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
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