Clostridium botulinum

Causes: Botulism

Introduction

Clostridium botulinum is a spore-forming, gram positive bacterium. It 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.

Pathogenesis

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.

Signalment

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.

Clinical Signs

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 flacid paralysis of legs and wings.

Diagnosis

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.

References

Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition), Elsevier Science.
Knottenbelt, D.C. A Handbook of Equine Medicine for Final Year Students, University of Liverpool.
Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial.
Piercy, R (2008) The Nervous System and Special Senses Study Guide, Royal Veterinary College.
Quinn, P.J., Markey, B.K., Carter, M.E., Donnelly, W.J., Leonard, F.C. (2007) Veterinary Microbiology and Microbial Disease, Blackwell Publishing.
Radostits, O.M, Arundel, J.H, and Gay, C.C. (2000) Veterinary Medicine: a textbook of the diseases of cattle, sheep, pigs, goats and horses, Elsevier Health Sciences.
Rycroft, A (2007) Principles of Microbiology Part I; Fundamentals of Veterinary Microbiology, Royal Veterinary College.