It is hard to believe that in most soils there exists a microscopic organism that is capable of killing a horse or a person just by a small amount getting into a cut or open wound. Clostridium tetani is the microbe that is responsible for causing the condition most commonly known as lockjaw in horses, but also known as tetanus in humans.
Tetanus or lockjaw is caused when an open wound, cut or lesion is in contact with the Clostridium tetani in the soil or on items that puncture the horse's skin. The microbe then produces a toxin in the body that binds itself to the nerves close to the injury, using them to travel up to the brain and spinal cord. Once in the brain or spinal cord the exotoxin begins to destroy the cells, resulting in several serious conditions.
The biggest and most problematic result of exposure to the toxin produced by Clostridium tetani is that the muscles of the body tense and are not able to relax. In horses, the strong muscles that control the jaw lock or tighten in the contracted position and cannot relax to allow the mouth to open. In addition the muscles of the legs contract as well, resulting in the horse standing with the legs out to the side, often described as the "sawhorse" stance. Horses will soon be unable to move or eat, plus they will be hypersensitive to sounds or movements. As the level of toxins in the body will vary, some horses will have mild cases of the symptoms while others will be severe. In the most severe cases the muscle spasms will result in seizures and death due to heart and respiratory failure.
Once a horse develops lockjaw there is little that can be done other than to try to manage any changes in the environment, keep the horse calm and stabled, and provide muscle relaxants to help to prevent injury to muscle mass. In addition antitoxins can be provided to help to neutralize the toxin, although the horse's body will also be working to neutralize the toxin. Most horses will need heavy feed supplementation and even IV or tube feedings to keep their metabolism functioning properly. About 20% of all horses that develop lockjaw can be successfully treated.
There is a way to prevent lockjaw or tetanus in horses, just as in humans. This includes a routine vaccination that includes an initial treatment and then a booster in three to six weeks, followed by an annual booster. Foals can have their first tetanus vaccination at four months, however if the mare has been vaccinated the foal will have some protection for the first few months of life. For unvaccinated mares and their foals an antitoxin can be administered that will provide short term protection until a routine vaccination schedule can be implemented.