Information on Botulism in Horses

Botulism is a rapidly progressive neuromuscular disease that can be fatal to horses.  It is caused by a spore-forming, toxin producing bacterium known as Clostridium botulinum.  The bacteria are wildly distributed in the environment worldwide and can be found in soil, agriculture products and in the intestinal tract of birds and animals.  Several different toxins, which are designated as types A, B, C, D, E, F and G, are produced by this bacterium.  The bacteria can survive in the soil for long periods of time because of its ability to form spores, which are very resistant to heat and cold.Type A botulism is more common in the western part of the United States whereas Type B botulism is more common throughout the entire United States with the highest concentration northeast of the Mississippi River (such as Kentucky and Pennsylvania).  In the adult horse botulism can occur by ingestion of the toxin in feed material or contaminated forage (“forage poisoning”) or wound infections.  In foals the most common cause of botulism is by ingestion of the spores (“Shaker foal syndrome”). Clinical signs of botulism can vary from subtle signs to horses found dead.  The first clinical signs can be seen within 12 hours to ten days following ingestion of the toxin.  Early clinical signs include generalized muscle weakness, slow eating and a shuffling gait with toe dragging.  In some cases sluggish pupillary light response may be detected within 6 to 18 hours after toxin ingestions.  Often, tongue tone, eyelid and tail tone are weak.  Prehension of food and ability to swallow are usually affected in horses with botulism.  Horses with botulism walk with a stiff, stilted gait and short “choppy” strides and oftentimes have muscle fasciculations.  Some horses may spend more time down and thus colic may be the initial complaint.  Some horses progress to recumbency within 12-24 hours.

There is no quick test to determine if a horse has botulism.  The diagnosis is based on history and physical exam findings.  The abrupt onset of diffuse symmetrical weakness that gradually progresses to recumbency in 1 to 4 days with normal mentation in the presence of an inability to swallow and decreased tongue tone should put the red flag up that botulism may be the cause.  The laboratory tests that are available require a specialized laboratory to look for spores or toxin in samples of blood, feed and gastrointestinal contents.

The fundamental principle of treating botulism is to neutralize the circulating toxin with administration of an antitoxin.  It is important that the antitoxin be administered early in the course of the disease otherwise the horse may die.  Other important aspects of treatment include proper nursing care, fluids and prevention of secondary problems.  In general the overall cost to treat a horse with botulism is very expensive.  Even with the antitoxin and appropriate treatment it can take 30-90 days for the horse to completely recover, if it survives.

A highly effective vaccine against type B botulism is available (BotVax B made by Neogen Corporation).  This vaccine is recommended for horses that reside in or are traveling to areas in which Botulism Type B is endemic (such as Kentucky).  Any horse traveling to Kentucky, including those competing in the Alltech FEI World Equestrian Games should be vaccinated for Botulism Type B. Adults should be immunized with an initial series of three doses given one month apart then boostered yearly.  Additional preventative measures include good husbandry.

Bonnie S. Barr, VMD, a graduate of the University of Pennsylvania School of Veterinary Medicine, is a board certified Internal Medicine specialist at Rood & Riddle Equine Hospital in Lexington, Kentucky. Dr. Barr has special interests in neonatology and infectious disease.   She raises Thoroughbreds and Quarter Horses with her husband.