Tetanus doesn’t grab many headlines these days. In this era of superbugs and COVID-19, a disease that can be prevented by vaccination and sensible wound management might seem almost tame.
But tetanus is anything but. Cases may not be as common as they once were, but this disease still poses a mortal threat to horses and humans alike. Clostridium tetani is an anaerobic organism, meaning it thrives in moist, low-oxygen conditions. So if the environment is right in a wound contaminated with C. tetani spores, the bacteria are activated, multiply and release powerful neurotoxins that cause painful muscle contraction and spasms. Often the muscles of the head and neck are among the most obviously affected, which is why tetanus is commonly called “lockjaw.” Horses with the disease often adopt a characteristic “sawhorse” stance, as well, as muscles in the back and torso seize. More than 50 percent of horses who contract tetanus die or must be euthanatized.
Thanks to vaccination, tetanus is rare among America’s horses, but it does occur. “I wish there were zero cases,” says Simon Peek, BVSc, MRCVS, PhD, DACVIM, of the University of Wisconsin-Madison. “It’s such a horrible disease that we’d prefer to never see it again. Yet we continue to have sporadic cases, and it’s always tragic when we do.”
So you are unlikely to ever see a case of tetanus firsthand, but you’ll still want to take the threat seriously. The frontline of defense is vaccination---it’s easy, effective and inexpensive. Beyond that, it’s wise to become familiar enough with tetanus to understand when horses are most at risk and why. To help you, we’ve provided an overview (see “In Focus: Tetanus,” page 30) along with the following collection of lesser known facts about this deadly disease.
1. Horses are particularly susceptible.
Horses are at higher risk of developing tetanus than other animals. First, as a species, horses are unusually vulnerable to the C. tetani infection---a relatively small amount of the toxins produced by the pathogen can be deadly. In contrast, chickens and other birds are highly resistant---a lethal dose is up to 300,000 higher per pound of body weight than for a horse. Likewise, it takes a fairly high dose of toxin to cause dogs and cats to develop tetanus.
Second, horses are very likely to be exposed to C. tetani. The organism’s spores are widespread in the environment; the soil in most regions is contaminated with them. C. tetani spores are often present in the digestive tract of animals, as well. “These soil bacteria can become part of normal flora in the horse’s intestine and are therefore present in manure,” explains Nat Messer, DVM, DABVP, professor emeritus of the University of Missouri. When deposited on the ground, the bacteria go dormant and can survive almost indefinitely.
Finally, many cuts, abrasions and other wounds occur on the very areas of the horse’s body where the risk of exposure to C. tetani is highest and where conditions are right for the organism to flourish: the lower legs. Because it is anaerobic, C. tetani cannot thrive in healthy, oxygen-rich tissues, so the horse’s lower limbs, which are not well-oxygenated to begin with, would provide a welcoming environment. In contrast, tetanus is less likely to develop from wounds to larger muscle groups elsewhere on the body that are well supplied with blood.
2. virtually any wound—not just punctures—can lead to tetanus.
In the classically imagined tetanus scenario, a horse steps on a rusty nail that pushes C. tetani spores deep into the resulting puncture wound, where the bacteria multiply and ultimately release the toxins that cause disease. In reality, though, tetanus can result from virtually any break in the skin that allows C. tetani spores to enter the body. In fact, health officials warn that superficial wounds that may be overlooked or less carefully cleaned pose an outsized tetanus risk compared to punctures or more severe injuries likely to receive prompt and thorough medical attention.
Tetanus is also a postpartum risk for mares and for their newborn foals. “Mares after foaling can develop tetanus from contamination of the uterus, and foals are at risk via umbilical infections---though these cases are less common than from puncture wounds,” says Messer.
Lastly, tetanus can occur after surgery, although modern veterinary practices have pretty much eliminated this threat. “Most veterinarians are fastidious about proper surgical technique and cleanliness,” says Peek, “and prior to doing procedures such as castration they make sure that the horse has had appropriate tetanus vaccination.”
3. One tell-tale sign can signal the onset of tetanus.
A stiff gait and/or hyperreactivity are often the earliest indicators of tetanus but they also are associated with a variety of other conditions, which can make the initial diagnosis difficult. “If a horse overreacts to visual or sound stimuli, and he travels with a choppy stride, you might think he is tying up or has laminitis or neck pain,” says Amy Johnson, DVM, DACVIM, of the University of Pennsylvania. “There are several things that might be suspected, rather than an early case of tetanus.”
Another common tetanus sign is spasm of the muscles on the head and face. “The horse develops a classic facial expression with ears erect and pointed backward,” says Johnson. “He looks like he’s grimacing, because the muscles of the lips are pulled back, showing the teeth.” (The term for this in human medicine is risus sardonicus, or “sardonic laughter,” which denotes an involuntary smile due to contraction of the muscles around the lips.)
“In milder cases a horse might not show all of these signs,” says Johnson. There is one indicator, however, that points pretty clearly toward tetanus: the visibility of the third eyelid. “If you wave your hand toward the eye, you’ll see the third eyelid flash up,” she says. “This is not something you would see in a healthy horse. And along with other clues, it will suggest a tetanus diagnosis.”
4. The disease’sincubation period can last several weeks.
The speed of tetanus onset is influenced by several factors, including the location of the wound, its severity and its level of contamination. If tetanus spores become lodged in well-oxygenated tissues, they may remain dormant after healing for long periods, until a bruise or another injury at the same site creates conditions that activate the organism and enable it to multiply.
