TETANUS
► Definition: a paralytic disease caused by toxins of the bacterium Clostridium tetani
► Transmission: C. tetani is an anaerobic bacterium that normally inhabits the equine digestive tract. But when the bacteria pass into the soil, they form spores that can survive for years. When the spores encounter an anaerobic environment---such as a puncture wound that heals over on the surface---the bacteria reactivate, grow and multiply, producing a potent neurotoxin.
The toxins enter the motor nerves and travel through them to the spinal cord. As they spread through the nervous system, the toxins interfere with the release of neurotransmitters, causing the skeletal muscles to lock into rigid spasms. In some cases, the spasms may be strong enough to fracture bones. Paralysis of the muscles of the head and mouth (called “lockjaw”) make it impossible for the horse to eat or drink. If the spasms affect the larynx and diaphragm, the horse will suffocate. The incubation period may vary, but tetanus usually develops within 10 to 14 days after the initial wound, which can be so
minor it was never even noticed.
► Signs: The first sign of tetanus is localized muscle stiffness in the vicinity of the wound where the bacteria entered as well as in the jaw, neck and hind limbs. The horse’s tail will be rigidly elevated and he will move with a stiff gait. Within another day, the horse will experience generalized stiffness throughout his body as well as hyperesthesia---a hypersensitivity to sensory stimuli---which causes him to be extremely reactive to sounds and touches. He may be unable to walk and will stand with a characteristic “sawhorse” stance with stiff legs, an arched back and a backwardly arched neck. The horse may develop the characteristic facial expression, called “risus sardonicus” or the “sardonic grin,” which consists of drawn back lips exposing the teeth, nostrils rigidly flared, erect ears and prolapse of the third eyelid.
WEST NILE VIRUS
► Treatment: Early intervention is critical to successful treatment of tetanus. An antitoxin is available, which is injected directly into the fluid surrounding the brain, but it neutralizes only those toxins that have not yet bonded to neural tissue. Other treatments include the administration of antibiotics and various musclerelaxing drugs and pain medications, along with intravenous fluids. Infected wounds also need to be drained and cleaned. If the affected horse cannot eat he needs to be fed via nasogastric tube. Even with aggressive treatment, the mortality rate of tetanus is 50 to 75 percent.
► The vaccine: The vaccine against tetanus is a formalininactivated adjuvanted toxoid---the toxin is chemically rendered harmless, mixed with an agent that stimulates the immune system and injected into the horse. Unvaccinated mature horses receive a two-dose series, spaced four to six weeks apart. Thereafter, a single annual booster is recommended for all mature horses. An additional booster is recommended if a horse sustains a wound or undergoes surgery more than six months after his last vaccination.
The AAEP recommends vaccinating a pregnant mare four to six weeks prior to delivery, both to increase the antibodies in her colostrum and to protect her if the delivery is difficult. Foals get a three-dose series starting at 4 to 6 months of age, with four to six weeks between the first and second inoculation, and then a third administered at 10 to 12 months.
Because C. tetani spores are present in soils, especially on farms with large animals, it’s a good idea for you to stay up-to-date on your tetanus shots, too.