THE WAIKATO CASE OF CLOSTRIDIUM BOTULINUM
The public may have many questions and concerns following the serious case of food poisoning, apparently from Clostridium botulinum, that has paralysed three people in the Waikato since mid-November. It is suspected, but not yet confirmed, that the source was the wild pork they ate just minutes before they became violently ill and collapsed. Such rapid onset is unusual and indicates that they either ingested a very large dose of the toxin or that it could have come from another source. Normally people become ill between 12 and 36 hours after botulinum poisoning.
The toxin some of these bacteria produce is the most powerful neurotoxin known. It blocks nerve communication, hence the patients’ unresponsiveness. Pigs are commonly resistant to the toxin when ingested. Avian die off events, such as those seen around ponds in warm weather and when the oxygen content is low, often include wild water birds, including duck species, but normally this is another variant of botulinum which is not considered a food safety threat to humans. However, precautions should be taken to protect human and pet health - don’t let your pets eat dead fish or ducks, don’t handle dead birds with your bare hands and don’t harvest dead or sick birds.
Cases of human food poisoning from botulinum are fortunately very rare and are usually associated with improperly processed canned meat, fish and vegetables which provide ideal conditions for the bacteria to grow.
“We should not make assumptions in this case without more information,” says NZFSSRC Associate Director and microbiologist researcher, Dr Gale Brightwell, from AgResearch Ltd. “And the key witnesses are as yet unable to tell us exactly what was eaten and how the food was procured, stored and prepared, and what else they may have eaten prior to that.” Samples have gone to a lab in Queensland for testing. In the meantime the three victims are receiving anti-toxin. The recovery period could be very long and recovery may not be complete.
C. botulinum is a spore-forming bacteria. The spores are everywhere in the environment, especially in soils. The hardy spores only germinate in certain conditions – they bide their time until the environment is the right temperature, with low levels or an absence of oxygen, which is likewise poisonous to anaerobic bacteria. These bacteria date back to a time on Earth before the atmosphere contained oxygen. The toxin they produce can survive high cooking temperatures. Not all these bacteria produce the toxin that causes this profound paralysis in humans.
Dr Brightwell’s research interests cover many aspects of food safety and microbiology. Her main area of research is associated with the safety of our meat production processes and microbial quality of meat products to make sure any risks from micro-organisms are minimised or eliminated. She is also Science Team Leader, Food Quality and Assurance, at AgResearch Ltd and Science Theme Leader – Mitigation for the NZFSSRC.
On behalf of scientists and staff at NZFSSRC, Dr Brightwell sends best wishes for a full recovery to the family of Subi Baba, Shibu Kochummen and Shibu's mother, Mrs Alekutty Daniel.
For more information about C. Botulinum, its incidence, effects, and methods of detection:
MPI risk assessment:
Microbial Pathogen Data Sheet: Clostridium Botulinum - MPI
C. botulinum is widely distributed in soils and in sediments of oceans and lakes. The finding of type E in aquatic environments by many investigators correlates with cases of type E botulism that were traced to contaminated fish or other seafoods. Types A and B are most commonly encountered in foods associated with soil contamination. In the United States, home-canned vegetables are most commonly contaminated with types A and B, but in Europe, meat products have also been important vehicles of foodborne illness caused by these types.
Measures to prevent botulism include reduction of the microbial contamination level, acidification, reduction of moisture level, and whenever possible, destruction of all botulinal spores in the food. Heat processing is the most common method of destruction. Properly processed canned foods will not contain viable C. botulinum. Home-canned foods are more often a source of botulism than are commercially canned foods, which probably reflects the commercial canners' great awareness and better control of the required heat treatment.
A food may contain viable C. botulinum and still not be capable of causing botulism. If the organisms do not grow, no toxin is produced. Although many foods satisfy the nutritional requirements for the growth of C. botulinum, not all of them provide the necessary anaerobic conditions. Both nutritional and anaerobic requirements are supplied by many canned foods and by various meat and fish products. Growth in otherwise suitable foods can be prevented if the product, naturally or by design, is acidic (of low pH), has low water activity, a high concentration of NaCl, an inhibitory concentration of NaNO2 or other preservative, or two or more of these conditions in combination. Refrigeration will not prevent growth and toxin formation by nonproteolytic strains unless the temperature is precisely controlled and kept below 3°C. Foods processed to prevent spoilage but not usually refrigerated are the most common vehicles of botulism.
Optimum temperature for growth and toxin production of proteolytic strains is close to 35°C; for nonproteolytic strains it is 26-28°C. Nonproteolytic types B, E, and F can produce toxin at refrigeration temperatures (3-4°C).