Clostridium tetani (Tetanus)
Authors: James Campbell
Tetanus was first described in
The Clostridia genus is a diverse group of anaerobic spore forming gram-positive bacilli. They are widely distributed in the environment, and are found in the intestinal flora of domestic animals, horses, chickens, and man. In Clostridium tetani endospores are produced which are wider than the bacillus giving rise to the characteristic drumstick shape. The most noteworthy toxin mediated diseases associated with infection by this genus are tetanus (Clostridium tetani), and botulism (Clostridium botulinum).
Clostridium tetani is an obligate anaerobic bacillus, which is gram positive if processed immediately but which may stain inconsistently from tissue samples (8). The bacilli are 2mm x 0.5mm in size and usually occur singly although occasionally in chains. They possess flagellae and are motile when young. Older organisms lose their flagella after the development of a spore. The spores are extremely stable, and although boiling for 15 minutes kills most, some will survive unless autoclaved at 120oC, 15psi, for 15 minutes, which ensures sterility.
If culture of Clostridium tetani is to be carried out it is best to use pre-reduced oxygen blood agar plates incubated under anaerobic conditions. These plates should then be taken to the bedside in an anaerobic jar and inoculated with any necrotic material from the wound and then placed back into anaerobic conditions thus minimizing the time that the organisms will be subjected to atmospheric oxygen. Two cooked meat dextrose broths should also be used and after inoculation if a red, hot nail is added to the broths then this will quickly reduce the oxygen level in the broths and aid the recovery of the organism. On blood agar medium Clostridium tetani grows as an extremely fine, swarming layer over the surface of the plate. The colonies are rarely more than 1mm in diameter, slightly raised and have a ground glass appearance with a filamentous edge. Non-motile variants can produce colonies lacking this filamentous edge. They will show incomplete or a-haemolysis initially, followed by complete haemolysis caused by the tetanolysin. In chopped meat dextrose broth the medium becomes turbid and shows gas production. If two cooked meat dextrose broths are inoculated, to aid in the identification, one can be heated to 80°C for 10 minutes. In this process the vegetative organisms are killed and only the spores will remain. If Clostridium tetani is present these spores can then be re-incubated to give a growth of the organism.
Clostridium tetani does not ferment lactose, maltose, fructose, arabinose, mannose or xylose, but does produce a greenish fluorescence in MacConkey's Media containing neutral red. Agglutination identification is possible and ten serotypes have been defined, although of little use clinically this may be useful in the investigation of an outbreak. In routine practice few attempts are made to culture Clostridium tetani; it is difficult to culture, a positive result does not indicate if the organism contains the toxin producing plasmid, and Clostridium tetani may be present without disease in patients with protective immunity. This paucity of information on the bacteria in vitro means very little is known about antimicrobial sensitivity patterns, important if resistance were to develop in Clostridium tetani. Similarly there have been very few attempts to quantify the toxin load and assess the prognostic significance of this. If large amounts are produced the toxin may be transported by blood and the lymphatics as well as by direct entry into nerve fibres, hence more rapid and wider dissemination of the effects of the toxin.
The toxin is encoded on a 75kb plasmid and transcribed as a single polypeptide with a molecular weight of 150000, the complete amino acid sequence of the toxin is known (19, 20, 21). The polypeptide undergoes post-translational cleavage into two subchains, the heavy and light chain linked by a disulphide bond. The carboxyl terminal part of the H chain mediates attachment to gangliosides (GD1b and GT1b) on peripheral nerves subsequently the toxin is internalized (12). It then is then moved from the peripheral nervous system to the central nervous system by retrograde axonal flow and trans-synaptic spread. The light chain of the toxin acts as a zinc metallopeptidase, which cleaves synaptobrevin (42), a single base pair mutation in the light chain abolishes this proteolytic activity (35). Synaptobrevin is an integral membrane component of synaptic vesicles, when cleaved these vesicles containing the inhibitory neurotransmitter g-aminobutyric acid (GABA) cannot fuse with the presynaptic membrane and release their contents into the synaptic cleft. The alpha motor neurons are therefore under no inhibitory control and undergo sustained excitatory discharge causing the characteristic motor spasms of tetanus. The toxin exerts is effects on the spinal cord, the brain stem, peripheral nerves, at neuromuscular junctions and directly on muscles. To what extent cortical and subcortical structures are involved remains unknown, certainly the toxin is a potent convulsant when injected into the cortex of experimental animals.
