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C. tetani is present in the feces and intestinal tracts of horses, and humans. Soil contaminated with horse feces commonly contains C. tetani spores. Therefore, humans working around horses and horse farms should seek immediate medical attention when injured, especially with penetrating wounds such as those caused by nails. Following a properly administered primary series of tetanus vaccine, virtually all persons develop a protective level of antitoxin (1). Booster vaccines for adult humans are recommended every 10 years (1).
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http://www.thehorse.com/ViewArticle.aspx?ID=5012
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Tetanus is caused by a naturally occurring bacteria called Clostridium tetani. This rod-shaped organism hangs out in the soil, tending to do best when it's warm and the soil is heavily cultivated. It needs only a small amount of soil moisture to survive, so is likely to be more prevalent in heavier soils as opposed to lighter types that dry out completely in the summer.
The tetanus bacteria is ingested by horses and can be found in their gut and their droppings. The spores themselves are not toxic to the animals. What they need are ideal growing conditions, and the source of an infection is nearly always through a wound. The wound may in some cases be so small that the source injury can no longer be found once a vet is called. It can also infect the horse through injuries to a mucus membrane.
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http://www.horsetalk.co.nz/health/tetanus.shtml
I've been around horses all my life. Every vet I've ever asked can state at length the dangers of tetanus in horses and humans. It would be different if they were free-roaming out in the wild like the mustangs, but modern horse-keeping on small acreage provides the IDEAL conditions for this bug to grow.
Also, vets state hooved animals in general are prone to picking it up from the soil because of the way they eat and their grazing patterns.
Even my holistic doc recommends this one vaccine for me because of my daily contact with horses.