Description

Clostridium botulinum are anaerobic, Gram-positive spore-forming rods, with the spores being very heat resistant. The trend towards vacuum packaging of chilled foods provides for a potential risk if temperature abuse is allowed to occur. They can be isolated from the soil and marine environment. Some strain (non-proteolytic) can grow slowly at temperatures down to 3.3°C. They usually will not produce toxins at pH values less than 4.6 and water activity values of less than 0.94. The toxin is one of the most potent toxins known and 10-6g is sufficient to kill an adult human. The toxin is easily destroyed by boiling for 10 minutes. Nitrate and nitrite are preservative which are effective in inhibiting the growth of C. botulinum.

Nature of acute disease

Foodborne disease caused by C. botulinum is referred to as botulism. It is caused by the ingestion of a neurotoxin produced by the microorganism in the food. The toxin can be destroyed by normal cooking procedures. Infant botulism can also occur which is thought to occur from the ingestion of C. botulinum spores with honey being the major food implicated. In the latter illness spores in the gastrointestinal tract will germinate and multiply, producing the toxin. There is now an antidote to the toxin which has reduced the mortality rate somewhat.

Characteristics of Illness

Symptoms: Include weakness, fatigue and dizziness, followed by blurred vision and progressive difficulty in speaking and swallowing. Weakening of the respiratory muscles is also observed and death may occur due to respiratory failure.

Onset of symptoms: Symptoms usually occur 18 to 36 hours after ingestion of the food containing the toxin, although cases have varied from a few hours to several days.

Duration of illness: If not treated botulism can cause death. Once treated recovery can take weeks to years depending upon the severity of the poisoning.

Infective dose: A very small amount (a few nanograms) of toxin can cause illness.

Diagnosis of human illness

Botulism is difficult to diagnose by clinical symptoms alone as it is often confused with other illnesses such as Guillain-Barré syndrome. The most direct and effective way is to demonstrate the presence of toxin in the serum or faeces of the patient or in the food that the patient consumed. Currently, the most and widely used method for detecting toxin is the mouse bioassay. This usually takes 3 days after isolation of the toxin.

Complications

Fatality rates for botulism are around 5-15%. Treatment requires the administering of antisera and the use of respiratory support systems.

Susceptible individuals

All people are believed to be susceptible to botulism.

Associated foods

It is generally thought that any low acid foods (pH above 4.6) can support the growth of C. botulinum and its subsequent production of toxin. Despite this an outbreak of botulism occurred in canned tomatoes, which had a low pH. It was thought that the production of toxin occurred due to the growth of mould increasing the pH of the product. Some foods that have been associated with cases of botulism include home preserves: meat, fish, rockmelon (cantaloupe), vegetables; soft cheeses, potato salad and garlic stored in oil. Infant botulism has mainly been associated with the consumption of honey.

Food analysis

Cultural methods can be used to detect the presence of the microorganism in foods. These tests can take up to 10 days. Mouse bioassay can then be used to test for the presence of the toxin. See Hocking et al (1997) for more details.

Outbreaks

The incidence of the disease is low, but the mortality rate is high if not treated immediately. There have been no reported cases of botulism in Australia since 1991 when a couple became ill from consuming home-preserved unacidified asparagus. In Australia between 1942 and 1984 there were 5 outbreaks of botulism, affecting 53 people with 9 fatalities. Foods implicated include canned vegetables (mushrooms and asparagus) and canned tuna.

Overseas outbreaks include:

Japan – 36 people became ill and 11 people died after eating fried stuffed lotus roots. The lotus roots were vacuum packaged and sold unrefrigerated. These conditions allowed for the growth and toxin production of C. botulinum.

Canada – At least 37 people were affected after eating at a restaurant that used an unacidified temperature abuse garlic in oil which had been stored at room temperature.

USA – Cases of botulism have occurred due to the consumption of potato salad. Three outbreaks have occurred, resulting in 50 people ill and one fatality. It is thought that the C. botulinum was present in the baked potatoes used for the salad. These had been stored at ambient temperatures for several days before use.

USA – sauteed onions were implicated in an outbreak in a restaurant in America. 28 people were affected, with one fatality.

UK – contaminated hazelnut yogurt was responsible for an outbreak resulting in 27 people ill and one fatality. The source of the C. botulinum was the hazelnut puree used in the yogurt.

Prevention

When processing shelf-stable products ensure that the heating processing is designed to destroy spores of C. botulinum. Any process used should be designed by a person with experience in the design of safe heat processes. Alternatively, the pH of the product can be adjusted to below 4.5 and a pasteurisation process is then sufficient. When making home preserves bottle only acidic fruit such as apples, pears, stone fruit, pineapple, berries etc should be used. Melons, tomatoes and many tropical fruits must have some acid added to them before they are bottled. Vegetables must be bottled in at least half vinegar and half water, and when making flavoured oils, use only dried herbs and vegetables or soak fresh herbs and vegetables in vinegar before adding to the oil. In manufactured meats, ensure correct levels of nitrites are used to inhibit the growth of C. botulinum. Do not stored perishable foods within the Temperature Danger Zone (5°C to 60°C). Always evenly reheat food to above 75°C. When reheating a in microwave, periodically stir the food to ensure even heating.

Further Information

Websites:

Communicable Diseases - Australia

FDA Bad Bug Book

Literature:

Hocking, A.D. et al. (1997). Foodborne Microorganisms of Public Health Significance. 5th ed. North Sydney. AIFST NSW Branch Food Microbiology Group.

Doyle, M.P. (1989). Foodborne Bacterial Pathogens. Marcel Dekker; New York.