The autonomic nervous system is also affected by the tetanus toxin, causing cardiac arrhythmias, severe sweating, and labile blood pressure. This is extremely difficult to manage and is a common cause of sudden death. As a consequence catecholamine levels are high and this may contribute to the high incidence of acute renal dysfunction seen in severe tetanus (14).
Tetanus typically follows deep penetrating wounds where anaerobic bacterial growth is facilitated. The most common portals of infection are wounds on the lower limbs, post-partum or post-abortion infections of the uterus, non-sterile intramuscular injections and compound fractures. However, even minor trauma can lead to disease and in up to 30% of cases no portal of entry is apparent (9). Tetanus has been reported following a myriad of injuries, including intravenous and intramuscular injections, acupuncture, ear piercing, and even from toothpicks. It can follow from chronic infections such as Otitis media (18, 40), and has been reported via a decubitus ulcer (36). Tetanus acquired following intramuscular injection with quinine is associated with a higher mortality than other modes of acquisition (53). In patients who have injuries more commonly associated with tetanus, deep wounds, contaminated with dirt, or faeces, should have the wound cleaned and be given antitoxin as well as active immunization.
The incubation period (the time from inoculation to the first symptom) can be as short as 24 hours or as long as many months after inoculation with Clostridium tetani. This interval is a reflection of the distance the toxin must travel within the nervous system, and may be related to the quantity of toxin released. The period of onset is the time between the first symptom and the start of spasms. These periods are important prognostically, the shorter the incubation period or period of onset the more severe the disease. Trismus (lockjaw), the inability to open the mouth fully owing to rigidity of the masseters is often the first symptom. Generalized tetanus is the most common form of the disease, and presents with pain, headache, stiffness, rigidity, opisthtonus, laryngeal obstruction and spasms. These may be induced by minor stimuli such as noise, touch, or by simple medical and nursing procedures such as intravenous and intramuscular injections, suction, or catheterization. The spasms are excruciatingly painful and can be uncontrollable leading to respiratory arrest and death. Tetanus can be localized at the site of injury causing local rigidity and pain. This form has the lowest mortality, although cephalic tetanus is local form with a higher mortality. A number of groups have attempted to devise scoring system to assess prognosis, the Dakar and Phillips scores are two examples (Table 1a and 1b). Both these scoring system are relatively simple schemes which take into account the incubation period, and or the period of onset as well as neurological and cardiac manifestations. The Phillips score also factors in the state of immune protection. The more clinical grading system developed by Udwadia is also useful (Table 1c) (48).
Respiratory failure, haemodynamic disturbance, and septicaemia, are the commonest causes of death. In those who survive sequaelae include contractures, chest deformities, fits, myoclonus, and the consequences of hypoxia. There is essentially very little information on follow up of patients after tetanus, particularly with regard to cognitive function. In one of the few studies to examine this question, Anlar found enuresis, mental retardation and growth delay to be frequent sequaelae after neonatal tetanus (4). This is an area of clinical research in tetanus, which deserves further attention.
The diagnosis is a clinical one, relatively easy to make in areas where tetanus is seen frequently, but often delayed in the developed world where cases are seen infrequently (43). The differential includes tetany, strychnine poisoning, drug induced dystonic reactions, rabies, and orofacial infection. The mainstay of management is supportive with adequate ventilatory support, sedation and muscle relaxation. The respiratory state should be assessed, the airway secured and ventilation initiated if necessary. A tracheostomy should be performed as soon as possible in generalized tetanus; this allows maintenance of the airway during laryngeal spasms and facilitates removal of secretions. Patients may need to be sedated, and paralyzed if mechanical ventilation is available. A nasogastric tube should be inserted and the wound cleaned and debrided if necessary. Passive immunization should be given (human tetanus immunoglobulin if available), and active immunization started. Antibiotics (Metronidazole or penicillin) should be given, and maintenance sedation continued. Sadly much of this ideal therapeutic approach is not applicable in hospitals where the vast majority of the world-wide cases of tetanus are seen. Adequate sedation and muscle relaxation is only possible if there are adequate facilities for ventilation, and equine antitoxin is much cheaper to produce than human.
