Ergotism is a worldwide disease caused by ingestion of sclerotia (ergots) produced by Claviceps purpurea, an endophyte fungus. Ergot sclerotias are hard, blackish-purple, elongated organisms which contain varying amounts of ergot alkaloids. They develop in place of the grain or seed of the flower heads of rye and other grains or forage grasses (brome, fescue, ryegrass, bluegrass). When crops are harvested, ergots will often be mixed in with the collected feed product.

Ingestion of the ergots from consuming contaminated grain, hay, straw, or grasses causes ergotism. Humans and all species of animals are susceptible to poisoning, however livestock are most commonly affected due to the processing methods of feedstuff that are incorporated into their feed.

There are four forms of ergotism which can occur in horses, these include: gangrenous or cutaneous, hyperthermic, reproductive, and convulsive.

Form	Description
Gangrenous or cutaneous	Associated with long-term ingestion of ergot-contaminated feedstuff. Results in the restriction of blood flow to parts of the body, causing tissue death, sloughing and foul odor from rotting flesh.
Hyperthermic	Affects the temperature regulation of the body. Affected horses are unable to cool themselves during high temperatures. It is associated with long-term ingestion of ergots.
Reproductive	Results in reproductive failure, abortion, and the ability of mares to lactate. Mares often birth small, unthrifty, or stillborn offspring. Sometimes offspring are born without any suckling reflex.
Convulsive	The result of acute ingestion of large amounts of ergots in a short period of time. It is the rarest form, and causes hyperexcitability, bizarre behavior, convulsions and death.