Before you give that tetanus shot you picked up at the livestock store to your horse, make sure you check the label. It could either be antitoxin or vaccine. Do you know the difference'
Tetanus vaccine, AKA toxoid, is used to stimulate the horse’s immune system to produce antibodies against the tetanus toxin. Tetanus occurs when a wound becomes contaminated with the bacterium Clodstridium tetanti, which produces a toxin harmful to the nervous system. Mounting an antibody response to a new challenge can take as long as two weeks to get in full swing. If the horse has been vaccinated, he will have memory cells in this immune system that recognize the toxin from the previous exposure to the vaccine. The vaccinated horse is therefore able to immediately start producing antibodies that protect him from the toxin.
Antitoxin is exactly what it sounds like, a substance (preformed antibodies) that can itself directly neutralize the toxin. It’s primarily used to provide some short-term immediate protection to injured horses that have never been vaccinated or are not up to date on their tetanus vaccine. Larger doses may also be used to attempt to treat a horse that actually has tetanus.
This sounds like a great safety net, but it’s far from foolproof. Because the tetanus toxin has a very high affinity for tissue, the antitoxin is not effective against the toxin unless it is floating free in the blood stream. Once it becomes bound to its target, the antitoxin has no effect.
There is also a risk of the horse developing liver disease a few weeks to months after being given tetanus antitoxin. This complication is called ”Theiler’s Disease.” The exact mechanism is unclear, but it is believed to probably be an autoimmune reaction. Tetanus antitoxin is made from the blood/serum of horses vaccinated against tetanus. The theory is that the horse reacts to the foreign proteins in the antitoxin and his own tissues are similar enough that he also starts to attack his own liver. The liver damage can be extensive, even fatal. This side effect occurs only with antitoxin, not the tetanus vaccine. Fortunately, it’s relatively rare.
Tetanus is much easier to prevent than treat, and the treatment itself can be harmful. If you do your own vaccinations, make sure you read the label closely. Both toxoid (vaccine) and antitoxin are widely available in stores and usually stocked side by side.