Botulism is Being Seen more Frequently in Horses

January 28, 2005 (published) | February 18, 2014 (revised)

Dr. Amy Johnson from the University of Pennsylvania indicated at the Veterinary Internal Medicine Conference that cases of botulism are increasing in horses. Botulism is a bacterial disease that produces toxins in feed, and these toxins are ingested when the horses eats. Botulism usually occurs when feeding round bales of hay or improper cured silage as well as hay that contains decaying dead animals such as mice in the hay.The toxin of botulism causes progressive muscle weakness and many horses will get down and be unable to rise. The most typical sign of botulism is the horse’s inability to swallow and use the tongue. Some horses also have difficulty breathing and have colic but horses with colic are in pain when lying down and horses with botulism are calm once they are lying down.

Testing for botulism is difficult because a mouse bioassay is the recommended test but it requires at least 5 days and can have false negatives. Many times the diagnosis is made on clinical signs and the fact that the disease may be commonly seen in the area. One of the exam tests used is the tongue test; for this test, the tongue is pulled from the mouth and if it hangs from the mouth for a longer than normal period, botulism is suspected. Treatment of botulism involves the use of antitoxin and supportive care. There a vaccine to prevent type B botulism but it does not protect against the other types. Unfortunately, adult horses only have about a 50% to 90% survival rate if infected with botulism.

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