Botulism Vaccine for Horses

Written by kathryn white
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Botulism Vaccine for Horses
You can prevent botulism in horses with a yearly injection. (vaccine image by Mykola Velychko from

Botulism contains one of the strongest toxins known to man--one milligram is enough to kill a horse weighing over 454kg. This frightening disease often arises when a baling machine traps dead animal parts in a bale of hay fed to horses. The resulting bacterial illness can wreak havoc on a horse owner's emotions and budget. However, scientists have developed a vaccination aimed at preventing it.

Threat of Botulism in Horses

Horses have a higher susceptibility to botulism than other domesticated animals such as pigs or cows. Contamination of feed with an anaerobic bacteria called clostridium botulinum causes the disease. This bacteria forms in rotting flesh, and more rarely in plants, which then get raked and baled with hay. Then, according to the Canadian Ministry of Agriculture, if a farmer bales the hay too wet or stores it in a silo, the bacteria multiply. Finally, when the horse ingests the bacteria, it consumes toxins that interfere with the transmission of signals between nerves and muscle. In some rare cases, a foal under eight months might grow the bacteria in its intestine, or a horse might become infected through a wound.

Symptoms of Botulism

A horse can develop symptoms anywhere between several hours to 10 days after it ingests the clostridium botulinum. Horses with botulism usually show muscle tremors, which is why the disease is sometimes called "the shakes." Other symptoms include general weakness, loss of muscle control in the tongue, inability to eat because of drool, poor eyelid and tail tones and stiff walking stride. Death often results from respiratory muscle paralysis. However, symptoms of botulism resemble those of encephalitis, rhinovirus, and West Nile virus and only a mouse toxin assay can detect the contaminant and positively diagnose a horse.

Expense of Treatment

Treating botulism in horses is expensive and unsure. The New Bolton Center at the University of Pennsylvania offers an antitoxin that works against all types (A-E), but this antitoxin costs £1,950. Furthermore, the survival rate with antitoxin, and 24-hour hospital care, is still only about 70 per cent, says Ken Marcella, D.V.M. This means that preventive care through correct feed storage and vaccination remains the most cost-effective measure.

The Botulism Vaccine

A botulism vaccine for horses comes from a "toxoid" which is a form of the toxin, botulinum, that normally causes the disease. Manufacturers create this alternate form that does not have the power to make the horse ill but does illicit an immune system reaction. Injecting the horse with the vaccine teaches its body to attack any future real toxins it might encounter. As type "B" botulism is the most common type in the United States (85 per cent of cases), the current vaccine available, BotVax B, addresses this strain.

Dosage of the Vaccine

According to its manufacturer, Neogen Corporation, the first botulism vaccine comprises a series of three 2ml doses, with each shot administered one month apart. Each horse should then also receive a yearly booster injection to keep the vaccination current. Dr. Marcella also advises that mares be given the vaccine 30 days before their due dates so they will pass on protection from botulism to their foals. Foals can then get shots as young as two weeks of age.


The only major side effect of the botulism vaccine for horses is a rare occurrence of anaphylaxis, or shock. It is important to discuss this possibility with the vet before having her administer the injection. Minor reactions might include local tissue reaction, heat and slight swelling. Certain geographic areas, including the northeast Ohio Valley, the mid-Atlantic states, and the Northeast have seen higher incidences of botulism and, therefore, horse owners in these areas should take this larger risk into consideration when thinking about vaccinating their horses. Cold, wet winters and travel to shows and sales also increase the risk for botulism and play a factor in deciding whether or not to vaccinate.

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