It's a scenario that's terrifying to imagine: Your horse's muscles are contracting in spasms, he's arching his back, and his body is rigid. The movements of his eyes and lips are unlike anything you've ever seen, rendering him unfamiliar. Brewing and multiplying deep in a puncture wound you might not even know he has is the dreaded Clostridium tetani, which is already wreaking havoc in his nervous system. He has tetanus. The good news is you're only imaging this, and generally tetanus, a disease long-dreaded in humans and other mammals, is preventable through vigilant vaccination.
Tetanus, often called lockjaw, continues to occur in horses unprotected by vaccination. This disease is caused by toxins produced by C. tetani, a spore-forming bacterium present in the digestive tract of many animals and in soils containing manure. Nat T. Messer IV, DVM, Dipl. ABVP, professor in equine medicine and surgery at the University of Missouri College of Veterinary Medicine, says these bacteria are always present in the environment.
"To decrease vigilance by not vaccinating is unwise," says Messer. "The only reason horse owners don't hear about cases is because so many horses are vaccinated. The vaccine is very effective, but like any vaccine is not 100% effective. I've seen cases of tetanus in horses that were appropriately vaccinated, and without vaccination there would be a much higher incidence of tetanus than what we see.
"Untreated, it is certainly fatal," Messer states. "There are treatment protocols that help, and some horses survive. Usually 50-75% of the horses that get tetanus will succumb to the disease, no matter what is done to treat them."
Surviving horses can return to normal, but it might take a very long time.
Wounds and Tetanus
Wound contamination is generally what leads to infection. A clean wound is not as likely to result in tetanus; it's usually a wound containing foreign matter such as soil--especially an enclosed puncture wound or contaminated surgical wound--that is most dangerous.
The incubation period for tetanus is usually one to three weeks, with some cases appearing even later after infection. The wound that allowed bacterial entry might be healed by the time infection is apparent. Spores can lie dormant in tissues after wound healing, and they might only produce toxins if the local oxygen level drops. If the wound damages tissue, it might create perfect conditions for spores to change/start multiplying immediately, or proliferation might be delayed until another injury at the same site causes additional damage.
"Often horses with the shortest incubation period and the most rapidly progressing clinical signs are the ones that don't sur-vive," says Messer. "If it's a short time between when clinical signs are first seen, and when the horse starts to go into spasms, that's a more serious form of the disease."
Much of the progression and outcome depends on how much toxin actually makes it to the spinal cord.
Tetanus is often caused by seemingly insignificant wounds. "Sometimes it's a wound that goes undetected, such as a puncture, or even a small cut that seals over, creating an anaerobic environment for bacteria to grow," says Messer. "There is still contamination within the wound, and the spores sporulate and develop, producing the organism that creates toxins. There are two toxins involved. One causes the clinical signs, and the other causes tissue damage, which creates a more favorable environment for the bacteria to grow."
That toxin continues to break down healthy tissue, enabling more bacteria to grow and produce the neurotoxin that causes clinical signs.
The toxins travel along the nerves until reaching the central nervous system. Then muscle spasms begin, and even mild stimulation can cause exaggerated responses. Amy Johnson, DVM, Dipl. ACVIM, a lecturer in the section of Medicine at the University of Pennsylvania's New Bolton Center, says the first sign you might see is a stiff gait; the horse travels with a choppy stride and might be reluctant to move.
"His muscles contract continuously, in spasm," she says. "Muscles of head and face contract, producing a classic facial expres-sion with ears erect and pointed backward, nostrils flared, and a sardonic grin--the muscles of the lips are pulled back like they're smiling, showing the teeth."
Another classic sign is prolapse of the third eyelid. "In subtle cases the horse might not show all of the signs, but if you make a threatening gesture toward his face or startle him, you'll see the third eyelid flash up," she adds. It rolls up over the eyeball and protrudes for a short time.
If the horse is agitated, not only will the third eyelid protrude, but he might also go into tetanic spasms. Messer explains, "The neck arches back (with head up) and the body becomes rigid." Front feet are farther forward than normal and hind legs farther back, and the tail sticks out straight behind. The disease was called lockjaw because jaw muscles become rigid and the horse can't eat. The horse might drool because he has difficulty swallowing.
"First signs generally show up around the head and face, then progress, but it may depend on where the wound occurred and how the toxin moves along the nerve pathways," says Messer. Signs can vary from horse to horse. "It is important to have a proper diagnosis, to make sure you're not dealing with some other neurological disorder. It fits into the differential of other neurological diseases that are characterized by stiffness and muscle rigidity. These might include equine motor neuron disease, botulism, or rabies," he says.
Treatment for tetanus involves intensive care. "You need to find the wound, if possible, and get it opened and cleaned up, removing all the necrotic tissue if you can. Make sure it is no longer an anaerobic environment," says Messer.
