Botulism has been in the news lately, with numerous outbreaks involving different species and some human food recalls. Botulism outbreaks are often pretty dramatic because of the number of individuals that can be involved, the severity of disease and the fact that it's often difficult to do much beyond damage control once the problem is recognized. Recently, there have been reports of widespread duck deaths along with a couple of different recalls and warnings in Ontario about potentially contaminated smoked salmon and improperly eviscerated salted fish.
On the equine front, there's also been a large botulism outbreak that is believed to have killed 23 horses in Maine (USA). The outbreak occurred over the last month and, as is typical, has been devastating because of the profound susceptibility of horses to botulinum toxin and the inability to do much to save the animals once it was realized that botulism was present.
In adult horses, botulism is caused by ingestion of food that's been contaminated with toxins produced by the bacterium Clostridium botulinum, as it grows. This relatively widespread bacterium doesn't normally grow and produce toxins in horse feed since it requires an oxygen-free environment and other specific conditions, but when these occur, the incredibly potent neurotoxin can be produced. Equine outbreaks are often associated with haylage or silage (which if improperly fermented allow for C. bolulinum to grow) or contamination of round bales (e.g. an smaller animal that died of botulism gets accidentally incorporated into the bale, where the toxins can persist and/or the bacterium can grow if the right environment is present deep within the bale). In this outbreak, silage is suspected to be the cause. The silage is being tested to confirm this suspicion.
You can never 100% prevent botulism, since strange sources are sometimes found, but avoiding high risk feeds (e.g. silage, haylage, moldy round bales), trying to ensure that dead animals do not get caught up in hay bales during the baling process and taking exceptional care when baling if botulism is present in wildlife in the area can help greatly. A vaccine is available but it only protects against certain types of botulism. If those types are the main types that cause disease in a given area, vaccination can be useful, but good feeding practices are the most important preventive measure.
Image: Horses at a round bale feeder (source: www.omafra.gov.on.ca)