Profile of a pathogen: Clostridium botulinum


The story behind one of the world’s most lethal bugs

Clostridium botulinum (C botulinum) falls into this latter category: a bacterium that produces neurotoxic proteins called botulinum toxins, which are so toxic they have been categorised by the World Health Organisation as one of “the most lethal substances known”.

Symptoms suggestive of Clostridium botulinum were first recorded in Germany in 1735. The first complete and accurate description of food-borne botulism came from district medical officer Justinus Kerner, who published a series of descriptions of possibly connected outbreaks between 1817 and 1822. The bacterium was identified in 1895, when Emile Pierre van Ermengem, a professor of bacteriology, was investigating a serious outbreak in the Belgian village of Ellezelles. He named it botulinum because of an apparent association to similar disease outbreaks linked to sausages—the Latin word for sausage being ‘botulus’.

Before the availability of modern medications, the mortality rate was around 60-70%. Today, it remains around 5-10% in the developed world.

Toxins in food
Infection most commonly arises when C botulinum bacteria produces toxins in foodstuffs prior to consumption. Early symptoms include fatigue, general weakness and vertigo, followed by blurred vision, dry mouth, and difficulty swallowing and speaking. Vomiting, diarrhoea, constipation and abdominal swelling may also occur. The disease can then progress to more specific weakness in the neck and arms, after which the respiratory muscles and muscles of the lower body are affected. Unlike many illnesses, there is no fever or loss of consciousness—the latter of which would, quite frankly, be a merciful thing.

C botulinum has found an impressive number of ways to infect humans. Infant botulism, which occurs mostly in those under six months of age, is caused by ingestion of C botulinum spores in the surrounding environment. These then germinate into bacteria and release toxins. Wound botulism is rare and occurs when these spores get into a muscle, usually through intravenous drug use, and are able to reproduce in an anaerobic environment. The symptoms are similar to food-borne botulism, but may take up to two weeks to appear. Inhalation botulism is rare and cannot occur naturally, but has been caused by the accidental inhalation of pre-formed toxins which have been found to be present in aerosols. It is also theoretically possible to ingest the pre-formed toxins in water. However, common water treatment processes such as boiling destroy the toxins, so the risk of this are very low.

Diagnosis is usually based on a clinical examination, followed by laboratory confirmation. Early treatment is essential. Anti-toxins such as the snappily named Botulism Antitoxin Heptavalent (A, B, C, D, E, F, G) Equine are available, and should be administered as soon as possible. Severe cases require extra treatment, and may include mechanical ventilation if the muscles controlling breathing have been affected. This may be required for weeks or even months. Unfortunately, while C botulinum is a bacterium, antibiotics are of limited use, only being effective against wound botulism. A vaccine has been developed, but its effectiveness is not fully understood and has demonstrably negative side effects.

Effective hygiene
Thankfully, botulism infections are rare. The most common type—food-borne botulism—is preventable with effective hygiene practices surrounding food preparation and storage. Even so, botulism outbreaks are officially designated public health emergencies that require a rapid response from the authorities.

It’s only the fact that C botulinum is an anaerobic bacterium, which can only grow in the absence of oxygen, that prevents it regularly becoming the star of a real-life horror movie. So next time you see green leaves photosynthesising gently in the sunlight, you can add keeping C botulinum at bay as another thing to thank them for.