The common complications in tetanus, such as nosocomial infection, bed sores, tracheal stenosis and gastrointestinal hemorrhage are often attributable to prolonged periods in intensive care. Secondary infections are a frequent complication, most commonly associated with the lower respiratory tract, catheterization, and wound sepsis. Gram negative organisms, particularly Klebsiella and Pseudomonas are common, Proteus and Staphylococcal infection are also frequently encountered. Rigorous attention to sterile technique and infection control is essential in a tetanus intensive care unit. In addition to these there are also problems unique to the disease. Cardiac and haemodynamic problems resulting from sympathetic over activity are seen in a significant number of patients with severe disease. Beta-blockers, magnesium, clonidine and labetolol have been used to treat autonomic dysfunction with mixed success (30, 32, 46, 51). The most complete work on the haemodynamic complications, by Udwadia, has shown that it is possible to substantially reduce the mortality in severe disease by careful attention to the patient’s haemodynamic and respiratory state (48). This seminal work deserves further attention, in particular elucidating the causes of sudden death that occur despite intensive monitoring. Acute renal dysfunction and failure is a common complication and is probably secondary to markedly labile blood pressure and severe autonomic dysfunction rather than to rhabdomyolysis or myoglobinuria. The pathology is acute tubular necrosis (44). The consequent metabolic derangement inevitably worsens the cardiac function, and hence the renal failure should be treated early with appropriate renal support either hemofiltration or dialysis. Unfortunately in most centers where tetanus is seen neither of these renal support systems are available. Other complications include haematemesis, compression fractures of vertebrae, constipation, and pulmonary emboli.
SUSCEPTIBILITY IN VITRO AND IN VIVO
The paucity of information on the bacteria in vitro means very little is known about antimicrobial sensitivity patterns, important if resistance were to develop in C. tetani.
Penicillin remains the standard therapy for tetanus in most parts of the world. The dose is 100000-200000 I.U.kg-1day-1 IM or IV for seven to ten days. Johnson and Walker were the first to report that intravenous administration of penicillin could cause convulsions, and went on to show, in animal models, that penicillin caused myoclonic convulsions when applied directly to the cortex (31). Penicillin became the standard model for induction of experimental focal epilepsy. The structure of Penicillin, distant to the b-lactam ring is similar to g-aminobutyric acid (GABA) the principal inhibitory neurotransmitter in the central nervous system. Penicillin therefore acts as a competitive antagonist to GABA. Penicillin does not readily cross the blood brain barrier, but in high cumulative doses it can cause CNS hyperexcitability. In tetanus this side effect of penicillin could synergize with the action of the toxin in blocking transmitter release at GABA neurons. Penicillin is bacteriocidal and acts by disrupting the cell wall causing it to become fragile and eventual rupture. Any change to the cell wall allows the release the exotoxin tetanospasmim which can increase the number of spasm that the patient will have. The intravenous route of administration also is problematic as C. tetani is a strict anaerobic organism and will not grow in the presence of oxygen and this means that the focus of infection must be anaerobic. C. tetani is not an invasive organism and does not move from the original focus of infection. Thus using the intravenous route it is doubtful how much of the antimicrobial reaches the focus of infection. Thus good tissue penetration is of prime importance.
