Botulism is a motor paralysis caused most commonly 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. It results in rapid death due to the paralysis of vital organs. Botulism is not usually an infection. The paralysis is usually caused by the consumption of the toxin in food. The frequency of botulism in animals is not known with accuracy, but it is low in horses.

There are 7 types of Clostridium botulinum; the C₁ toxin is seen in most animal species, although types A and B may also be implicated in horses. The usual source of the toxin is decaying carcasses or vegetable materials such as decaying grass, hay, grain, or spoiled silage.

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 up to about 8 months of age and results in the shaker foal syndrome. 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 common 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.

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.

Last full review/revision July 2011 by Otto M. Radostits, CM, DVM, MSc, DACVIM (Deceased); Delores E. Hill, PhD; Barton W. Rohrbach, VMD, MPH, DACVPM; Charles J. Issel, DVM, PhD; Max J. Appel, DMV, PhD; David A. Ashford, DVM, MPH, DS; Daniela Bedenice, DrVetMed, DACVIM, DACVECC; Farouk M. Hamdy, DVM, MSc, PhD, MPA (Deceased); Kenneth R. Harkin, DVM, DACVIM; Johnny D. Hoskins, DVM, PhD; Eugene D. Janzen, DVM, MVS; Jodie Low Choy, BVMS; John E. Madigan, DVM, MS; Dale A. Moore, MS, DVM, MPVM, PhD; J. Glenn Songer, PhD; Joseph Taboada, DVM, DACVIM; Charles O. Thoen, DVM, PhD; John F. Timoney, MVB, PhD, Dsc, MRCVS; Ian Tizard, BVMS, PhD, DACVM; Brian J. McCluskey, DVM, MS, PhD, DACVPM; Bert E. Stromberg, PhD; Peter J. Timoney, MVB, MS, PhD, FRCVS