Tetanus, one of the oldest recorded diseases of horses, is caused by the bacterium Clostridium tetani.
Clostridium tetani is an anaerobic bacterium that is widespread in soil and can be carried in the intestinal tracts of many species.
The most common route of infection is through wounds, particularly deep puncture wounds that provide the anaerobic conditions favoured by the bacterium. The bacteria localise and multiply in the damaged tissue and produce a toxin known as a neurotoxin. Surgical procedures, post-surgical wounds and retained foetal membranes in the mare may also act as a route of entry for the harmful bacteria.
The majority of mammals are susceptible to the neurotoxin produced by the bacteria but horses and people are particularly sensitive to the effects of the toxin. Tetanus has a worldwide distribution. In general the occurrence of Clostridium tetani in the soil and the incidence of tetanus in man and horses is higher in warmer parts of the world.
The neurotoxin is absorbed by nerves in the affected area and travels up the nerve to the spinal cord. The toxin interferes with the release of inhibitory neurotransmitters in the nerve endings leading to excessive muscle contractions and spasm. Excess toxin can also be disseminated to the bloodstream and is carried to the central nervous system where it exerts its effect.
The incubation period for the disease is highly variable and ranges from one to several weeks. The clinical signs of the disease relate to the effect of the neurotoxins on the neuromuscular system. Muscle stiffness involving the neck and back muscles resulting in "a rocking horse stance" or stiffness of the masseter muscles resulting in "lock jaw" are often the initial presenting signs. Affected horses are also often recognised due to characteristic facial changes including erect ears, dilated nares and prolapse of the third eyelid, which may be exaggerated if the animal is tapped lightly on the side of the face.
As the condition progresses the stiffness becomes more generalised and even minor stimulation can result in tetanic muscular spasm. These episodes of muscular spasm continue to increase in intensity and frequency. The respiratory muscles may also be affected leading to increased respiration rate and eventually respiratory failure. In advanced cases the heart rate is increased, may be irregular and generalised sweating is evident. Horses with tetanus become seriously ill very rapidly and the fatality rate is high. In the advanced stage of disease the muscular spasms continue until seizures and recumbency develop, followed by death after a number of days.
Diagnosis is often based on clinical signs and history of a recent penetrating wound. Occasionally the penetrating wound may have gone undetected or may even be fully healed by the time clinical signs of tetanus appear. If the wound is accessible then demonstration of the bacterium in gram stained smears or by culture may be possible.
Treatment and Control
Treatment must be initiated as early as possible in the course of the infection to be effective. Antibiotic therapy is used to kill the toxin-forming bacteria in conjunction with tetanus antitoxin to combat the circulating tetanus neurotoxin. Variable results in the treatment of tetanus in horses have been achieved when tetanus antitoxin was administered into the subarachnoid space to supply the central nervous system with antitoxin. Sedatives and muscle relaxants are also used to negate the effects of the toxin. Supportive fluid therapy and the use of slings may also be necessary.
Good husbandry techniques to avoid the introduction of infection include management of the horse’s environment to prevent wounds and unnecessary trauma and the regular cleaning of stables with a suitable disinfectant. Surgical procedures and foaling should be carried out as aseptically as possible and horses should be turned out on clean pasture as soon as possible following the procedure.
Good management of wounds can be an aid in protecting your horse against tetanus. Any wound or surgical site may become contaminated; therefore contact your veterinary practitioner immediately for advice on protecting your horse against tetanus.
Even with intensive treatment the mortality rate remains high in horses. Vaccination is therefore essential to protect your horse from the most serious consequences of infection.
Both the mare and young foal are particularly susceptible to disease in the time around parturition. For this reason mares should be vaccinated to reduce the risk of mortality due to clostridial contamination of retained foetal membranes. An added benefit of vaccination of the mare is the transfer of antibodies to the naïve foal via the colostrum in early life. The colostrum derived antibodies protect the foal during the first four to six months of life before the foal’s own vaccination programme can begin. It is also common practice to give foals tetanus antitoxin at birth especially if the dam is unvaccinated.
A variety of tetanus vaccines are available on the market either as single component vaccines or as combined influenza and tetanus vaccines. MSD Animal Health markets both options. The duration of immunity of vaccines is variable. It is advisable to consult the product SPC or your veterinary practitioner for advice on correct vaccination timing and regimes.