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Botulism in Horses & Other Mammals

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Information below provided by Dr. Robert MacKay, Dr. Sarah Reuss and Dr. Chris Sanchez of the UF Large Animal Medicine Service.

What is botulism?

Botulism is a disease caused by ingestion of botulinum toxin, one of the most potent toxins known. It can affect all mammals, however horses are one of the most susceptible species. Botulism can affect both adult horses and foals (called “shaker foal syndrome”).

What are the clinical signs of botulism?

Signs vary in severity depending on the amount of toxin ingested.

  • The main sign is flaccid paralysis (weakness with decreased muscle tone). This may start off looking like a stiff, stilted gait but progresses to muscle tremors, weakness, and then recumbency (inability to rise).
  • Some affected horses may just spend more time lying down.
  • Other signs of paralysis involve the eyelids and tongue. When looking closely at the eyes, the pupils will be dilated and the blink will be weak. The horse often has trouble chewing and swallowing. Tail tone will also be decreased.
  • Once the horse is recumbent, issues with bladder paralysis and colic can result.
  • Ultimately, horses may die due to paralysis of the respiratory muscles.
  • One important thing to note that helps differentiate this disease from other neurologic diseases is that the horse’s mentation will be normal. They will be aware of their surroundings and often want to eat.

What causes botulism?

Botulism is a disease caused by toxins produced by the bacteria Clostridium botulinum. There are 8 different types of botulism that produce toxins of varying potency. Types A, B, and C are the types that usually affect horses. Horses can acquire disease three different ways. The most common route is by eating feed that is contaminated with toxin already in it that has been released by bacteria. This can be from spoiled feed or feed contaminated with an animal carcass. They can also acquire disease by ingesting the bacteria which then sporulate within the GI tract and release toxin inside the animal. This is more common in foals and is due to type B toxin. Rarely, horses can acquire botulism if a wound becomes contaminated with bacteria that then release toxin. The toxin itself acts at the junction between nerves and muscles so that the muscles don’t get the signal to contract when they should, hence the “flaccid paralysis.”

Can an infected horse spread botulism?

No. Botulism is only acquired from ingestion or infection of a wound. It is not spread directly from horse to horse, or from horses to people.

How can I prevent my horse from getting botulism?

Do not feed any obviously spoiled grain, or feed in which any animal carcasses are found (rats, birds, squirrels, etc). Also avoid feeding silage or haylage to horses as, if improperly fermented, it can harbor Clostridial organisms. Do not feed hay from plastic-wrapped bales if the plastic is torn.  There is a vaccine against one type of botulism (Type B). This vaccine is given in certain geographic areas (central Kentucky, the mid-Atlantic) to reduce the risk of “Shaker Foal Syndrome.” Mares should be vaccinated with a 3 dose series in the last 3 months of pregnancy. Unfortunately, this vaccine is not effective against other types of botulism.

How is botulism diagnosed?

Botulism is diagnosed based on known risk factors and clinical signs. There is no blood test for botulism. As many horses with botulism have difficulty eating, a grain test can easily be performed to help make a diagnosis. Give 8 oz of grain in a pan, and a normal horse should be able to eat it in less than 2 minutes. If they take longer, that may be a sign of botulism, especially if they have other signs of weakness or recumbency. The bacteria may be able to be cultured from the feces, more frequently in foals than adult horses. If the horse does not survive, a post-mortem exam will usually not find anything obviously abnormal. Contents of the gastrointestinal tract can be tested for toxin to confirm the diagnosis.

What if my horse gets botulism?

If you think your horse may have botulism, call your veterinarian immediately. While you wait, try to keep the horse as quiet as possible. You do NOT want to make affected horses walk any further than absolutely necessary, as they will quickly use up their energy reserves. There is an antitoxin available that has improved survival over the years. The antitoxin helps by binding to any toxin that is still in the bloodstream. Once the toxin has bound to the nerve cells, it will not be removed by antitoxin. Therefore, antitoxin will not reverse clinical signs, and the earlier it can be given, the better. Because the antitoxin is expensive and botulism is a relatively rare disease in Florida, most veterinarians do not keep this in stock. The University of Florida maintains a limited supply of the antitoxin in our pharmacy that we are happy to get to your veterinarian if needed. Nursing care is also very important to prevent eye ulcers, sores on the body, and pneumonia if the horse is recumbent. Foals may be put on mechanical ventilation if necessary, and this procedure is also available at the University of Florida Large Animal Hospital.

Contact the UF Large Animal Hospital at 352-392-2229 or your primary care veterinarian for additional questions.

Video of horse suffering from botulism