If an injury or circumstances put a horse at risk for tetanus and his vaccination status is unknown, his treatment regimen may include tetanus antitoxin. Derived from the blood of horses immunized against tetanus, the antitoxin can halt or slow the progression of disease by binding to toxins circulating in the horse’s body, which prevents them from damaging additional neurons. Administering the antitoxin can help to slow the progress of tetanus in a horse who is not yet, or is only just, starting to show signs.

“This is still done by a number of veterinarians if you don’t know the history or you know that the animal has not been vaccinated, and the horse needs to have surgery or suffers a wound,” says Amy Johnson, DVM, DACVIM, of the University of Pennsylvania.

But antitoxin carries the risk of rare but significant complications. “It is a biologic product that could trigger an anaphylactic reaction,” says Johnson, “and mature horses can develop hepatic [liver] disease—serum hepatitis—a month or two after the treatment.”

Johnson emphasizes that serum hepatitis, also called Theiler’s disease, is a serious and usually fatal disease, but it is an extremely rare complication of tetanus antitoxin administration. Nat Messer, DVM, DABVP, professor emeritus of the University of Missouri, agrees, noting that the chances a horse might develop serum hepatitis and die are around one half of one percent, compared to at least a 50 percent chance of dying if he develops tetanus, says Messer: “The incidence of serum hepatitis is very low when giving antitoxin, but owners do need to be aware of that risk.”

Though these complications are rare, they are one reason why antitoxin is rarely administered to horses who have been vaccinated against tetanus in the past. Instead, a horse who turns up with a puncture wound will likely be given a vaccination rather than antitoxin if his annual booster is overdue. “That would be as effective as antitoxin, and safer,” says Messer. “In my view, antitoxin should only be used if you know for sure that the horse has not been vaccinated, and has a risky wound, such as a puncture in the foot; then you should probably give antitoxin, to be safe.”