Metronidazole is a safe alternative, and may now be considered as the first line therapy. Following rectal administration metronidazole is rapidly bioavailable and causes fewer spasms than repeated intravenous or intramuscular injections. Ahmadsyah was the first to compare penicillin and metronidazole, and showed a reduction in mortality in the metronidazole group (7% compared to 24%) (2). In a much larger study Yen recruited over 1000 patients and showed that there was no significant difference in mortality between the penicillin and metronidazole group (54). However, the 533 patients randomized to Metronidazole required fewer muscle relaxants and sedatives compared to 572 patients randomized to penicillin. This may be explained by the action of penicillin at GABAnergic synapses and may therefore apply to the third generation cephalosporins. The structure of these drugs is similar to that of penicillin and ceftazidime has been shown to induce absence seizures with spike and wave discharges (29). If metronidazole is available and applicable this should be considered as the drug of choice in the treatment of tetanus. The dose is 400mg rectally every 6 hours, or 500 mg every 6 hours IV for 7-10 days. Erythromycin, tetracycline, vancomycin, clindamycin, doxycycline and chloramphenicol would be alternatives to penicillin and metronidazole if these were unavailable or unusable in individual patients (6, 10). There is little or no indication for the use of other antibiotics in the management of tetanus. There is a need for an up to date assessment of the antimicrobial sensitivity patterns of clinical isolates of Clostridium tetani. Pyridoxine (Vitamin B6) is a coenzyme with glutamate decarboxylase in the production of GABA from glutamic acid, and increases GABA levels in animal models. In an unblinded open trial 20 neonates with tetanus were treated with pyridoxine (100 mg on day-1) and compared with retrospective records. The mortality in pyridoxine treated group was reduced (22). The role of pyridoxine in the management of neonatal tetanus should be re-examined in a blind randomized trial.
Corticosteroids have been shown to be of benefit in tetanus, however, as is often the case in studies on this disease the trials have not recruited enough patients to be convincing or have been inadequately controlled. In two studies, betamethasone has been shown to reduce the mortality, but only in small numbers of patients (11, 41). Corticosteroids are not recommended in the management of tetanus until further blinded controlled studies are conducted in large enough numbers to show significant differences.
Passive immunization with human or equine tetanus immunoglobulin shortens the course and may reduce the severity of tetanus. The human antiserum is isolated from a pool of plasma derived from healthy human tetanus immune donors, and has a half-life of 24.5-31.5 days. The equine (or bovine) form, widely available throughout the developing world has a higher incidence of anaphylactic reactions, has a half life of only 2 days, but is much cheaper to produce.
In established cases patients should receive 500-1000I.U.kg-1 of equine antitoxin intravenously or intramuscularly. Anaphylactic reactions occur in 20% of cases. In 1% they are severe enough to warrant adrenaline, antihistamines, steroids and intravenous fluids. If available 5000-8000 I.U. of human anti-tetanus immunoglobulin should be given intramuscularly, this has a lower incidence of side-effects. Anti-tetanus toxin was first used in 1893, and there was a dramatic fall in the incidence of disease amongst soldiers in World War 1 following its introduction. Although the antiserum will have an effect only on circulating and unbound toxin (demonstrated in the serum of only 10% of cases at presentation and in 4% of cerebrospinal fluid (50). It should be administered to all patients with tetanus. Whether it should also be infiltrated locally at the portal of entry is unclear and should be examined prospectively. For prophylaxis 1500-3000 I.U equine or 250-500I.U human anti-tetanus immunoglobulin should be given.
Passive immunization should be administered as soon as possible after the injury. Once the toxin is bound and internalized it will clearly have no effect. The blood level of passive antitoxin to protect a man against tetanus is approximately 0.1 I.U.ml-1. When 3000 I.U. are administered intramuscularly maximum levels are reached in 24-48 hours and adequate levels are maintained for 10-15 days. It is not easy to assess the optimal dose of anti-sera to give for prophylaxis. Extrapolation from animal work would suggest that these doses are too low and that 50 000 I.U. would afford greater protection, however at such doses the incidence of side effects is higher. The side-effects can be either acute anaphylactic reactions or delayed serum sickness. The former has an estimated incidence of 1: 200 000 individuals; the overall frequency of all reactions is approximately 5%. The incidence of immediate reactions can be reduced by simultaneous (or 15 minutes prior to use) injection with an antihistamine (promethazine). The use of the Besredka rapid desensitization method does not necessarily prevent anaphylactic reactions. The use of human tetanus immunoglobulin is very rarely associated with anaphylactic reactions, creates a longer duration of protective immunity and one can use lower doses (500-1000 I.U.). It is the passive immunization of choice; unfortunately, it remains unaffordable in many parts of the world.
