Tetanus

M W ODENDAAL AND N P J KRIEK

GENERAL INTRODUCTION: ENDOSPORE-FORMING GRAM-POSITIVE RODS AND COCCI Neurotoxin-producing group of Clostridium spp.

Introduction

Tetanus is a non-contagious, almost invariably fatal neurointoxication, particularly of horses and sheep, occasionally of cattle, goats, pigs and humans, and rarely of dogs and cats. The disease is caused by Clostridium tetani and usually develops after deep, penetrating wounds have been contaminated by the organism. Necrotic tissue in such tissue injuries creates the necessary anaerobic environment which allows the organism to grow vegetatively and produce its potent neurotoxin, tetanospasmin, which causes the rigidity and muscle spasms characteristic of the disease.

Owing to its distinctive clinical features, tetanus is usually readily recognizable, particularly in the later stages of the disease. It was described by Hippocrates in the early records of medical history and, even in modern-day human and veterinary medicine, it remains an important clinical entity that requires prophylactic vaccination. In the developing countries it is a serious public health problem, with 50 000 human fatalities being reported each year.²⁷ Sporadic cases or outbreaks of tetanus occur in humans and animals throughout southern Africa.^{15, 39}

Aetiology

Clostridium tetani is a Gram-positive bacillus in young cultures, becoming Gram-negative in cultures that have been incubated for longer than 24 hours. The bacteria occur singly or in pairs, and measure 0,5–1,7 × 2,1–18,1 μm.⁷

The bacterium is moderately fastidious in its anaerobic requirements. After two days’ incubation on blood agar, the colonies are 2 to 5mm in diameter, slightly raised, feathery, semitranslucent and grey. They have an irregular or rhizoid margin and are surrounded by a zone of haemolysis.^{7, 33} Most C. tetani cells are motile due to the presence of peritrichous flagella which allow them to swarm on media with a moist surface. Non-motile strains also occur. The most characteristic microscopic morphological feature of C. tetani is the development of round to spherical terminal spores which cause the bacterium to bulge, giving it a typical ‘drumstick’ appearance. The spores are unable to retain Gram’s stain, and are resistant to many standard disinfection procedures, including steam heat at 100 °C for 30 minutes, but can be destroyed by autoclaving at 115 °C for 20 to 30 minutes. Spores germinate under anaerobic and aerobic conditions in the presence of certain substrates.³³

Clostridium tetani grows well on most laboratory media, its nutritional requirements being satisfied by various growth factors in the peptones and tissue extracts present in the media. The optimum temperature for growth is 37 °C, though there is still moderate growth at 30 °C, but no growth at 25 °C. Its growth in cultures is also inhibited by the presence of 20 per cent bile or 6,5 per cent sodium chloride.

The organism is metabolically relatively inactive and does not produce acid from carbohydrates, lecithinase or lipase, and neither does it hydrolyse esculin or reduce nitrate. ⁷ It is a typical example of a non-saccharolytic and only slightly proteolytic Clostridium. Certain strains may, however, ferment glucose and inositol, and most strains hydrolyse gelatine³⁴ and produce indole^.25

Clostridium tetani is susceptible to metronidazole, penicillin, tetracycline, erythromycin, chloramphenicol, clindamycin and augmentin, but it is resistant to the aminoglycosides and most of the cephalosporins.^{7, 25}

The vegetative forms of C. tetani synthesize two very potent toxins: tetanospasmin and tetanolysin. A structural gene on a plasmid codes for the production of the toxins, the absence of the gene being the reason for the occurrence of non-toxigenic strains of C. tetani.¹¹ Tetanolysin (a haemolysin) is an oxygen-labile, membrane-damaging toxin that plays no role in the pathogenesis of tetanus.³⁵ The main toxin, tetanospasmin, is a potent neurotoxin and is produced intracellularly in the bacterium, being discharged into the surrounding medium as the organisms die, undergo autolysis and disintegrate. It is very susceptible to heat, being destroyed by 20 minutes’ exposure at 60 °C,⁹ and can be produced in vitro and purified within six days.²⁴ Tetanospasmin has a molecular weight of 150 000 Daltons, and is composed of a heavy chain (100 000 Daltons) and a light chain (50 000 Daltons) linked by a single, disulphide bond. It appears that the light chain is the toxic portion of the molecule and that the interchain disulphide bond plays an important role in membrane translocation of the toxin into nerve cells.²⁸ In mice, tetanospasmin is lethal at a dosage of 1 ng/kg body weight, whereas the lethal dose is 0,3 ng/kg in guinea pigs and varies from 0,5 to 5 ng/kg in rabbits.¹² More details of the structure and actions of tetanus toxin can be obtained from a number of reviews.^{21, 22}

Epidemiology

Tetanus occurs wherever animals are farmed. Susceptibility varies considerably among the different mammalian and avian species, with horses and humans being the most sensitive to the effects of the toxin, followed, in order of sensitivity, by sheep and goats, mice, rats, rabbits, monkeys, dogs, cats, pigs and cattle, while birds are the most resistant.^{1, 16, 34}

The spores of C. tetani are present in soil, dust, and the faeces of most herbivores^{6, 39} but their distribution is unequal, which...