Equine Tetanus: Signs and Treatment

It's a beautiful sunny day and being that nothing exciting is happening in the pasture today, you decide to lie down and roll a bit. In the midst of an extremely satisfying roll, there is a sudden sharp pain in your rump area, but it goes away as fast as your initial perception of it--no big deal. Unbeknownst to you, that sharp pain was a small piece of old and dirty wire coated with a plethora of bacteria that violated your skin and about two inches of muscle tissue. The skin wound quickly seals over and heals, but a small population of bacterial spores germinates and starts to proliferate in the damaged muscle tissue. As this family of bacteria begins to flourish in the muscle tissue, a small protein that is manufactured by the bacteria is released and eventually enters the bloodstream. During the elapsed time, you hardly notice a thing wrong; you go about the daily routine of eating and whatnot. The protein now has made its way to the spinal cord and is starting, ever so slightly at first, to wreak havoc on your nervous system's control over your muscular system. You start to feel stiff and cannot see well as your third eyelid protrudes up and obscures vision. The slightest noise startles you and makes all the muscles in your body contract and spasm; you want to flee in panic, but the more you panic, the more your muscles spasm. It becomes impossible to move or even eat, and you stand with a wad of half-chewed grass clenched between your teeth. Eventually when startled, you fall, unable to rise again. In the end you fearfully suffocate from an inability of your muscles to allow you to breathe, and you die -- the grass still clenched between your teeth.

Michael A. Ball, DVM

Horse suffering from tetanus--note the hay in clinched jaws and the "flashing" of the third eyelid. The ears have had cotton placed in them to reduce sound stimulation of the muscle spasms.

This is my visualization of what a horse dying from tetanus experiences. Some might feel that this is an over-dramatized rendition, but having personally observed five horses die from tetanus, I believe this horror to be real. The saddest part is that it could have been prevented.

Tetanus is caused by the bacteria Clostridium tetani that when viewed under the microscope have a characteristic rod-like shape that often is reported 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 (the absence of oxygen is necessary for it to live). But, 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.

Spore formation is an extremely good mechanism this bacteria has evolutionarily developed in order to survive. Commonly found in soil, the spores of Clostridium tetani have been shown to be viable for years. They are very resistant to heat and drying. In fact, the spores have been shown to be resistant to being killed at temperatures of 100ï¿½ F for up to an hour. In addition, it has been shown that the spores of Clostridium tetani are very resistant to chemical destruction--it can take up to several hours of direct contact with some very harsh disinfectants to destroy the spore.

This organism was first isolated in 1884 from mice that had been inoculated with common soil. Five years later, the organism was obtained in pure culture by making use of its heat stability, which killed other organisms and "selected" for the heat-resistant Clostridium in necrotic tissue. In 1890, there was presentation of scientific work that had identified bacterial toxins, the first of which was the toxin produced by Clostridium tetani.

As mentioned before, there can be large amounts of Clostridium tetani in soil, especially on farms. The organism is considered to be a primary contaminant of soil, but an increased amount on farms can be related to the fact that it is very common to find this organism in the manure of horses and cows. (Have you had your tetanus booster?). Infections in both humans and animals typically occur via contamination of a wound with the organism. Due to the anaerobic nature of the organism, the deep puncture wounds that have some tissue damage associated with them often are involved. However, it is not always puncture wounds and dirty pieces of metal that are the inciting cause; surgical wounds also can be a source of the infection. It is very important for you to keep specific vaccination records, indicating the accurate date of vaccination, so when your veterinarian needs this information, it is easily at hand.

The toxins produced by Clostridium tetani, and its relative Clostridium botulinum, are two of the most potent toxins currently known; a quantity the size of a pin head easily can cause disease and death. There actually are several toxins produced by Clostridium tetani. Some of these toxins serve to destroy tissue and decrease the blood supply locally, a process that further improves the environment for growth and replication of the organism. This neurotoxin has been shown to enter the bloodstream from a site of infection and bind to the many nerves that are associated with muscle tissue. From the site of the nerve-muscle junction, the toxin travels up the nerves and binds tightly to very specific areas of the spinal cord.

