Many people are familiar with the term "botulism" in reference to a form of human food poisoning. The disease can also affect horses and, as in people, it is commonly fatal.
Botulism is caused by a bacterium in the Clostridium family, Clostridium botulinum. Two other Clostridia also can cause botulism, C. barati and C. butyricum. Tetanus is also caused by a Clostridium bacterium, C. tetani, but the symptoms of tetanus and botulism are very different. Both organisms produce toxins that attack the nervous system, but tetanus causes the animal to be rigid while botulism produces profound weakness.
The earliest obvious symptoms of botulism involve the nerves that control the horse's ability to swallow and move his tongue. He may have trouble holding his head up, and anal and tail tone are often decreased. These early symptoms are easily mistaken for encephalitis or viral nervous system infections, or even rabies.
As the disease advances, the horse gets progressively weaker and has trouble supporting his weight. Eventually the horse goes down, is unable to rise on his own, and paralysis spreads to the muscles he uses to breathe-the diaphragm and muscles between the ribs (intercostals). If the disease runs its natural course, the horse will die either of suffocation or when the heart muscle fails.
The Most Common Strains
There are seven different strains of C. botulinum bacteria, but the two most frequently involved in horses are type C (West Coast, Florida, New England) and type B (mid- Atlantic and Kentucky). This is important in formulating vaccines and antiserum.
The majority of cases of botulism arise from eating foods contaminated with the bacteria, and then suffering the effects produced by the toxins. This particular bacterium exists in two forms: as an inactive spore (organism "sleeping" inside a tightly sealed shell), and as the activated, reproducing form. It only grows under conditions of very little to no oxygen. The spores of this organism can be found in the environment, even in the intestinal tract of many animals. However, they do not become activated in a healthy body.
One of the most common ways horses become poisoned by the toxin is if a small animal is caught in baling or harvesting equipment. When that animal dies, Clostridium botulinum spores inside its intestinal tract become activated as the body oxygen levels drop. The reproducing organism then produces toxin, which contaminates the hay or grain. Feeding silage (fermented, high-moisture fodder, stored in a process called ensilage) or haylage (ensiled forages, made up of grass, alfalfa, and alfalfa/grass mixes) to horses is risky since the organism thrives under those conditions. It also may be activated in the depths of large round bales under ideal conditions.
Dead animals contaminating a water supply can have the same effect. Botulism can also be seen in horses on pasture; some farms experience recurrent problems with it. Once in the soil, the organism reverts to its spore form and becomes very resistant to the environment. Ideal weather conditions (e.g., under the protection of a cover of decaying vegetation) may sometimes activate the spores, but exactly how or when this occurs is unknown.
Much less common is botulism that results from contamination of a wound or activation of the organism inside the intestinal tract. Experts believe that this form of botulism is what causes Shaker Foal Syndrome, a condition in very young foals (10 days old or younger) that causes rapidly progressive weakness and often death. Vaccinating broodmares at a time that will result in good levels of antibodies against the botulism toxin has been successful in reducing or eliminating this problem on some Kentucky breeding farms.