Editor’s Note: This is an excerpt from Understanding Equine Preventive Medicine by Bradford G. Bentz, VMD. 

Botulism is a disease caused by a toxin produced by the bacterium Clostridium botulinum.  As in the case of tetanus, toxin is produced by a clostridial organism. However, instead of the spastic paralysis produced by tetanus, botulism produces flaccid paralysis and weakness when nerve transmission to the muscle is inhibited by the binding of toxin and blockage of nerve-to-muscle transmission.  Botulism is the most potent biologic toxin known, and horses are highly sensitive to its presence and effects.  The clinical effect on the horse may become severe enough to cause the horse to become recumbent and eventually to die from paralysis of respiratory muscles.

There are three major forms of botulism, which are categorized by the mode of entry of the toxin. Ingestion of pre-formed endotoxin (forage poisoning) from contaminated feedstuffs may be the most common means of entry. Ingestion of the Clostridium botulinum spores may lead to production of toxin within the gastrointestinal tract, as is most commonly the mechanism in foals (shaker foal syndrome).  These types of botulism may be associated with contaminated feed or hay and may be a particular concern for horses being fed round bales or grazing in pastures with animal carcasses or decaying plant material. Finally, infection of a wound with Clostridium botulinum may lead to the production of the toxin after the spores vegetate.

There are eight recognized types of botulism toxin. Horses appear to be most susceptible to types B and C.  Almost all cases of shaker foal syndrome appear to be caused by type B toxin, and foals between two weeks and seven months of age in Kentucky and Mid-Atlantic seaboard states appear to be at greatest risk. However, shaker foal syndrome is reported sporadically in other areas of the country