Botulism is a serious disease that is accompanied by symptoms of paralysis with possibly fatal consequences. Botulism may occur in both animals and humans. The disease is usually contracted by ingesting food or water contaminated with the toxins produced by Clostridium botulinum bacteria, also referred to as botulinum neurotoxins, BoNT. Botulism is primarily a form of food poisoning. Wageningen Bioveterinary Research (WBVR) conducts laboratory diagnosis on test material from humans and animals suspected of suffering from botulism.
Transmission of botulism
Clostridium botulinum bacteria are soil bacteria that are widely present in the Netherlands. C. botulinum bacteria may form toxins under anoxic conditions.
Waterfowl and fish may die in large numbers. Large numbers of chickens and cattle kept in barns may also show symptoms of the disease, often with fatal consequences, whereas with horses botulism frequently occurs in individual cases. Botulism rarely occurs in humans.
Paralysis of the muscles
The symptoms of botulism are caused by toxins binding to the nerves. The botulinum toxins prevent signals from the nerves reaching the muscles, which results in paralysis. The muscles that are paralysed differ from species to species, the type of toxin and the quantity of toxin taken in by the body. Once paralysis has taken hold, recovery is slow. The link between the nerves and the muscles has to be formed again. If no measures are taken, botulism is frequently fatal.
The bacterium and toxic substance
The poisonous substance (toxin) is produced by the Clostridium (C.) botulinum bacterium and on occasion by related species of Clostridium, such as C. baratii or C. butyricum. Seven different toxins are distinguished, identified with the first letters of the alphabet: A, B, C, D, E, F and G. The most frequently occurring are the types C, D, or their intermediate forms (mosaic variants) CD and DC. Types A, B, and E are indicated now and then as well.
Humans are susceptible to the A, B, E and F toxins. Farm animals and (water)fowl are susceptible primarily to the C and D toxins, while the B-toxin is found in horses now and then . In fish and piscivorous birds, type E can be fatal. Dogs are much less susceptible to the botulinum toxins, but botulism may occur in rare occaisions.
Toxin types F and G do not occur in the Netherlands as far as we know.
Botulism in animals
Best known is botulism in waterfowl. It can lead to mass die-off, especially if the outside temperature remains above 25 degrees over a period of days. Other animals in which botulism is found regularly include horses, cattle and chickens, as well as occasionally fish, mink and dogs.
Botulism can lead to mass die-off among waterfowl. C. botulinum occurs naturally in the environment inhabited by waterfowl. As a result, the bacteria also enter the birds' gastrointestinal tracts. Under normal circumstances, this is not a problem; however, if the bird dies, and the outside temperature rises above 25° C for some time, then C. botulinum can replicate in the carcass. The larvae of insects feeding on the carcass take up these toxins. These larvae are not susceptible, but may carry extremely high concentrations of the toxins. These larvae (small toxin bombs) are then fed on by waterfowl, with the result that they die of botulism. A vicious circle arises in this way, with waterfowl dying off in large numbers if ther is no intervention. This is also the most important reason for mainly dead waterfowl to be cleared, in order to prevent large-scale outbreaks.
How is botulism recognised in waterfowl?
The most visible symptom in waterfowl is paralysis of the neck and wing muscles, often followed by death. A large number of waterfowl dying and observation of symptoms of paralysis during or shortly after a warm period are strong indications of botulism. Poisoning may proceed so rapidly, as a result of the high concentration of toxins, that no birds are found alive.
Botulism is rare in humans
Luckily, botulism occurs only rarely in humans, primarily as food poisoning. The Clostridium botulinum bacterium has then had the opportunity to replicate in food and to produce toxins. Homemade preserves that have been insufficiently heated pose a known risk.
Treatment of poisoning
Botulism is a form of poisoning that is difficult to treat. Once the toxin has penetrated to the nerve ends, there is no medication available to counter its effects. Success in treatment depends heavily on the quantity of toxins taken in and the stage the disease has reached. Successful treatment depends on rapid diagnosis. However, the prognosis for animals with botulism is poor in general.
The treatment of animals with botulism consists in stopping use of the current feed (and destroying this feed) and going over to feed that is uncontaminated with botulism toxins. Secondly, the animal's symptoms must be treated by maintaining the moisture and energy balance. Antiserum may be administered to bind the toxins present in the bloodstream; this makes sense only during the acute phase of the disease just after the symptoms first show.
The antiserum is not registered as an animal medication and falls under the cascade provision for animal medicines (Art. 22, Section 1 of the Veterinary Medicines Decree (Diergeneesmiddelenbesluit)). Antiserum against the toxins of type A, C, D and E is available through a vet from Wageningen Bioveterinary Research (WBVR). An evaluation is made in consultation with a WBVR expert on whether treatment with antiserum is appropriate. Stocks of antiserum are limited, and it is not intended for the treatment of entire populations.
If botulism occurs in waterfowl, the dead birds must be removed as quickly as possible to prevent these animals transmitting botulism to others. Dead waterfowl can be reported to the manager of the water body where the animal was found, usually the water authority or the municipality (with larger bodies of water, usually Rijkswaterstaat). They will have the animals removed and tested for botulism. Apart from removing carcasses, ensuring adequate water flow is advised. This leads to a rise in the oxygen content, a decline in the water temperature and a lowering in the concentration of spores and toxins. Extremely high concentrations of toxins are often present in carcasses. For this reason, precautionary measures should be taken on contact with dead animals, such as wearing protective (disposable) gloves.
The role of Wageningen Bioveterinary Research
Wageningen Bioveterinary Research has a long tradition in testing for botulism. WBVR is the national reference laboratory for botulism in both humans and animals. The institute carries out diagnosis and provides advice on problems with botulism.
We conduct diagnosis on testing material from humans and animals suspected of suffering from botulism. The C. botulinum bacterium is indicated by means of PCR tests. The test can be carried out on serum, faeces, liver or gut content. There are currently two PCR tests available; one for indicating the presence of types C and D (including the mosaic variants CD and DC), and another test for C. botulinum of types A, B, E and F. These tests can also be used on the feed components or feed in order to trace (the source of) the botulism.
Toxins can be indicated by means of a toxin and typing assay. This test is usually conducted only after the presence of the bacterium has been shown by a PCR test.