It survives in the environment in spore form which is inactive, or in the gut of some animals. It is usually introduced into tissues through wounds (especially deep puncture wounds), and if there is no oxygen present, the spore activates and the bacteria multiply in the wound. The bacteria release a potent neurotoxin (a nerve poison) which are absorbed by motor nerves (nerves that serve voluntary muscles) and pass up to the spinal cord and brain, causing spasmodic tonic contractions of the voluntary muscles. Minor stimulation of the horse can trigger these contractions, which eventually become continuous.
Signs seen usually start 10 to 14 days after the original injury, but can vary from one to several weeks. These start as localised stiffness, often involving the muscles of the jaw ('lockjaw') and neck, the hindlimbs and the region of the infected wound; this progresses to general stiffness within a day or so, and the horse becomes more easily stimulated into more general violent spasms by sudden movement or noise. The ears are firmly erect, the tail stiff and extended, the nostrils dilated, and the third eyelid prolapsed. Turning. Walking and backing are difficult, the head and neck is extended, and the stiff legs make the horse take a 'sawhorse' stance. Sweating from pain and apprehension is common, and heart rate and respiratory rate are increased. About 80% of horses will die from this condition; the 20% that survive go through a long (4 to 6 weeks) recovery period.
Prevention is achieved very well by having your horse vaccinated against this disease by your vet. (two inoculations 6 weeks apart followed by a booster vaccination a year later, followed by a booster every 5 to 10 years). If your horse has not had a vaccination course as above, and suffers a wound, your vet can administer a tetanus antitoxin (tetanus antibodies) at that time to give a temporary protection lasting 2 weeks. The vaccination course can be started at the same time, the vaccination stimulates your horse to produce its own antibodies, but this process takes 2 - 3 weeks to start. If your horse has tetanus spores in the wound, these antibodies will be too late. Mares should be booster vaccinated 4 - 6 weeks prior to foaling to give the foal protection through colostrum for its first few weeks of life.
Treatment is difficult, expensive, and has at best a 20% chance of success. In the meantime, your horse will go through severe pain. So it pays to discuss the option of treating your horse very carefully with your vet. If attempted, treatment includes high doses of tetanus antitoxin into the bloodstream and possibly the spinal column behind the skull. Tranquillisers, plus drainage and cleansing of the infected area, and penicillins are also used. Good nursing is necessary; placing the horse in a quiet, dark stall, with food and water high enough for the horse to get without lowering its head. Tube feeding may be necessary, as are the use of slings