Botulism is caused by a potent toxin produced by the bacteria Clostridium botulinum. This bacteria is anaerobic, meaning it only lives where there is no oxygen and can be found in soils, wounds and decaying organic material. The spores of this bacteria can lie dormant in the environment for years and can tolerate climate extremes. Under the right conditions, the spores become active and produce a very potent toxin.

There are several types of toxins, all of which cause similar symptoms but which come from different sources. Either the toxin itself is ingested (type B or C) and absorbed in the intestine, the bacteria is ingested and the toxin is formed in the intestine (type B), or the bacteria grows in an infected wound and the toxin taken up by the blood stream.

Signs of intoxication include progressively worsening muscle weakness, and ultimately paralysis, resulting in a down horse that cannot rise. Classic signs are very slow eating and lack of tongue strength.

Botulism is hard to definitively diagnose, and is often diagnosed by exclusion of other diseases.

In Shaker Foal Syndrome (type B), foals ingest botulinum spores from the soil. The spores grow in the intestine, forming the toxin that gains access to the bloodstream. This condition is most common in the mid-Atlantic States, especially Kentucky.