Botulism is a gradually progressive neuromuscular disease that is associated with a high mortality rate in horses. It is caused by ingestion of the botulinum toxin or Clostridium botulinum spores or it can be absorbed through wound contamination with the bacterium. The toxin is extremely potent, and it acts by interfering with nerve function, resulting in respiratory and musculoskeletal paralysis. There are several different strains of C. botulinum which produce different types of toxins.
Horses are one of the most susceptible animal species to infection with C. botulinum.
Infected horses develop fluctuating muscle weakness and fatigue. Hay can sometimes become contaminated with the bacteria during the raking and baling process. C. botulinum is found worldwide and is commonly found in the environment, inhabiting soil, sewage, and marine sediments, as well as the intestines of decaying carcasses or vegetation. The botulism bacteria grows on food sources which are above a pH of 4.5 and that are in an anaerobic (non-oxygen) environment.
In 2015, there was an outbreak of type-D botulism in 12 horses living in Israel. All of the horses showed clinical signs of progressive weakness, decreased tongue and tail tone, and ataxia. Within 9 days after being infected, 10 out of the 12 horses died, even with supportive care provided.