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Dr Catherine Conlon Botulism - where does it come from and what are the risks?

The public health expert examines botulism and its symptoms, how it develops and what harm it can cause.

FRENCH AUTHORITIES HAVE been left reeling this week following a botulism outbreak that led to the death of a woman and the serious illness of others. The outbreak has been connected to a wine bar in Bordeaux while thousands of Irish people travel there for the Rugby World Cup.

One woman who died from complications of botulism there has been reported as married to an Irish man from the west of Ireland. Her husband is believed to be in intensive care in a hospital near Paris. The HSE previously confirmed a small number of Irish people are being treated in French hospitals for botulism.

The outbreak centred on home-preserved canned sardines served at the Tchin Tchin Wine Bar in the city. The HSE advised that only people who ate sardines at the bar between Monday 4 September and Sunday 10 September and who feel unwell should seek urgent medical care at an Emergency Department.

“It is important to emphasise that this advice only applies to those who ate sardines at the premises during the time specified” advised the Director of Health Protection Surveillance Centre, Dr Greg Martin. ‘There are no other circumstances in which a person can be affected by this incident.”

What is botulism?

Clostridium botulinum is a bacterium that produces dangerous toxins (botulinum toxins) under low oxygen conditions. Botulinum toxins are one of the most lethal toxins known with the ability to block nerve functions and lead to respiratory paralysis.

Human botulism can refer to foodborne botulism, infant botulism, wound botulism, inhalation botulism or other types including iatrogenic (caused by medical activity including examination or treatment).

Foodborne botulism, the reported source of this outbreak in France and caused by improperly processed food, is a rare but potentially fatal disease if not diagnosed rapidly and treated with antitoxin. Homemade tinned, preserved or fermented foodstuffs are a recognised source of foodborne botulism. It is an intoxication usually caused by ingestion of potent neurotoxins, formed in the contaminated food.

Thankfully, person-to-person transmission of botulism does not occur. But how do these toxins get into food? Spores produced by the bacteria Clostridium botulinum are heat-resistant and exist widely in the environment including in soil, river and seawater. In the absence of oxygen, they germinate, grow and excrete toxins.

botulismfoodpoisoningfromcannedfoodsconcept-handholdsrustytin Shutterstock / Dmitriev Mikhail Shutterstock / Dmitriev Mikhail / Dmitriev Mikhail

Botulinum toxins are ingested through improperly processed food in which the bacteria or the spores survive, then grow and produce the toxins. The growth of the bacteria and the formation of toxins happens most often in lightly preserved food and in inadequately processed home-canned or home-bottled foods.

Importance of acid

C. botulinum will not grow in acidic conditions and therefore the toxin will not be formed in acidic foods. However, a low acidity will not degrade any pre-formed toxin. Combinations of low storage temperature and salt contents and /or low acidity (pH) are also used to prevent the growth of the bacteria or the formation of the toxin. The botulinum toxin has been found in a variety of foods including low-acid preserved vegetables such as green beans, spinach, mushrooms, and beets; fish, including tinned tuna, fermented, salted and smoked fish; and meat products, such as ham and sausage.

Predictably, the food implicated differs between countries, local eating habits and food preservation procedures. Occasionally, commercially prepared foods may be implicated. Though spores of C. botulinum are heat-resistant, the toxin produced by the bacteria growing out of the spores under anaerobic conditions is destroyed by boiling.

Ready-to-eat foods in low-oxygen packaging are more frequently involved in cases of foodborne botulism.

The European Centre for Disease Prevention (ECDC) reported five confirmed and four probable cases of botulism in Spain between the 21 June and the end of July this year. The outbreak was linked to the consumption of packaged Spanish potato omelettes from different brands and supermarkets in various parts of Spain. One of the suspected brands of omelettes was linked to a factory in Segovia. The factory ceased operation and recalled all suspected omelettes made before 18 July as a precaution. It is now back in production.


Foodborne botulism frequently presents with descending flaccid paralysis that can progress to respiratory failure. Early symptoms include fatigue, blurred vision, dry mouth, difficulty swallowing and speaking. Symptoms may include vomiting, diarrhoea, constipation or abdominal pain swelling.

Progressive weakness may occur in the neck and arms followed by weakness of the respiratory muscles and lower limbs. There is no fever or loss of consciousness.

Symptoms usually appear within 12-36 hours (within a range of 4 – 8 days) after exposure. Mortality is high if prompt diagnosis followed by appropriate and immediate treatment of antitoxin and intensive respiratory care is not given. The disease can be fatal in 5 – 10% of cases.

manwithstomachpainonwhitebackground Shutterstock / BonNontawat Shutterstock / BonNontawat / BonNontawat

Infant botulism occurs most commonly in infants under six months of age. Different from foodborne botulism caused by ingestion of pre-formed toxins, infant botulism occurs when C. botulinum spores are ingested and germinated into bacteria that colonise in the gut and release toxins.

In most adults and children older than about six months, this would not happen because natural defences in intestines that develop over time prevent germination and growth of the bacterium.

Although there are several possible sources of infection for infant botulism, spore-contaminated honey has been associated with a number of cases. That is why parents and caregivers are warned not to feed honey to infants until they are at least one year old.

Wound botulism can occur rarely when the spores get into an open wound. Symptoms are similar to foodborne botulism but may take longer (up to two weeks to appear). Wound botulism has been associated with intravenous drug use.

Inhalation botulism is also rare and does not occur naturally. It can be associated with accidental or intentional events (such as bioterrorism) that result in the release of toxins in aerosols. Following inhalation, symptoms become visible between 1-3 days, with longer onset times for lower levels of intoxication.


Adverse effects of the pure toxin have been reported as a result of its medical and or / cosmetic use in patients. The bacterium C. botulinum is the same bacterium that is used to produce Botox, the pharmaceutical product used clinically and also widely popular for cosmetic enhancement. Botox treatments use the purified and heavily diluted botulinum neurotoxin type A.

asceneofmedicalcosmetologytreatmentsbotoxinjection Shutterstock / Kamil Macniak Shutterstock / Kamil Macniak / Kamil Macniak

The European Centre for Disease Prevention (ECDC) states that while it is rare, individuals receiving botulinum neurotoxin (BoNT) injections for cosmetic purposes (facial wrinkle lines) or therapeutic treatments such as muscle spasticity may develop botulism if they are injected with excessive doses of BoNTs.

In February and March 2023, 87 cases of botulism linked to the intragastric injection of BoNT were reported In Germany (30), Austria (1), France (1), Switzerland (2) and Turkey (53). The cases were linked to medical interventions aimed at helping them lose weight. The cases were reported to have received BoNT injections for the treatment of obesity at two private hospitals – one in Istanbul and one in Izmir.

The symptoms ranged from mild to severe and several cases were hospitalised, with a number reported to have treatment in intensive care and treated with botulinum anti-toxin.

The ECDC strongly encourages EU/EEA citizens to avoid intragastric treatments with BoNT for obesity in Turkey as this is associated with ‘a significant risk of developing botulism.’

Dr Catherine Conlon is a public health doctor in Cork and former director of human health and nutrition, safefood.


Dr Catherine Conlon
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