Introduction

Home canning offers a great way of preserving excess produce, as well as preserving family recipes and cultural traditions. Many find home canning to be a rewarding and enjoyable hobby. However, improper canning poses a serious health risk, botulism, caused by neurotoxins released by bacteria that can grow within canned food. If you are interested in trying home canning, it is important to understand the risks and learn how to use proper techniques, to keep you and your family safe.

Understanding botulism

Botulism, a rare but potentially fatal illness, is caused by a toxin produced by a type of bacteria called Clostridium botulinum.¹ Botulinum toxin is one of the most dangerous neurotoxins in existence, it works by blocking the release of acetylcholine, a neurotransmitter involved in muscle movement, which ultimately leads to muscle paralysis.²

A person with botulism can experience a range of symptoms, including:

• Nausea, vomiting, stomach cramps and diarrhoea
• Fatigue and muscle weakness
• Blurred or double vision
• Vertigo (a sensation of motion or spinning)
• Dry mouth and difficulty swallowing (dysphagia)
• Drooping eyelids (ptosis), facial muscle weakness and facial paralysis
• Slurred speech
• Difficulty breathing

Without treatment, these symptoms can increase in severity. Muscle weakness and paralysis spread from the head down through the body (descending flaccid paralysis), eventually resulting in death.

Human botulism can occur in multiple different ways,³ and these all present with unique symptoms:

• Foodborne botulism⁴ occurs when a person ingests food that is contaminated by the toxin, most commonly improperly canned foods. This type of botulism is characterised by descending flaccid paralysis. It often results in both gastrointestinal symptoms (e.g., nausea, vomiting, diarrhoea) and neurological symptoms (e.g., blurred vision, ptosis, slurred speech, muscle weakness and paralysis), which generally begin 12-36 hours after ingestion of the contaminated food
• Wound botulism⁵ can happen if a wound becomes infected by C. botulinum bacteria spores, which then release the toxin within the body This is rare, but most commonly seen in injection drug users. This form of botulism also results in neurological symptoms, but it can take up to 2 weeks for these symptoms to appear
• Infant intestinal botulism⁶ occurs if an infant consumes C. botulinum bacteria spores by eating contaminated food, such as honey, or by putting contaminated soil in their mouth. These spores then colonise the child’s intestines and begin to produce the toxin. The spores are harmless to adults and older children, however, young children (usually under the age of 1) are vulnerable to infection, as their immune systems have not fully developed. Parents often notice their child exhibiting gastrointestinal symptoms (such as vomiting, diarrhoea and constipation) and other symptoms (such as a weak cry, ptosis, excessive drooling and shallow breathing)
• Intestinal colonisation botulism/adult sporadic botulism⁷ is similar to infant botulism but occurs in adults and children over the age of 1. Like infant botulism, it involves the ingestion of spores that germinate and produce the toxin within the intestines. While this is very rare, certain conditions, such as inflammatory bowel disease (IBD), or previous gastric surgery can predispose an individual to developing intestinal botulism. As with the other forms of botulism, this generally results in a mixture of gastrointestinal and neurological symptoms

Unfortunately, botulism symptoms can easily be confused with the symptoms of more common conditions, such as stroke, myasthenia gravis or Guillain–Barré syndrome. This can delay patients from receiving the correct treatment.

Treating botulism

Botulism is classed as a medical emergency, requiring immediate treatment. Key treatment options include:⁸

• Administration of the botulinum antitoxin. This antitoxin contains antibodies, proteins that protect the body from unwanted substances, which bind to the botulinum toxin thereby preventing it from attacking further nerve cells⁹
• Supportive care, for example, mechanical ventilation, for patients who have respiratory failure, and intravenous nutrition, for patients who are having difficulty swallowing
• Rehabilitation, such as long-term physiotherapy, to help patients regain their muscle strength and function

The introduction of modern treatment options, particularly botulinum antitoxin, has helped to reduce mortality (death) rates from roughly 60% to 10%.¹⁰

Botulism and home-canned foods

C. botulinum bacteria is anaerobic, meaning it grows in the absence of oxygen, and it thrives best in low-acid environments.¹¹ Improper canning of food, particularly low-acid foods like meat, fish and vegetables, can create the ideal environment for the growth of this bacteria.

If foods are not heated to sufficiently high temperatures during the canning process, the bacterial spores are not destroyed. Then, the sealed, oxygen-free environment inside the container provides the perfect opportunity for the bacteria to multiply and produce their potent neurotoxin. Therefore, extreme caution must be taken when home-canning, especially with foods with a pH higher than 4.6.

If you decide to try home-canning, there are some tips and practices you should know and follow:

• A pressure canner, which should reach 116-121°C in temperature, is highly recommended
• Be aware that, while low-acid foods are considered the highest risk, even acidic foods, such as tomatoes, can pose a risk
• Use fresh, high-quality produce without signs of spoilage, and thoroughly wash and peel fruits and vegetables before canning
• Sterilise your containers and maintain a clean working area to reduce chances of contamination
• Label and date your containers, and store them in a cool (10°C - 21°C), dark place out of direct sunlight
• Look out for signs of contamination, such as cracked, leaking or bulging containers, spurting of liquid or foam upon opening, or any signs of food discolouration, mould or a bad smell. If in doubt, throw it out
• Boil home-canned foods before eating them. The length of time you should boil them varies depending on your altitude. At less than 1,000 feet, boil for 10 minutes, for each additional 1,000 feet of elevation, add an extra 1 minute to the boiling time

FAQ’s

Can botulism be fatal?

Yes, botulism can be fatal, especially if left untreated.

How do I know if I have botulism?

Identify the symptoms. Botulism can cause gastrointestinal (e.g., nausea, vomiting and diarrhoea) and neurological (e.g., drooping eyelids, vertigo, vision disturbances, muscle weakness and paralysis) symptoms. A key symptom of foodborne botulism is descending flaccid paralysis, muscle weakness and paralysis that begins at the head and spreads down the body. If you suspect you have botulism, you should visit the Accident & Emergency department of your closest hospital as quickly as possible.

Can you get botulism from canned tomatoes?

Yes, while low-acid foods are the most likely to be contaminated, even higher-acid foods, like tomatoes, can harbour the bacteria.

Is botulism always fatal?

No, people do survive botulism, especially when treated with antitoxin and supportive care (e.g., mechanical ventilation). 

Summary

Home canning can be an excellent way to store produce, but it is crucial to follow safe canning practices to prevent the growth of botulinum bacteria. This bacterium thrives in low-oxygen and low-acid environments. Improperly canned foods can harbour these bacteria, which produce a dangerous and potentially deadly neurotoxin. Consuming this toxin leads to botulism, a disease that causes gastrointestinal and neurological symptoms, including paralysis. Without treatment, botulism can be fatal.