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The bacteria that cause botulism are always present in soil, so horses are never far from the source of the deadly neurotoxin produced by these microbes. However, horses that develop botulism have often developed signs after eating contaminated hay. Frequent culprits could be large round bales that contained too much moisture when baled, or hay that harbored a dead animal that was accidentally picked up by the baling machine. The bacteria thrive in damp, warm, anaerobic places, and among horses that ingest the toxin, up to half may die, according to results of a study conducted at the University of Pennsylvania’s New Bolton Center.

In the study, researchers looked at the records of 92 horses brought to the center because of signs of botulism. Treatment was provided for 86 of the horses. Survival rate was excellent for the 37 horses that were able to maintain a standing position, but of the 49 that could no longer stand, 40 died. The survival rate for the group of treated horses was 48%. Horses that were given an injection of antitoxin within the first six hours of treatment were 121 times more likely to survive than those that did not receive the antitoxin.

Horses are more sensitive than some other animals to botulin neurotoxin. Muscle weakness, the most common sign, is first seen as a lolling tongue, drooping eyelids, and poor control of the tail. As the weakness progresses, horses lose the ability to swallow, and eventually are unable to support themselves in a standing position. By the time the condition affects the muscles used in breathing, the horses usually won’t recover.

Treatment is most effective if it is started early. Antitoxin doesn’t immediately cure the horse but does stop progression of the disease. Affected horses often need to be fed and watered through a stomach tube for a week or more until they regain the ability to chew and swallow. Complications like pneumonia, colic, and pressure sores may occur in recovering horses, which had an average hospital stay of 14 days in this study.

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