Botulism is a rare but serious illness caused by a toxin produced by the bacterium Clostridium botulinum. This toxin can cause paralysis and even be fatal if not treated promptly. While the botulinum toxin is extremely potent and dangerous, botulism itself is relatively rare. This is because the bacteria require specific conditions to grow and produce the toxin. The most common sources of foodborne botulism are improperly handled home-canned foods, particularly low-acid foods like green beans, beets, and corn. However, many other foods have also been linked to botulism outbreaks.
What causes botulism?
Botulism is caused by botulinum toxin, produced by the anaerobic bacterium Clostridium botulinum when it grows. This bacterium is very common in the environment, found in soil and water all over the world. However, it only produces the toxin when the conditions are right – it requires an anaerobic (no oxygen), low-acid environment with temperatures above refrigeration. The spores of C. botulinum are heat resistant and can survive boiling, which allows them to persist in home canned goods if the contents are not processed correctly. Once inside the low-acid, oxygen-free environment of a sealed jar or can, the spores can germinate and start producing the deadly botulinum toxin as they multiply.
Types of botulism
There are three main forms of botulism:
- Foodborne botulism – caused by consuming foods containing the botulinum toxin
- Wound botulism – caused when the bacterium colonizes a wound and produces toxin within it
- Infant botulism – caused when infants ingest C. botulinum spores, which then germinate and multiply in the gut
Foodborne botulism is by far the most common of these forms. Infant botulism can occur when babies eat honey containing C. botulinum spores. While adults have mature gut microflora that inhibit C. botulinum growth, infants do not.
Most common food sources
The vast majority of foodborne botulism cases are associated with home-canned foods that have been improperly processed. The most common sources include:
Home-canned vegetables
Low-acid foods like green beans, carrots, beets, and corn are frequent sources of botulism when home-canned. If these vegetables are not processed at a high enough temperature and pH to destroy C. botulinum spores, they can allow the bacteria to thrive once sealed in a jar. This is the source of many botulism outbreaks traced back to community potlucks, church gatherings, and family meals.
Home-canned meat and seafood
Meats and seafood are also low-acid and require meticulous canning methods to avoid botulism. C. botulinum growth can occur in things like canned tuna, salmon, chicken, beef, and other meats if strict hygiene is not followed.
Garlic and herbs in oil
Homemade herb and garlic infused oils are risky due to the anaerobic environment under the oil. Botulism outbreaks have occurred from consumption of garlic-in-oil mixtures as well as other herb oils. While commercially prepared oils contain preservatives and are processed safely, homemade infused oils can allow growth of the toxigenic bacterium.
Other sources
While home canning is the most frequent cause, many other contaminated foods have caused botulism. Other sources include:
- Smoked fish or meat – such as smoked salmon, whitefish, or ham
- Honey – a risk for infants due to potential spore contamination
- Sausages and meat products – if made under insanitary conditions
- Improperly handled baked potatoes – wrapped in aluminum foil while still hot
- Tomatoes – if the pH is raised by mold growth
- Corn syrup – if stored improperly
- Fermented fish
- Sealed rice dishes
- Anything with conditions that allow C. botulinum growth and toxin production
Key factors enabling growth
For C. botulinum to grow and produce toxin, a few key conditions must be present:
- Anaerobic environment – botulism is rare in acid foods due to oxygen permeation
- Low acid – optimum pH is 4.6-7.0, cannot grow below pH 4.6
- Temperatures above refrigeration – minimum growth temperature is around 3°C
- Time – takes 12-18 hours at optimum growth temperature to produce toxin
- Moisture – minimum aw of 0.93
- Lack of competitors – C. botulinum is sensitive to lactic acid bacteria and other competitors
Home canning often provides the perfect storm of conditions. When low-acid foods are canned without pressure cooking, spores can survive and populate the anaerobic, sealed jars. Time and warm storage temperature allows rapid growth and toxin production. Commercial processing avoids this by using pressure and time to destroy spores, acid to prevent growth, and preservatives.
Most frequent symptoms
Botulism causes flaccid paralysis by blocking acetylcholine signaling at neuromuscular junctions. Initial symptoms typically include:
- Double vision
- Blurred vision
- Drooping eyelids
- Slurred speech
- Difficulty swallowing
- Dry mouth
- Muscle weakness
Without treatment, symptoms may progress to paralysis of arms, legs, trunk, respiratory muscles, resulting in death. Infant botulism can cause poor feeding, constipation, general weakness, and floppiness in babies.
Diagnosis and treatment
Botulism is diagnosed based on symptoms and tests to detect botulinum toxin in blood, stool, or food samples. Treatment involves antitoxin injection to neutralize circulating toxin, as well as supportive respiratory care. Antibiotics are not used, as killing the bacteria just releases more toxin. With proper intensive care, the paralysis slowly improves over weeks as the neuromuscular connections regenerate.
Prevention tips
Since most foodborne botulism comes from home canning, some key prevention tips include:
- Use pressure canner for low-acid foods like meat and vegetables
- Follow validated canning instructions precisely
- Check seals on jars – discard if loose or bulging
- Do not consume damaged or spoiled canned goods
- Keep oils infused with garlic/herbs refrigerated
- Avoid giving honey to infants under 1 year old
- Use care when preparing and wrapping baked potatoes
- Refrigerate leftover foods promptly
Conclusion
In summary, while botulism is quite rare, it can be prevented by being careful when canning and preparing foods in the home. The most frequent sources are improperly handled home-canned vegetables, meats, and fish. Following proper canning technique, avoiding letting foods sit out, refrigerating promptly, and being cautious with homemade infused oils can help keep this dangerous toxin out of foods.