Merck Manual

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Pet Owner Version

Botulism in Horses


Henry R. Stämpfli

, DMV, Dr Med Vet, DACVIM-LAIM, Department of Clinical Studies, Ontario Veterinary College, University of Guelph

Reviewed/Revised May 2019 | Modified Oct 2022

Botulism is a rapidly fatal disease with motor paralysis. It is commonly caused by eating food contaminated with the toxin (a type of poison) produced by Clostridium botulinum bacteria. This organism grows rapidly in decomposing animal tissue and sometimes in plant material, such as decaying grass, hay, grain, or spoiled silage. It results in rapid death due to the paralysis of vital organs. Botulism is not usually an infection but a poisoning. The frequency of botulism in animals is not known with accuracy, but it is low in horses.

The signs of botulism are caused by muscle paralysis. They include paralysis that becomes progressively more severe, disturbed vision, difficulty in chewing and swallowing, and overall weakness. Death is usually due to paralysis of the lungs or heart. Diagnosis of this condition is difficult, and it is often made by excluding any other possible causes of paralysis and by association with a likely source of the toxin.

Botulism may also originate in 2 other ways. Clostridium botulinum has occasionally been found to grow in the gastrointestinal tract and produce toxins there. When the toxins are released, they cause typical botulism. This occurs in foals usually less than 4 weeks old and results in the shaker foal syndrome. Foals may be found dead without previous signs, but most often, foals show signs of paralysis that slowly progresses. Stilted gait, muscle tremors, and the inability to stand for more than 4 to 5 minutes are key signs. Other signs include difficulty swallowing, constipation, dilated pupils, and frequent urination. As the disease progresses, labored breathing with extension of the head and neck, rapid heart rate, and respiratory arrest occur. Death occurs most often 1 to 3 days after signs are first noted. Botulinum antitoxin has been used for treatment, with varying success. Early administration to foals, before they can no longer stand, is reported to be successful. Full supportive care is also important. In endemic areas, such as Kentucky, vaccination with type B toxoid appears to be effective.

A third form of botulism occurs in humans—and sometimes in adult horses—when the bacterium grows and produces toxin in a wound. This is referred to as wound botulism.

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