But when external conditions change in its favor---that is, in anaerobic conditions with the right amount of moisture---C. botulinum emerges from its dormant state and multiplies rapidly. As each individual bacterium matures and dies, it releases its deadly toxin.

Seven distinct types of botulinum toxin have been identified---designated by letters from type A through G---but only types A, B and C are likely to produce illness in horses in the United States. Types A and B both reside in soil, but your risk of encountering them depends largely on where you live. Type A is more common in the West, and type B is seen more frequently east of the Mississippi River, especially in Kentucky and the Mid-Atlantic States. Type C is found in animal carcasses and bird droppings, which can be anywhere. However, up to 85 percent of all cases of equine botulism are caused by type B, which means that the risks are highest for horses in the eastern United States.

Botulinum toxin can cause illness in three ways:

* Food poisoning (botulism). Botulism is most likely to occur in horses who eat forage stored in a moist, anaerobic environment that encourages the proliferation of C. botulinum. This might occur, for example, if hay is baled while still moist or stored improperly; the wetness at the center of the bale causes spoilage and creates the ideal conditions for C. botulinum. Improperly processed haylage or silage--fermented forages normally fed to cattle---may also cause botulism in horses, as can clumps of grass clippings left by mowers. A far less common threat is feed or forage that has been contaminated by bird droppings or an animal carcass.

* Toxicoinfectious botulism ("shaker foal" syndrome). Foals are vulnerable to this form of botulism when they ingest the endospores as they nibble on grass or other things in their environment. The bacteria may activate and form colonies in gastric ulcers or the intestines.

* Wound botulism. Dirt and contaminants can carry endospores into a wound; if the surface heals over, an anaerobic environment may be created that allows the bacteria to gain a foothold within the surrounding tissues. This is more likely to occur with punctures and other deeper wounds.

A deadly threat

No matter how the botulinum toxin gets into the horse's body, the effects are the same. The toxin binds to the synapses of the nerves that control the muscles, blocking the transmission of nerve impulses to the muscles. With no source of input, the muscles go flaccid, causing paralysis. Signs may appear within hours or days and often begin with the inability to swallow. A foal might have difficulty nursing.

As the toxin spreads, the effects begin to appear throughout the body, with signs such as muscle tremors, generalized weakness, a limp tail and gait issues. The severity and extent of the paralysis depends upon the amount of the toxin that a horse consumes. If he ingested only a little, he may just become less active and eat less before recovering after several days. A large dose of botulinum toxin will likely cause a horse to become recumbent. In the most serious cases, the cause of death is often suffocation, as the toxin paralyzes the muscles that facilitate breathing.

The early signs of botulism---difficulty swallowing, lack of eating, lying down, flaccid muscles---can look like other conditions, such as choke, colic or neurological disorders. Signs more specific to botulism include... click here for the rest of the article.