Complete human immunoglobulin now can be engineered in vitro and designed for specific antigens (34). This raises the possibility of producing human antibodies specific for the tetanus toxin, free from the risks of infection, easy to store, and potentially available at a cost affordable in the developing world. Owing to its smaller size it is possible that the antigen-binding domain of the immunoglobulin, the Fab fragment, may gain better access to the toxin, and so enhance neutralization. Fab fragments can be produced from donors, but the engineered approach to antibody production would facilitate this.
Intrathecal therapy with anti-tetanus serum has been subjected to a number of clinical trials. A meta-analysis has concluded that there is currently no evidence of a beneficial effect in neonates or adults using equine or human tetanus immune globulin, and that the safety of their use intrathecally remains unproved (1). In addition to passive immunization active vaccination needs to be administered to all patients not previously vaccinated, so called active-passive immunization. This adds to the short-term immunity (passive), and to long term humoral and cellular immunity (active). As the former is declining the latter appears and thus avoids a window of non-protection. From experimental work in animals it is clear that the toxoid starts acting a few hours after injection and before a humoral response is detectable. Presumably the toxoid saturates the ganglioside receptors and prevents wild type toxin binding. The toxoid and the human (or equine) anti-tetanus immunoglobulin should be administered at different sites on the body to prevent interaction at the injection site. If both are to be administered together no more than 1000I.U human or 5000 I.U equine should be administered, higher doses can neutralize the immunogenicity of the toxoid.
Adequate sedation is essential in tetanus but is a double-edged sword. Benzodiazepines are the most commonly used sedative agents. Diazepam has a wide margin of safety, can be given orally, rectally or intravenously and is a sedative, an anticonvulsant and a muscle relaxant. It is also cheap and available in most parts of the world. However it has a long cumulative half-life (72 hours) and has active metabolites, in particular oxazepam and demethyldiazepam. Invariably in the doses required to achieve adequate control of spasms (often up to 3-8mgkg-1 day-1 in adults) respiratory depression, coma, and medullary depression are common. Establishing the correct therapeutic window is extremely difficult, particularly in patients requiring prolonged support. Midazolam and propofol are alternatives, but these are often not available or not affordable in regions where tetanus is seen frequently (25, 38). Neuromuscular blocking agents such as pancuronium and vecuronium are used in centers with adequate facilities for mechanical ventilation. Tetanus patients require sedation in addition to muscle paralysis.
Magnesium sulphate has been used both in ventilated patients to reduce autonomic disturbance and in non-ventilated patients to control spasms (5, 30). Magnesium is a presynaptic neuromuscular blocker, blocks catecholamine release from nerves and adrenal medulla, reduces receptor responsiveness to released catecholamines and is an anticonvulsant and a vasodilator. It antagonizes calcium in the myocardium and at the neuromuscular junction and inhibits parathyroid hormone release so lowering serum calcium. In overdose it causes paralysis and probably sedation/anesthesia, though this is controversial. In the paper by James, patients with very severe tetanus were studied and magnesium was found to be inadequate alone as a sedative relaxant but an effective adjunct in controlling autonomic disturbance (30). Serum concentrations were difficult to predict and regular monitoring of serum magnesium and calcium levels were required. Muscular weakness was readily apparent and ventilation was required in all cases. Attagyle studied patients at an earlier stage of the illness yet all cases were probably severe (5). They used similar doses of magnesium to try to avoid sedatives and positive pressure ventilation and reported successful control of spasms and control of rigidity. Magnesium concentrations were predictable and readily kept within the therapeutic range by using the clinical signs of the presence of a patella tendon reflex. In both studies the absence of hypotension and bradycardia was in contrast to results with beta blockade. Both groups agreed that tidal volume and cough may be impaired and secretions increased: tracheostomy is mandatory and ventilatory support must be readily available. More work is necessary to define the role of magnesium both with regard to the physiological effect it exerts on neuromuscular function in the presence of tetanus and secondly to establish what role if any it has in the routine management of severe tetanus.
