Many years ago, some long-time clients of the vet clinic brought in one of their pasture horses to be examined. The horse had developed a rather marked lameness in its hind end, and rather than let a bad problem get any worse, they thought it best to get the horse treated. Once at the vet clinic, dad began to do a thorough exam to determine the cause of this lameness. When he picked up the horse’s left hind foot, it was then that he found out why the horse was having difficulty putting weight on that left hind: a nail had pierced the frog of the foot and was still deeply embedded.

Dad extracted the nail and reviewed the horses’s vaccination records. He noted that the horse had not been vaccinated regularly, so he opted to keep the horse for a day or two. He was worried that such a deep puncture might go on to cause additional, more critical complications in the horse.

Several days later, sure enough, the horse’s muscles began to stiffen markedly. Its lips retracted, its jaws stiffened, and its tail began to lift. Dad knew what was happening with the horse: it was infected with equine tetanus.

Just What Is Tetanus?

Most people know of tetanus as what you might get for stepping on dirty, rusted nail that punctures your foot. But did you know that horses can become infected with tetanus, too?

Tetanus is caused by the bacteria Clostridium tetani, which is a normal inhabitant of a horse’s gastrointestinal tract. When viewed under the microscope, Clostridium tetani have a characteristic rod-like shape that is often said to look like a tennis racket or drumstick. The bacteria stain darkly purple with the Gram method of staining. The round bulging end of the organism, the shape likened to a tennis racket or drum stick, is called the terminal spore. Like all the Clostridium species of bacteria, Clostridium tetani is considered an anaerobic organism, meaning that oxygen must be removed in order for it to live. However, despite the fact that oxygen is not needed for life, the organism can survive in the spore state. The spore can be viewed as a seed that, when the environmental conditions are right, can germinate into a viable bacterium capable of replication and spore formation.

Signs of Tetanus

For a horse infected with tetanus, there very clear signs:

• Colic
• Muscle stiffness
• “Sawhorse” stance–the horse’s legs are extended in a tight, rigid fashion
• Retracted lips
• Flared nostrils
• Ears erect
• Stiffened jaw
• Elevated tail
• The eye retracts into the socket and the third eyelid spasms (this is commonly known as “flashing”)
• Sensitivity to sound

It may be as little as a few days to as long as a few weeks from the time the horse is infected with Clostridium tetani to showing signs of actual, but once the organism begins to multiply and produce toxin, the visibility of clinical signs can be rapid and progressive. Sometimes, the first obvious clinical sign noticed might be colic followed quickly by a vague stiffness. If the local site of infection is in a limb or foot, there can be an associated lameness. Also, some of the early, and sometimes subtle, signs are a slight retraction of the lips and flaring of the nostrils, a pulled downward and back position to the ears, a difficulty in opening the jaws, and a slight elevation of the tail.

Typically, within 24 hours of the initial signs, there is a generalized spastic activity of the large muscles required for standing; the legs are extended tightly in a ridged fashion, and the horses are said to adopt a “sawhorse” stance. In addition, the retraction of the eyeball itself into the socket and spasms–often referred to as “flashing”–of the third eyelid over the eye can occur. All of the muscle spasms can be evoked by sudden sound, movement, or touch. These horses can become recumbent and often will lie on their sides with their legs extended in rigid extension and their neck extended in an upward manner.

Mortality

Respiratory failure is the primary cause of death in horses infected with tetanus. Because tetanus affects the horse’s muscles adversely, the muscles of breathing such as the diaphragm and those located in between the ribs, also are affected, and respiratory distress can occur. Infected horses are also prone to developing pneumonia from the aspiration of feed material or water.

Unfortunately, horses infected with the tetanus have a 50% and higher mortality rate, and treatment and long-term care is both intensive and expensive. It should be noted that the survival of horses is strongly associated with that horse having been previously vaccinated for tetanus.

Preventing Tetanus in Your Horse

Fortunately, equine tetanus is relatively rare due to aggressive vaccination programs such as that advocated by the American Association of Equine Practitioners, “Tetanus toxoid is a core equine vaccine and is indicated in the immunization program for all horses.” However, for wild horses or horses in areas where vaccination programs are not as common, equine tetanus remains an issue.

Horse owners often think that because human tetanus shots are good for several years, then the same is true for horses. This is not the case. Equine titers lose strength after eight months, hence the mandate of annual equine tetanus shots. If a horse suffers a wound or undergoes surgery and has not had a tetanus shot within the past six months, conscientious veterinarians will issue a tetanus shot as a precaution.

This tetanus vaccination schedule was developed by the American Association of Equine Practitioners.

• Adult horses, previously vaccinated against tetanus: Vaccinate annually. Horses that sustain a wound or undergo surgery 6 or more months after their previous tetanus booster should be revaccinated with tetanus toxoid immediately at the time of injury or surgery. Note: The severity of the wound does not predict the risk for development of tetanus. Superficial wounds have resulted in clinical tetanus in horses.
• Adult horses, previously unvaccinated against tetanus, or of unknown vaccinal history: Administer a primary two-dose series of tetanus toxoid with a 4-to-6-week interval between doses. Protective concentrations of immunoglobulin are usually attained within 14 days of the second dose of vaccine. Vaccinate annually thereafter. Tetanus antitoxin is indicated to provide passive immunity in situations where the horse is at risk of tetanus infection and has not been immunized according to labeled recommendations for tetanus. If the veterinarian determines that administration of tetanus antitoxin is indicated, then it should be administered in one site and the initial dose of a priming series of tetanus toxoid vaccinations in a distant muscular site. The rare, but fatal, risk of Theiler’s disease consequent to the use of tetanus antitoxin needs to be taken into consideration when determining if use is indicated.
• Pregnant mares previously vaccinated against tetanus: Vaccinate annually 4-6 weeks before foaling, both to protect the mare should foaling-induced trauma or retained placenta occur and to enhance concentrations of colostral immunoglobulins.
• Pregnant mares unvaccinated against tetanus or of unknown vaccinal history: Administer a two-dose primary series of tetanus toxoid with a 4 to 6 week interval between doses. Revaccinate 4-6 weeks before foaling.
• Foals of mares vaccinated against tetanus in the prepartum period: Administer a primary three-dose series of tetanus toxoid beginning at 4 to 6 months of age. A 4-to-6 week interval between the first and second doses is recommended. The third dose should be administered at 10-12 months of age.
• Foals of unvaccinated mares or mares of unknown vaccinal history: Administer a primary three-dose series of toxoid beginning at 1-4 months of age with 4-week intervals between doses. Serologic data indicates that a three-dose initial series produces a more consistent anamnestic response in all foals, regardless of the age at which the series is initiated. Tetanus antitoxin is indicated to provide passive immunity in situations where a foal is born to a non-vaccinated mare and is at risk of tetanus infection.(See Tetanus antitoxin above.)
• Horses having been naturally infected with tetanus and recovered: Revaccinate annually.

And while you’re at it, when is the last time that you had a tetanus booster? Ask your (human) doctor for one the next time you’re in for a physical.

References

• www.aaep.org/info/tetanus
• http://www.thehorse.com/articles/10604/equine-tetanus-signs-and-treatment