Botulism in Horses

August 1, 2010 (published)

Today on Texas Vet News I am going to talk about a toxin that affects horses as well as many other animals and humans called botulism. Botulism is a disease caused by a bacterium that produces a potent toxin. Although there are different types of botulism, only types a, b, and c commonly affect horses. The most common mode of botulism poisoning is called forage poisoning and is due to decaying vegetable matter in the hay or small animals like dead mice that were accidentally trapped in the hay when it was baled. The bale of hay sets up an environment without oxygen that allows the bacterium called clostridium to grow and produce the toxin that is ingested when the horse eats the hay.

Foals can develop a type of botulism called toxicoinfectious botulism that can lead to shaker foal syndrome. The bacteria are ingested by the foal, grow in the foal's intestine, and produce the toxin. Adult horses have an intact GI tract that prevents this specific syndrome in adults. Wound infections with clostridium botulinum organisms can occur and produce the toxin although another clostridial organism that produces tetanus is much more common with wound infections. The botulinum toxin is a neurotoxin that prevents nerves from stimulating muscles and paralysis develops. Most horses have the inability to swallow and overall muscle weakness. There is an antitoxin available to treat these horses but it will not help with toxin that is already at the nerve terminal. If you are in an area where botulism is common, there is a vaccine for horses to help prevent it.

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