Tetanus toxoid is produced by formaldehyde
treatment of the toxin, its immunogenicity improved by absorption with
aluminium hydroxide. Alum-absorbed tetanus toxoid is very effective at
preventing tetanus with a failure rate of 4 per 100 million immunocompetent
individuals. In the
Reactions to the tetanus toxoid are estimated to be 1 in 50 000 injections, although most are not severe, local tenderness, oedema, flu like illness and low grade fever are the most frequently encountered. Severe reactions such as the Guillian-Barré syndrome and acute relapsing polyneuropathy are rare (24, 27). In recent years there have been a number of reports from Australia and USA of tetanus occurring in patients over the age of 50 (26). In a survey from the USA 59% of women and 27% of men from an urban geriatric care centre did not have adequate anti-tetanus titres (3). For every child in the USA who dies of a vaccine-preventable disease, about 400 adults’ die of such a disease (23). There is a strong argument for the introduction of a vaccination strategy for the immunization of all adults at the age of 50.
Neonatal tetanus can be prevented by immunization of women during pregnancy. Two or three doses of absorbed toxin should be given with the last dose at least one month prior to delivery. Immunity is passively transferred to the fetus and protective antibodies will persist long enough to protect the baby. There is no evidence of congenital anomalies associated with tetanus toxin administered during pregnancy (45).
The influence of human immunodeficiency virus (HIV) infection on the transplacental transfer of tetanus specific maternal IgG is of critical importance. Polyclonal hyperimmunoglobulinaemia is common in HIV and may limit the transfer of protective maternal antibodies, as may HIV infection per se. The level of anti-tetanus antibody levels was lower in babies born to 46 HIV infected women than in a control HIV negative group, although still above 0.01 IU/ml (16). Approximately 10% of babies born to mothers with a placenta heavily infected with plasmodium falciparum my fail to acquire protective levels of tetanus antibody despite adequate maternal levels (7). The antibody response to tetanus vaccination is reduced in HIV-infected adult individuals with a CD4+ lymphocyte count ≤ 300x 106/L (33). HIV infected individuals who completed their vaccination course prior to acquisition of HIV should maintain adequate levels of protection against tetanus.
The World Health Assembly resolved to eliminate neonatal tetanus by 1995. Three years later the infection still kills over 400,000 babies a year. A safe effective vaccine has been available for most of this century. If any disease epitomizes the health care disparity between developing countries and the difficulties in overcoming that inequality, then tetanus is that disease. It is entirely preventable worldwide. The priorities must be in prevention; universal vaccination, and the development of simpler immunization schedules with longer protection. This will always be unfortunately relevant until vaccination covers the global community. The cost of this disease is high in both physical and financial terms. The health cost from a stay in an intensive care unit can range from nosocomial infections, mainly pneumonia (47) caused by multi-drug resistant bacteria to extreme pressure sores. The nursing has to be of highest quality to prevent the later as patients with severe tetanus are ridged and difficult to maneuver to avoid constant pressure on one specific body area. This lack of movement increases the chance of fluid collecting in the lungs with possible pneumonia but Parry and others showed that there was no difference in outcome by positioning the patient in a semi-prone position as opposed to prone (28). Financially it has been shown that in Viet Nam the approximate cost of treatment was 75$ US/day with an average stay of 29 days with a resultant cost of 3,480$ US (Campbell et al in press). The average weekly wage is 145 – 200$ US thus the financial burden is unsustainable for the patient and their family. On top of this there is a knock on effect in loss of productivity of the patient in the work place. Compare this to the cost of 5 vaccinations (~25$ US), the difference is marked. Parts of the developing world will continue to see large numbers of patients with tetanus. Further work on pragmatic solutions applicable in these countries is needed on how to reduce the high mortality. A better understanding of C. tetani, the toxin, its effects on the central and autonomic nervous system, and cardiac and respiratory function is needed. There is a tendency to accept a high mortality from tetanus, Udwadia and his colleagues have shown in India that it is possible to substantially reduce the mortality even in the absence of fully-fledged intensive care units (49).
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