Botulism in Horses

Horse laying down

Diagnosing Botulism in Your Horse

Botulism is a neurologic disease caused by bacteria (Clostridium botulinum) that is fatal in horses. Horses are the most susceptible species to this particular bacterium and researchers are still trying to figure out why this is. This bacterium is classified as a “Category A” because it can be used as a bioterrorism weapon and is a food-safety concern for humans.

Botulism is described as types A through G. Types A and B are the most common in the Equine species and can be ingested with the forages that horses eat. Type B is the only type that is currently preventable through vaccination. Type C is usually caused by contamination of food sources with dead animals. Types D through G have never been reported in the United States. Horses can contract Botulism through ingestion, wound contamination or by having an immature digestive tract (this is most common in foals with immature immune systems).

There are several clinical signs that you may notice if your horse were to contract the Botulism bacterium. They are:

  • Muscle weakness that worsens with time- this could even lead to paralysis and recumbency (laying on their side)
  • Difficulty swallowing
  • Weak to no tongue tone
  • Ileus (normal intestinal mobility is lost)- this can include constipation or even colic
  • Weak eyelid tone
  • Dilated pupils or pupils that don’t respond quickly to light
  • Respiratory distress
  • Death

Sometimes Botulism can be improperly diagnosed as Colic because the clinical signs such as being restless and lying down often are very similar.  There is one very important difference between Botulism and Colic. Horses that have Botulism are typically uncomfortable when they stand but are much more comfortable after lying down. Horses with colic will continue to fidget and throw themselves around.

A definitive botulism diagnosis can be made by performing a mouse bioassay test. This test is performed using a sample from the infected horse. This sample is sent to a laboratory and exposed to a mouse. If the mouse dies after the exposure, the horse has Botulism. Unfortunately, this test takes 5-14 days to complete which makes it relatively ineffective since treatment needs to be started as soon as possible. Because of this, veterinarians typically base their diagnosis on the horse’s clinical signs and history. The most important questions your veterinarian may ask you have to do with what your horse is eating.  It is more likely for your horse to contract Botulism if you’re feeding them from large round bales, silage or haylage.

Two other things that veterinarians can look at when trying to determine whether this illness is Botulism or not is to do a “tongue stress test” or a grain test. With the tongue stress test the vet will determine how well the horse can pull its tongue back into its mouth after they’ve pulled the tongue out of the horse’s mouth. Horses with Botulism will not be able to effectively retract their tongues. The grain test consists of giving the horse 8 ounces of sweet feed and seeing how long it takes them to eat it. A healthy horse takes an average of less than 2 minutes to eat all the grain, but a horse with Botulism can take upwards of 15 minutes and often allows grain to fall out of its mouth.

The treatment for Botulism is to administer an antitoxin and to provide supportive care. The antitoxin is meant to halt the progression of the toxin in the horse’s body, but it doesn’t reverse the illness or treat the horse’s clinical signs. Supportive care can involve:

  • Deep bedding/padding
  • Repositioning often to prevent pressure sores
  • Leg wraps
  • Monitoring defecation and urination patterns
  • Eye protection
  • Antibiotics if the horse develops a secondary infection

Botulism is a very serious disease and the antitoxin should be administered as soon as clinical signs are present. Time is of the essence when a horse is sick with Botulism and ultimately even an hour can make quite a difference in their prognosis. In addition to an antitoxin, the infected horse can also receive the Type B vaccine, which can induce the proper immune response needed to help overcome Botulism. This could also prevent the horse from being infected in the future. If you suspect that your horse has Botulism or you need help to diagnose an illness in your horse please contact River Road Veterinary Clinic.

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