil and boil 10 minutes at altitudes below 1,000 feet. Add an additional minute boiling time for each additional 1,000 feet of elevation. Heat fish to an internal temperature of 185 ° F (as measured by a meat thermometer) in a 350 ° F oven.

For low acid foods that are pickled, vinegar must be added in sufficient quantity to lower the pH to below 4.6. The vinegar in pickling recipes must be at least 5% acidity. Use only recipes known to provide sufficient acidity, such as those provided at extension offices or the USDA Complete Guide to Home Canning .

Botulism has occasionally been caused by foods that were not vacuum-sealed by canning. Smoked fish can develop anaerobic conditions under the skin and in the visceral cavity. Baked potatoes wrapped in foil and meat pies have also been the cause of botulism. A large pot of sautéed onions left in a warm place resulted in botulism cases from persons eating the onions. Garlic in oil also caused multiple cases of botulism at a Canadian restaurant. Pickled eggs left at room temperature developed botulism in the yolk, which had been pricked with a toothpick, before the acid pickling solution was able to equilibrate throughout the egg.

To prevent botulism toxin from forming in non-canned foods, low-acid foods need to be refrigerated after they are cooked. Long-term storage of smoked fish should be in the freezer. This is because type E Clostridium botulinum , which is found in fish, can produce toxin at refrigerator temperatures. Smoked fish can be safely stored in the refrigerator for up to two weeks.

Botulism symptoms include difficulty in swallowing, speech, and respiration and double vision. Respiratory failure may cause death. Before 1950, fatality rates from botulism were about 50%, but with availability of antitoxin and modern respiratory support systems, the death rate has decreased to about 10%.

Additional information on botulism can be found in The Bad Bug Book , http://www.fda.gov/food/foodsafety/foodborneillness/foodborneillnessfoodbornepathogensnaturaltoxins/badbugbook/default.htm.