Tetanus is a neurological condition that is caused by a bacterium (Clostridium tetanum) that liberates toxins, into wounds that they infect. Typical wounds include sole abscesses, heel lacerations, and wire cuts in adults, and the umbilicus in foals. These bacteria can be found everywhere in the environment, being residents of the soil. Indeed, Clostridium tetanus (C tetanum) are a constituent of dust, and like Salmonella, they can live comfortably in the intestinal tract of horses. It's not hard to imagine, therefore, that C tetanum can be found all over a horses' skin as well. As a result, wounds are readily contaminated with these bacteria. Occasionally, the intestine gets overgrown with C tetanum and the bacteria release the toxins. Toxins travel to the spinal cord, where they have the effect to block the calming function of certain spinal nerves. This is why you see such severe muscle spasms in the condition. Later, these spasms lead to paralysis and death.
All horses should be properly vaccinated against tetanus. Most horses that develop tetanus in this day and age have not been vaccinated against tetanus because the horse was either overlooked or could not be caught for the vaccine treatment. As a species, horses are very susceptible to this disease.
Signs of Tetanus
An unvaccinated horse that is infected with Clostridium tetani will start to demonstrate symptoms of 'tetanus' about 2 weeks after the wound was created. Owners may not be able to recall a recent injury, especially since the inciting wound may have been tiny, or unnoticed, and certainly could have healed over.
In the early stages of tetanus, signs include protrusion of the third eyelid, twitching and spasm in the ears, drawing back of the lips, flaring of the nostrils, the appearance of a stiff neck and abnormal carriage of the tail.
As the problem develops, further signs include: marked stiffness in the limbs ("sawhorse stance"); difficulty swallowing and drinking ("lockjaw"); difficulty standing; profuse sweating; grunting; falling; and respiratory distress. Affected horses respond to abrupt noise and startling by flashing their third eyelids, or worse, by falling into lateral recumbency and manifesting seizures. It's important to be quiet around these horses for this reason.
Horses affected with severe tetanus often die, even with treatment. With exhaustive and expensive treatment lasting several months, it is sometimes possible to successfully treat horses affected with tetanus. Treatments are aimed at reducing further production and release of toxins by bacteria and supportive care to prevent self-injury, asphyxiation, dehydration, and seizures. These include wound debridement, mega-doses of penicillin, muscle relaxants, tranquilizers (promazines), an anti-seizure medication such as valium, intravenous fluids/electrolytes, nutritional support, and the administration of tetanus anti-toxin, either intravenously or into the spinal fluid. The reason that veterinarians do not enjoy a high degree of success in treatment of tetanus is because the toxin ("tetanospasmin") binds very tightly to nerve endings in the spinal cord, and is therefore sequestered from the effects of treatments.
The bottom line is that we should never see tetanus. Vaccination against tetanus is both extremely safe and effective. The recommendation is that all broodmares get vaccinated regularly, and toward the end of pregnancy to confer antibodies for the foal. The foal is then vaccinated at 6 and 7 months of age, at one year, then annually. All other horses should be vaccinated annually.
If he has not been vaccinated for tetanus for greater than one year, he should receive an anti-toxin as well as tetanus toxoid at the time the wound is first seen. Failure to vaccinate may lead to tetanus, and so vaccination is considered imperative to good management.