Botulism poisoning is caused by a bacterium, Clostridium botulinum (Cb). Cb is found naturally in the soils throughout the United States. The source of the poisoning can be contaminated feed, wounds, or in the case of foals, bowel overgrowth of the bacteria. There are five different types (A through E) of botulism poisoning, though symptoms are similar in all five. The incidence most years may be fewer than a few hundred cases a year in the continental US though large outbreaks have occurred when large numbers of horses are exposed to a contaminated feed. The incidence by state over one 19-month period is as follows:

• KY: 18
• OH: 13
• MD: 7
• PA: 6
• CA: 5
• TN: 5
• States with fewer than 5 deaths but had cases are: FL, VA, NJ, DE, NC, and IN.

Botulism is a poisoning caused by a gram-positive anaerobe bacterium and occurs commonly in the soil and in intestinal contents of most animals. In order to reproduce it requires low oxygen tension and prefers necrotic tissue. There are eight strains that each produces different toxins that vary in potency and are distinct antigenically. Of the eight strains the B strain and the C strain seems most prevalent and it is important to note that the antisera and vaccines do not cross protect between these two most common strains so vaccination and treatment must be strain specific or cover all the possibilities.

The very potent toxin is elaborated by the Botulism bacterial organism whenever it finds conditions suitable for growth. The toxin interferes with the normal transmission of nerve impulses to the muscles resulting in a progressively worsening flaccid paralysis. There are at least three ways for animals to contract the poisoning:

• Ingest the preformed toxin in carcass contaminated feed, forages or pastures contaminated by carrion bird feces, or from poorly insiled feeds like silage or haylage.
• The botulism organism can grow in the necrotic tissue of a wound.
• It may be possible that poisoning in horses could occur from toxin produced intestinally.

Shaker foals are most likely poisoned from infected wounds and possibly gastric ulcers were the source of the toxin. Type B is most common in foals.

Many cases of botulism have been traced to exposure to feed contaminated with the carcasses of dead small animals. Poorly put up silage or haylage has also been proven as a cause and even a pile of rotting grass may produce enough toxin to kill a horse. It has been hypothesized that the stools of healthy cats and carrion eating birds could contain enough toxin to kill a horse and should be considered a possible hazard.