Botulism

Definition/Overview

Botulism is a neuromuscular disease characterized by flaccid paralysis that is caused by neurotoxins produced by strains of Clostridium botulinum. Horses are one of the most susceptible species, with both individual and group outbreaks reported.

Etiology

Clostridium botulinum is a Gram positive, sporeforming anaerobic bacterium. Spores are found in the soil throughout most of the world with the distribution of strains dependent on temperature and soil pH. Seven serotypes of botulinum neurotoxin exist and are labeled A, B, C1, D, E, F and G, all of which have similar toxicity. There is geographic variation in the predominant serotypes. In North America, botulism in horses is most often caused by type B toxin and less often by toxin types A and C1.

Pathophysiology

There are two main forms of botulism. Toxicoinfectious botulism, also known as ‘shaker- foal syndrome’ occurs almost exclusively foals as a result of overgrowth of C. botulinum in the intestinal tract, followed by production of neurotoxins. The disease most often affects fast growing foals from 1-2 months of age, although cases outside this age range have been seen. The mature, protective gastrointestinal microflora of adult horses typically prevents overgrowth of C. botulinum following ingestion. In adult horses, botulism occurs following ingestion of pre-formed toxins in feed. Spoiled hay or silage are most commonly implicated in botulism caused by types A and B. Silage with a pH greater than 4.5 is favorable for sporulation and toxin production. This is known as “forage poisoning”. It has also been suggested that birds may be able to carry preformed toxin from carrion to the feed of horses. Type C botulism is associated with ingestion of feed or water contaminated by the carcass of a rodent or other small animal. Less commonly, botulism can occur when neurotoxins are produced in wounds infected with C. botulinum. Proliferation of C. botulinum type B organisms in gastric ulcers, foci of hepatic necrosis, abscesses in the navel or lungs and wounds in skin and muscle have been associated with toxicoinfectious botulism.

Botulinum neurotoxins bind to presynaptic membranes at neuromuscular junctions, irreversibly blocking the release of the neurotransmitter acetylcholine resulting in flaccid paralysis. Botulinum neurotoxin has also been linked to equine grass sickness. [...]