Disease: Botulism
This page provides general information about this condition; reveal the text by clicking on the green headers. Press releases, results from DWHC investigations as well as other useful documents and relevant literature can be found at the bottom of the page.
Pathogen
Botulism is the potentially fatal disease of humans, other mammals, birds and fish that is caused by intoxication with the toxin produced by Clostridium botulinum bacteria. This toxin acts on the nervous system, blocking communication between nerves and muscles leading to weakness that progresses to paralysis. There are 7 different types (A-G) of this bacteria, classified according to the different forms of toxin that they produce. Although the Clostridium bacteria produce the toxin, it is ingestion of the toxin and not an infection with the bacteria that leads to disease.
Clostridia are a special kind of bacteria that thrive in nutrient-rich environments without oxygen (anaerobic), for example in rotting vegetation or animal carcasses. Another characteristic of these bacteria is the ability to survive for long periods in non-favourable conditions (i.e. nutrient poor) such as in soils and pond/lake sediments by forming spores. These spores are inactive forms of the bacteria that can reactivate when conditions support bacterial growth; it is the active form that produces botulinum toxin. Some human activities can increase the risk of spore activation and toxin production, for example the flooding and drying of wetlands or extensive pesticide usage can result in the death of fish and birds leading to an increase in carcasses providing the substrate for bacterial growth (2).
Susceptible species
Different species show varying susceptibility to the 7 different toxins. In general, types A,B,E and rarely F cause disease in humans. Type C is more commonly implicated in disease in many species of mammals including horses (also affected by type B), cattle (also affected by type D), dogs and in poultry (also affected by type A). Type C botulinum toxin is also responsible for many mass die-offs in wild birds and avian botulism is considered to be one of the most important diseases of migratory birds. Type E tends to be associated with disease and die-offs in fish, amphibians, and fish-eating birds.
Dogs, cats and pigs tend to be relatively insensitive to intoxication with botulinum toxins, however in recent years there have been some reports of individual cases (type C) in dogs, possibly due to ingestion of spoiled meat or contaminated wildlife carcasses.
Signs in animals
Intoxicated animals show weakness progressing to recumbency and characteristically a protruding tongue. Death is typically due to paralysis of the respiratory muscles and may occur before clinical signs are observed, particularly in wildlife populations, in which mass die-offs of fish or waterbirds are often the first sign of intoxication.
Infection of animals
Animals become intoxicated (not infected) via ingestion of the pre-formed botulinum toxin in contaminated food: For example, in grazing animals this could be through ingestion of bird carcasses in grassland or hay; in fish-eating birds this may result from ingestion of fish that have fed on botulinum-toxin-containing material; in waterbirds this can be through ingestion of maggots that have fed on fish carcasses in which C. botulinum bacteria were actively producing the toxin.
Another form of botulism referred to as toxicoinfectious botulism, results from growth of C. botulinum bacteria in the tissues of a living animal; as this can only happen in anaerobic conditions, this form is typically only seen where there is existing tissue damage that has impaired the flow of blood and therefore the delivery of oxygen to the site. Examples of possible sites include gastric ulcers, areas of tissue death in the liver and intestines (possibly associated with parasitic infection), abscesses in the lungs or navel, or areas of skin and muscle that have been severely damaged (1). This is thought to be equivalent to wound botulism in people.
Symptoms in people
In addition to general weakness with difficulty in breathing and ultimately paralysis, some of the most obvious symptoms of botulinum intoxication in people result from the effects of the toxin on the nerves and muscles of the face; these include droopy eyelids, blurred vision, slurred speech and difficulty in swallowing.
Infection of people
Botulinum toxin can be absorbed into the body across any mucosal surface but the most common route of intoxication in people is via the intestinal tract following ingestion of improperly preserved or poorly prepared foods.
Preventative measures
As the spores of C. botulinum are found ubiquitously in wetland areas, elimination of this toxin-producing bacteria is impossible. However, environmental management can help reduce the creation of conditions suitable for bacterial growth and toxin production. This could include controlling pesticide usage and restricting flooding/drainage procedures to avoid the death of large numbers of birds and fish and prevent the build-up of rotting matter that provides the ideal growing conditions for these bacteria. Local authorities should regularly monitor and remove wild bird and fish carcasses and pet owners should prevent their animals from eating rotting animals.
Suggestions for minimising the risk of intoxication with botulinum toxin in humans are available on the website of the US Centers for Disease Control and Prevention.
- Merck veterinary Manual
- Friend, Milton. & Franson, J. Christian. & Geological Survey (U.S.). Biological Resources Division. (1999). Field manual of wildlife diseases general field procedures and diseases of birds. [Washington, D.C : U.S. Dept. of the Interior, U.S. Geological Survey pp. 271-281.
External information
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