15.20F: Tetanus
- Describe the mode of transmission and mechanism of action for Clostridium tetani
Overview
Tetanus is a medical condition characterized by a prolonged contraction of skeletal muscle fibers. The primary symptoms are caused by tetanospasmin, a neurotoxin produced by the Gram-positive, rod-shaped, obligate anaerobic bacterium Clostridium tetani .
Infection and Symptomatic Effects
Infection generally occurs through wound contamination and often involves a cut or deep puncture wound. C. tetani is not invasive, and the infection is normally confined to a wound. Here the bacteria multiply and produce tetanospasmin, which is able to travel throughout the body. Tetanospasmin is an A-B toxin. The B subunit binds to the receptors on motor neurons, while the A subunit induces endocytosis to enter the neuron. Early symptoms of the disease include restlessness, irritability and difficulty swallowing. As the infection progresses, muscle spasms develop in the jaw (thus the name “lockjaw”) and elsewhere in the body. Infection can be prevented by proper immunization and by post-exposure prophylaxis. Tetanus often begins with mild spasms in the jaw muscles (lockjaw). The spasms can also affect the chest, neck, back, and abdominal muscles. Back muscle spasms often cause arching, called opisthotonos. Prolonged muscular action causes sudden, powerful, and painful contractions of muscle groups. This is called tetany.
Tetanus affects skeletal muscle, a type of striated muscle used in voluntary movement. The other type of striated muscle, cardiac or heart muscle, is not affected by the toxin because of its intrinsic electrical properties. The incubation period of tetanus can be long, and may be as long several months, but is usually about eight days. In general, the further the injury site is from the central nervous system, the longer the incubation period. The shorter the incubation period, the more severe the symptoms.
Sources of Infection
Tetanus is often associated with rust, especially rusty nails, but this concept is somewhat misleading. Objects that accumulate rust are often found outdoors, or in places that harbor anaerobic bacteria, but the rust itself does not cause tetanus nor does it contain C. tetani bacteria. The rough surface of rusty metal merely provides a prime habitat for a C. tetani endospore to reside, and the nail affords a means to puncture skin and deliver endospore into the wound. An endospore is a non-metabolizing survival structure that begins to metabolize and cause infection once in an adequate environment. Because C. tetani is an anaerobic bacterium, it and its endospores survive well in an environment that lacks oxygen. Hence, stepping on a nail, rusty or not, may result in a tetanus infection, as the low-oxygen (anaerobic) environment is provided by the same object that causes a puncture wound, delivering endospores to a suitable environment for growth.
Diagnosis, Treatment, and Prevention
There are currently no blood tests that can be used to diagnose tetanus. The diagnosis is based on the presentation of tetanus symptoms. Diagnosis does not depend upon isolation of the bacteria, which is recovered from the wound in only 30% of cases and can be isolated from patients without tetanus. The “spatula test” is a clinical test for tetanus that involves touching the posterior pharyngeal wall with a sterile, soft-tipped instrument, and observing the effect. A positive test result is the involuntary contraction of the jaw (biting down on the “spatula”); a negative test result would normally be a gag reflex attempting to expel the foreign object.
Unlike many infectious diseases, recovery from naturally acquired tetanus does not usually result in immunity to tetanus. This is due to the extreme potency of the tetanospasmin toxin; even a lethal dose of tetanospasmin is insufficient to provoke an immune response.Tetanus can be prevented by vaccination with tetanus toxoid. The CDC recommends that adults receive a booster vaccine every ten years, and standard care practice in many places is to give the booster to any patient with a puncture wound who is uncertain of when he or she was last vaccinated, or if he or she has had fewer than three lifetime doses of the vaccine. The booster may not prevent a potentially fatal case of tetanus from the current wound as it can take up to two weeks for tetanus antibodies to form.
A person infected with C. tetani can be treated with antibiotics, which will kill the multiplying bacteria but will have no effect on the endospores or the toxin. To combat the effects of the toxin, tetanus immune globulin (TIG) antitoxin can be given to the patient. These antibodies are able to neutralize the tetanospasmin if they are not already bound to motor neurons, and can confer passive immunity.
Key Points
- Infection generally occurs through wound contamination and often involves a cut or deep puncture wound.
- There are currently no blood tests that can be used to diagnose tetanus.
- Tetanus can be prevented by vaccination with tetanus toxoid.
Key Terms
- tetanus : A serious and often fatal disease caused by the infection of an open wound with the anaerobic bacterium Clostridium tetani, found in soil and the intestines and feces of animals.
- opisthotonos : A tetanic spasm in which the body is bent backwards and stiffened.
- neurotoxin : A toxin that specifically acts upon neurons, their synapses, or the nervous system in its entirety.
- tetanospasmin : The A-B toxin produced by C. tetani which is responsible for the symptoms of tetanus.