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Equine Botulism Overview

Botulism (Latin, botulus, "sausage") also known as botulinus intoxication. Botulism is a disease that affects not only horses but also a wide variety of animals. It is a tiny microorganism that is a silent, but deadly killer, the casual agent identified as Clostridium botulinum. It is closely related to the bacterium that causes tetanus only it is more deadly. As with the tetanus bacteria, which are always present in the environment, so it is with Clostridium botulinum. Once the horses have the toxins in the system the incubation period for the organism is from 24 hours to several days. When it is within the horses system, the toxins reproduce repeatedly and rapidly in the horse's gut. Diagnosing horse ailments requires a vet. (Click here for a list of Florida veterinarians)
Botulism Types
Eight distinct toxins of botulism have been identified, they are A, B, Ca, Cb, D, E, F and G. Some bacterial subtypes can produce more than one type of toxin. Types C and D are most common in Australia. The organism seems to grow best in neutral or alkaline conditions. It grows and produces toxins in anaerobic (minus oxygen) environments, such as both decaying vegetable matter and animal carcasses.
Botulism Causes
There are three basic modes of intoxication:

  1. Ingestion of a pre-formed toxin, also know as forage poisoning.
  2. Growth of the agent in the gastrointestinal system, also known as toxicoinfectious botulism.
  3. Contamination of wounds, also known as wound botulism

The ingestion of pre-formed toxins is the usual route of infection in adult horses. This would normally occur when the animal actually consumes bits of decayed or decaying material that harbored the bacteria. This explains why one horse will die from the disease, but another in the same paddock won’t. The toxins have to be eaten to have any effect.
Gastrointestinal Toxicoinfection
In cases where toxicoinfection is the route of attack, the horse ingests the spores that then germinate and produce toxins within the gastrointestinal tract. This route is most often the implication of the Shaker Foal Syndrome. Occurring at three to eight weeks of age, causing muscle tremors thus resulting in the name.
Wound Infection
Wound infection is less common; toxins are produced at the site and absorbed into the horses system.
Clinical Symptoms
Dr. Hutchins (1994) prepared a report on the clinical signs the yearlings presented at Newmarket. He reported the following; the indication of Botulism is that the horse shows mild depression; muscle fasciculation’s of varying severity seen most commonly affecting the tricep muscles, muscles of the forearm, muscles of the flank and hindquarters. There were signs of a mild wobbly gait, prolonged periods of sternal and lateral recumbency with difficulty in rising and the tongue hanging out. Eventually the horse cant get up and may die of breathing failure

The toxins affect the nerve endings where it joins onto the muscle of the horse and blocks transmission from the nerves to the muscle similar to symptoms of "tying up". The yearlings at the sales exhibited signs of dilation of the pupils and difficulty with the prehension of food in the later stages. Self-inflicted head trauma was common due to repeated and futile efforts to raise the head or neck.

The treatment used at the 1994 Yearling sales was unsuccessful in most of the cases. But that was due to not knowing what type of botulism was evident until after the incident. There are two sorts of treatment, first being supportive care which is given to the horse that have gone too far, making them as comfortable as possible and the second treatment is a more specific treatment with antiserum. But the problem being that there eight distinct toxins and a particular antiserum will only match the distinct toxin (Source - Hutchins, 1994)