Update on botulism

Introduction

Botulism is a syndrome of diffuse neuromuscular weakness which affects most mammals. The horse appears to be one of the most susceptible species. The disease is caused by one of the neurotoxins produced by the Gram-positive spore-forming anaerobic bacterium Clostridium botulinum. The bacterium produces 8 different serotypes of neurotoxins (A, B, C1, C2, D, E, F and G). Most cases of equine botulism result from serotypes B and C, although type A and possibly type D cases have also been reported. This presentation will concentrate on the syndrome of equine botulism seen in the adult horse.

Pathogenesis

Botulism occurs by 3 routes of infection: 1) ingestion of preformed toxin (‘forage poisoning’), 2) ingestion of spores leading to toxicoinfectious botulism (usually in foals, largely in the USA; ‘shaker foal’ syndrome) and, more rarely, 3) wound botulism through C. botulinum contamination of anaerobic wounds with subsequent bacterial growth and release of toxin. The disease can occur in outbreaks, as has been seen in the USA and Australia, and as sporadic cases as seems to be the case more often in the UK. The highest risk to the horse exists in contamination of feedstuffs (e.g. by rodent carcasses) and spoiled silage and haylage. Botulinum toxin acts primarily presynaptically at the peripheral cholinergic neuromuscular junction where it blocks the release of the neurotransmitter acetylcholine. […]