“A lot depends on the level of contamination of the wound and the amount of toxin being produced, and the location---and how long it has been going on,” says Messer. More foreign material in a wound is likely to mean more C. tetani spores and faster proliferation. On the other hand, the quick discovery, cleaning and treatment of a wound will reduce the threat of tetanus.
5. The progression of tetanus depends on where the organism enters the body.
The specific signs in a developing case of tetanus depend on which nerves are affected first. Initially the neurotoxins produced by the C. tetani organisms are absorbed by the motor neurons in the vicinity of the original infection site, and they then spread along the nerve cells.
In “ascending tetanus,” the in-citing wound is located on a hoof or lower leg, and the neurotoxin travels up the peripheral nerves to the segments of the spinal cord that control that limb. This often results in muscle spasms that primarily affect the limb that sustained the wound.
In “descending tetanus” more neurotoxin is produced than can be absorbed by neurons in the local area, so the excess is carried away by the lymph system and bloodstream, making its way to the brain and/or spinal cord. In these cases, muscle spasms may quickly become evident in the head and jaw.
6. The bacteria that cause tetanus and botulism are closely related.
Both Clostridium botulinum and C. tetani produce potent neurotoxins but their effects on the body are in direct contrast to one another---C. botulinum produces a toxin that inhibits muscle contraction causing paralysis, while C. tetani produces a toxin that prevents muscles from relaxing, causing spasticity.
The neurotoxin produced by C. tetani binds to nerve cells to prevent the release of a neurotransmitter that normally enables muscle contractions to cease. As a result, the muscles controlled by these nerves become locked into painful spasms---and sometimes the contractions are powerful enough to break bones.
The C. botulinum neurotoxin has the opposite effect---it prevents muscles from contracting. As a result, victims of botulism may lose the ability to swallow and ultimately breathe, as the muscles of the diaphragm become flaccid.
7. tetanus has a high mortality rate, but it is survivable.
By the time outward signs of tetanus appear, a horse is in serious trouble, and saving him will require heroic effort. The earlier treatment begins, the better his chances. “If you can treat the horse before the toxin has time to make it into the spinal cord, the horse will likely do better than if a wound isn’t recognized until most of the toxins have already made it to the spinal cord and are bound to those cells,” says Johnson. “If there is a treatment delay, the horse is less likely to recover.”
There is no cure for tetanus. Instead, the goal of supportive care and other measures is to reduce the severity of the signs and keep the horse alive while the disease runs its course over many days to several weeks. Treatment commonly includes:
• vaccination. Even after a horse begins showing signs of tetanus, vaccination can help to boost his immune system’s response to the disease.
“The amount of toxin necessary to cause clinical disease is less than that required to promote an immune response for future protection,” says Peek. “Thus, when we see a case of clinical tetanus, we also vaccinate the horse.” This may help to increase the horse’s immune response to the disease even while it is in progress. “We then come back and give him a booster again in about four weeks, if he survive, mimicking the primary series you would normally do in a foal, or an unvaccinated adult,” Peek adds.
• antibiotics. “We recommend treating with antibiotics to kill any clostridial bacteria that are still present,” says Johnson. “Usually penicillin is the antibiotic recommended for this purpose.” Penicillin---our first mass-produced antibiotic---is very effective against organisms like clostridia as well as other anaerobic agents. But penicillin does pose challenges, says Peek: “Ideally the horse would be in a hospital situation, with an intravenous catheter and receiving the more expensive IV penicillin. Many people can only afford the conventional and cheaper procaine penicillin, given intramuscularly. This can be painful for the horse, giving large doses into the muscle in what is already a very sore and anxious horse, yet this may be a compromise that has to be made. Sometimes we also administer penicillin directly into the wound, as well as systemically, to get a greater local effect.”
Another antibiotic option is metronidazole. “Penicillin may promote convulsive signs in a horse with tetanus,” says Messer. “In certain instances metronidazole may be a better choice, and sometimes they are used together.”
• magnesium sulfate. Intravenous administration of magnesium sulfate has shown promise in human medicine. “This is a tactic that has been gaining more favor in treating horses,” says Messer. “The magnesium seems to interfere with the action of the toxin at the cell membrane and helps overcome some of the neurological signs ---neutralizing some of the spastic effects of the toxin. It has shown some promise, giving us another tool for treating the horse, to increase the chance for survival.”
• antitoxin. A purified serum derived from the blood of other horses who have been immunized against tetanus, antitoxin contains antibodies that provide immediate but short-term “passive immunity.” These antibodies can prevent further damage by binding with any toxins still circulating in the horse’s body and preventing them from attaching to neurons, but it cannot undo damage that has already occurred. Still, administering the antitoxin may help to slow the progress of tetanus in a horse who is not yet, or is only just starting, to show signs. “It’s controversial whether administering large doses of antitoxin are helpful once actual signs begin,” says Messer.
When you consider the wide distribution of C. tetani spores and how terrible the disease they can cause, the choice to vaccinate against tetanus is an easy one. Indeed, the tetanus vaccine is designated as a “core” vaccination by the American Association of Equine Practitioners, meaning that it is recommended for all horses regardless of age, activity level or location. Beyond vaccination, learning the basics---and even some lesser-known facts---about tetanus can help you protect your horse.