Caused by a toxin produced by the spore forming anaerobic bacterium Clostridium botulinum which is part of the same family as Clostridium tetani, the causative organism of Tetanus. The bacteria grow in conditions where oxygen is absent such as in rotting vegetation or inside the decaying bodies of small mammals like mice and rabbits. Contamination of hay or other feedstuffs by the bacteria or it’s spores or toxin is the route of poisoning. Poorly ensiled forage or haylage is a possible source as is hay, especially large round bales, contaminated by dead rodents. Often the preformed toxin is consumed but foals may eat the bacterial spores which then develop in their intestine and produce toxin. Botulinum toxin A is the most toxic natural substance known, interfering with nerve impulse transmission, and causing flaccid paralysis.
In horses, signs include weakness, ataxia, difficulty prehending and swallowing food, recumbency with inability to rise, and loss of muscle tone of the jaw, tongue, and tail. Foals may show signs of ‘’Shaker Foal Syndrome” including sudden death, motor paralysis, with inability to suck or stand for more than a brief period. Breathing difficulty, constipation and dilated pupils develop later along with frequent urination, elevated heart rate and fatal respiratory arrest. Samples for testing should include the suspect feed, serum, and intestinal contents. Treatment is by intravenous botulinum antitoxin if available, but prognosis is poor.
Care should be taken with ensiled haylage which has an unpleasant odor like compost or mouldy grass clippings. Haylage should smell sweet and fruity, and any which does not should be discarded. Even a tiny amount of toxin can be lethal from a small area of spoiled feed in a bag or bale. The incidence of Botulism in Australia is rare, perhaps in part because the use of ensiled forage feed is also uncommon here.