Botulism is caused by the toxin Clostridium botulinum. Horses are among the most susceptible species. The toxin may be produced in food, such as silage or vacuum-packed moist hay, which has been contaminated by decaying matter containing the organism. Also known as "Shaker foal" disease, the toxicoinfectious form of botulism affects foals, with the highest incidence in the United States seen in Kentucky and the mid-Atlantic region.
Clostridium botulinum is a spore-forming anaerobic bacterium which reproduces in decaying animal or plant matter. Under favorable conditions of warmth and humidity spores multiply and produce a highly lethal toxin. There are at least 7 different toxin types. Botulinum neurotoxins (BoNTs), produced by spore-forming anaerobic Clostridium botulinum, are the most toxic substances known. Ingestion of preformed toxin is the major route of infection in adults.
The rapidity of onset, in addition to severity of clinical signs, is toxin dose related. Affected foals are usually less than 8 months and often less than 2 months old. Signs include impaired sucking, inability to swallow, decreased eyelid and tail tone and dilated pupils. There is progressive muscular weakness and tremors, leading to collapse and inability to rise. Muscle tremors over the shoulders and flanks are evident. Saliva drools from the mouth. The gait is weak, shuffling and unsteady. Death results from respiratory paralysis within 24 to 72 hours of the onset of clinical signs.
Equine antitoxin (pentavalent vaccine against A-E toxin types) can only target the toxins at extracellular level, and can not reverse the paralysis caused by botulism. Intensive care is often difficult to provide and always expensive in the face of a guarded to poor prognosis. Treatment often is unrewarding unless a case is identified early and the proper antitoxin is readily available.
Prevention involves common sense approaches to feeding and care of the horse and, where possible, judicious use of vaccination in endemic areas.