In birds, also known as: Limber Neck — Alkali Poisoning — Western Duck Sickness
Clostridium botulinum is a spore-forming, gram positive bacterium.
There are 7 types of C. botulinum, A to G, differentiated on the antigenic specificity of the toxins. Types A, B and E are most important in people. C and D are most important in animal species.
The bacterium is oval with subterminal endospores. C. botulinim spores survive boiling for hours. Botulism is caused by the ingestion of the neurotoxin produced by the pathogen.
Clostridium botulinum is found ubiquitously in the soil worldwide. It is one of the most lethal substances known on earth as only 1.0 micrograms is the lethal dose for humans. Therefore the disease is zoonotic and a public health issue.
The bacterium has been implicated in equine grass sickness.
The toxin works by binding to gangliosides at the neuromuscular junction and irreversibly inhibiting acetyl choline (Ach) release, following absorption into the bloodstream. This inhibition of Ach, causes flaccid paralysis and death.
There are three ways that botulism can arise, firstly and most commonly (only one of the three that does occur in the UK) is via ingestion of the preformed toxin. Secondly, the spores can be ingested and germinate in the intestine releasing the toxin that is then absorbed, but this tends only to occur in neonates or young stock. Thirdly, the bacteria can enter the body through a wound and then proliferate again, once inside the body. The last two types of infection are rare and neither occurs in the UK. In the UK, infection usually arises when animals e.g. cattle or horses have ingested big bale silage, that has been contaminated with soil or carcasses. Carnivores can get infected by consuming rotting carcasses.
Birds become infected by consuming maggots from rotting fish carcasses. Hot weather and stagnant water favour anaerobic conditions and the production of toxins.
Any animal of any age, sex or breed can contract this disease. In the UK, access to big bale silage is the main cause of the disease, hence cattle and horses are most likely to develop the disease in this country. However the incidence is quite low in these species.
Type C C. botulinum causes a very high incidence of disease in wild waterfowl, with thousands of birds dying each year in outbreaks in the USA and Canada.
The characteristic clinical sign is generalised flaccid paralysis. Most clinicians consider the reduction in tongue tone to be the most characteristic sign (pull tongue out of side of mouth and let go; the animal will not pull it back in cases of botulism). Sudden death may be the only clinical sign in some cases, especially farm animals.
One may also notice dilated pupils, dry mucus membranes, decreased salivation and dysphagia in farm animals.
Another presentation is with incoordination and knuckling followed by flacid paralysis and recumbency. Paralysis of respiratory muscles leads to death.
Birds present with flaccid paralysis of legs, wings and neck. In waterfowl, neck paralysis can lead to drowning.
History of big bale silage or knowledge of previous Clostridium botulinum on farm, plus characteristic clinical signs are indicative of the disease.
Any differentials should be ruled out. Faecal samples can be taken for detection of the toxin.
The bacterium may be identified via mouse inoculation with infected serum, toxin detection by PCR or ELISA. Toxin neutralisation tests in mice are also available.
Treatment and Control
Polyvalent antiserum to Clostridium botulinum can be given, but this will only work very early on, when the toxin is free (unbound). When the toxin has fixed to the gangliosides, antiserum is not effective.
In countries where botulism is endemic, a toxoid vaccine is available for livestock. This is not the case in the UK.
Management of the recumbent animal, plus fluid therapy and nutrition can be undertaken if considered economically viable. Most livestock and horses that contract the disease are destroyed on humane grounds.
In birds, nursing and supportive care may be effective if the bird can still walk. Antitoxins are not practical, and vaccination with type-C toxoid has been used in commercial settings but is not cost-effective for wild species. Control involves the removal of dead birds from the water or poultry house. Poultry houses should be cleaned and disinfected. In waterfowl outbreaks, birds should be dispersed from affected areas and shallow water areas should be eliminated to prevent conditions favourable to the decay of vegetation and invertebrates.
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Kahn, C. (2005) Merck Veterinary Manual Merck and Co
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