How quickly you begin treatment, or how swiftly you recognize, clean, and treat the wound, can make a difference in how severely the horse is affected. Johnson explains, "If you can treat the horse before the toxin has time to make it into the spinal cord, the horse will do better than if a wound is not recognized until the toxins bind to cells in the spinal cord."
It is important to combat the infection with antimicrobial therapy. Penicillin has been the traditional drug of choice, and many veterinarians still use it because it is effective against anaerobic infections such as clostridial ones.
Messer notes, "Currently some veterinarians are using metronidazole instead of penicillin, since the latter might tend to promote some of the convulsive signs in a horse with tetanus. Many people still use penicillin to treat suspected tetanus cases, but in certain instances metronidazole may be a better choice."
If the horse has not been vaccinated, or you don't know his vaccination history, and he sustains a deep wound or starts showing signs of tetanus, a veterinarian should treat him with antitoxin, says Johnson. "This will bind the toxins before they get to the spinal cord. If they've already made it there, antitoxin can't stop the disease, but can neutralize any toxins that have not yet made it to the spinal cord," she says. Therefore, promptly administering antitoxin might make the disease less severe.
Various combinations of sedatives and muscle relaxants will minimize spasms in the horse and keep him quiet. "We often give acepromazine. Sometimes valium helps, or even methocarbamol--a muscle relaxant," says Johnson.
Messer says putting the horse in a dark, quiet place is also helpful, to minimize stimuli that might cause him to go into spasms. "It's important to avoid a lot of handling of the animal," he explains.
The horse will need intravenous fluid therapy if he is unable to eat; this is less stressful to the horse than tube feeding. Using a sling is counterproductive because the horse usually resists, and this might stimulate more spasms. "I prefer to have the horse in a well-bedded stall, and make sure he is turned over frequently enough to prevent pressure sores," says Messer.
Intensive nursing care must continue until the horse is past the danger stage. Some won't recover, and some injure themselves during a spasm. "I've seen older horses break vertebrae by going into a severe spasm," says Messer. "They probably had more brittle bones (or some osteoporosis) compared with a young animal, and when they went into a tetanic spasm it caused a compression fracture of the thoracic vertebrae."
Johnson says, "It's important to vaccinate the horse you're treating. The amount of toxin that will produce clinical signs of tetanus does not always confer immunity (meaning just because a horse has clinical disease, it doesn't mean he'll be made immune to tetanus afterward). If the horse has tetanus, you should go ahead and vaccinate at the same time you are doing all the other treatment. This will make sure he won't get the disease a second time."
Vaccination with toxoid is usually very effective protection against tetanus. If a horse suffers a wound and has not had a booster during the past six months, most veterinarians advise giving a booster at that time. "How often to booster remains controversial," says Messer. "There are proponents of minimal vaccination who say protection against tetanus lasts for a number of years, so you don't need to vaccinate every year. Currently, the AAEP recommends annual vaccination (www.aaep.org/tetanus.htm)."
Johnson says, "If a horse is to undergo a surgical procedure and has not been vaccinated within the past six months, we usually booster that horse early."
Messer says for horses that don't have a history of immunization, the safest thing to administer would be antitoxin. "This is controversial, however, because tetanus antitoxin is not benign," says Messer. "There's risk for liver disease and serum hepatitis (which is fatal). If there was no chance the horse had been vaccinated, and it had a wound that was risky, such as a puncture in the foot, you should probably give that horse antitoxin, to be safe."
In this instance the veterinarian should inform the horse owner that there's a very small risk for serum hepatitis when using antitoxin. "There might be half of one percent chance that a horse might get serum hepatitis and die, but if you don't give it, and the horse develops tetanus, the horse has at least a 50% chance of dying," says Messer. "Tetanus antitoxin is a USDA-approved product for prevention of tetanus in nonimmunized horses.
"If a horse has been vaccinated in the past and just slipped by for a year--if the owner forgot to vaccinate--then giving a booster of toxoid would be adequate," Messer says.
It is important to keep up a broodmare's toxoid boosters so you don't ever have to give her or her foal antitoxin after birth (the incidence of serum hepatitis is much higher in postpartum broodmares). "People have a tendency to give broodmares tetanus antitoxin if there is no history of vaccination, but they need to be vaccinating those mares with toxoid during the last trimester of pregnancy," stresses Messer. "If a mare is vaccinated appropriately, and her foal gets antibodies via colostrum, the foal will be protected until six months of age, at which time it should be started on an immunization program."
If you follow vaccine guidelines and manufacturers' recommendations for vaccinating horses, the chances of your horse getting tetanus are very small.
About the Author
Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.
POLL: University Equine Hospitals