Tetanus is a condition caused by the bacteria Clostridium tetani which produces spores. It is found in the soil and also the intestinal tracts of animals so will be in the droppings left on the pasture.
Even retired horses or companion ponies that never leave the yard are still at risk. The horse becomes infected through a wound but for the bacteria to multiply and the spores to grow there must be reduced oxygen level at the wound site, for example a deep puncture wound or even a foot abscess. As the bacteria multiply a neurotoxin is released and this causes the symptoms that are seen in an infected horse. This incubation of the bacteria will take around 10 days so you often have to look for old wounds that may not have seemed a concern at the time.
The neurotoxin causes over stimulation of muscles. Commonly involved muscle groups are the jaw, neck, hind legs and areas around the wound. So this tends to present as lock jaw and a very stiff gait when walking. They also become very over sensitive to noise and sudden movements.
Some more specific symptoms include stiff erect ears, dilated nostrils and a prolapsed third eyelid. As the muscle spasms progress the horse will become recumbent and as the respiratory muscles become involved breathing is compromised resulting in death.
Tetanus can occasionally be treated successfully if the symptoms are recognised very quickly and the horse is started on an intensive antibiotic and tetanus anti-toxin protocol. However, the best way to protect your horse against tetanus is to vaccinate. This can either be as part of the flu/tet combination vaccine or as a single vaccine. Once the initial course of vaccines is started boosters are only required every two years. So it is recommended that even horses that do not leave the yard are still covered for tetanus.