At Work In The Body

In order to understand how the toxin produces its effects, we must first discuss the normal interaction between the nervous system and the muscular system. A muscle can contract via conscious or unconscious direction. An example of an unconscious contraction is the reflex associated with getting burned--the signal enters your finger, goes up the nerve in your arm to the spinal cord, relays there (just like an electrical switch), and sends a signal right back down another nerve triggering the muscles in your arm to contract and jerk away from the hot stuff. In the relay area of the spinal cord, it also sends a signal up to your brain just to let you in on what's going on. On the conscious side of things, you want to pick up that cup of coffee and the brain sends a signal down the spinal cord. It relays there, and then signals are sent to the muscles, causing them to contract.

The "relay" area of the spinal cord is the point where the Clostridium neurotoxin has its effect. In this area of the spinal cord there are cells that act to inhibit or dampen excitatory signals. Many of the interactions between the nervous system and the muscular system are not "all on" or "all off" events--they are a summation of excitatory and inhibitory stimuli occurring concurrently. It is the modulation of signals by these so-called "interneurons" that help to coordinate the whole neuromuscular interaction.

The neurotoxin "binds" to these interneurons and inactivates their ability to inhibit excitatory signals--with that delicate balance disrupted, the excitatory signals predominate and cause intense and persistent contraction of the muscles. This is an important point, as once the toxin is "bound" to the cells in the spinal cord, there is no therapeutic intervention to dislodge it--the effect must slowly wear off.

As mentioned before, the organism can infect wounds of all kinds, but the puncture wounds or those with heavy trauma to the tissue, where an anaerobic environment is created, are more probable. Infection of the umbilicus with Clostridium tetani in a newborn also can be a cause of tetanus. In addition, Clostridium infection related to trauma during the birth process or a retained placenta also is an occasional cause of tetanus in the post-partum mare.

Signs Of Disease

From the time of inoculation of tissue with Clostridium tetani to the time of toxin producing infection might be as long as weeks, but once the organism begins to multiply and produce toxin, the course 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 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.

The muscles of breathing, the diaphragm and those located in-between the ribs, also are affected and respiratory distress can occur. In fact, horses which die from tetanus generally do so from respiratory failure. They also are prone to developing pneumonia from the aspiration of feed material or water.

Tetanus Treatment

The degree of disease is related to the degree of infection, the quantity of toxin produced, and the immediacy of therapeutic treatment. By and large, the therapy is supportive once the diagnosis is made. If possible, the site of infection is identified and addressed by drainage, flushing, and antibiotics--Clostridium species generally are quite sensitive to penicillin. Sometimes the site cannot be located or easily attended without causing extreme distress to the horse. The use of sedatives and muscle relaxants can give some relief to these signs, and placing cotton in the ears also can help with respect to sound-induced spasms. Stabling or hospitalization in a dark, quiet stall with as little handling and disturbance as possible is desirable.

Good footing is very important. If an affected horse manages to lie down, it can have extreme difficulty getting back up; slippery footing can result in needless trauma and, at worst, a fracture.

If the diagnosis is made early, a commercially manufactured antitoxin can be administered, but remember that once the toxin is bound to the nervous system, the antitoxin will have no effect. The antitoxin does serve to neutralize any toxin that is still circulating in the bloodstream at the time of its administration. Another important aspect of therapy is to maintain hydration and nutritional needs. Depending on how seriously affected, these horses might need intravenous fluid therapy and a nasogastric feeding tube.

The mortality rate for horses suffering from tetanus is reported to be as high as 50%. It can take up to two weeks to see any improvement, and clinical signs can persist for a month or more. The general belief is that if a horse with tetanus survives for seven days, there is a fair to good prognosis. However, this also is a time when the exhausted animal might start to lie down and get up more frequently, increasing the chances for trauma related to that activity.

As can be seen, tetanus is a horrible disease that is expensive to treat and, regardless of extreme therapeutic intervention, carries a high mortality rate. The vaccine is not perfect (there have been reports of vaccinated animals developing the disease), but it is extremely good. Horses with unknown vaccination histories should be vaccinated and boosted in accordance with your veterinarian's recommendations. A pregnant mare in her last month of gestation can be boosted to ensure both her protection and the development of the foal's immunity via its ingestion of colostrum after birth. If the mare was not boosted and the vaccination status is unknown, the newborn foal is generally given a dose of antitoxin and vaccinated at the same time shortly after birth and boosted accordingly. In addition, booster vaccination is generally recommended prior to any surgical procedure or after injuries if vaccination status is not current. Again, check with your veterinarian regarding specific vaccination protocols.

About the Author

Michael Ball, DVM


