Toxicity Caused by Use of Bacterial Toxins | Toxicology

After reading this article you will learn about the toxicity caused by use of bacterial toxins.

Chiefly two types of toxins namely exotoxins (protein toxins) and endotoxins are produced by bacteria. Exotoxins are proteins excreted by bacteria in the growing medium. They are specific to tissues and cells, highly toxic, unstable at 60°C and highly antigenic.

They can be converted to toxoid by chemical treatment.

Based on their specific actions they may be:

(i) Cytotoxic e.g. staphylococcus alpha and beta toxins, CI. welchii (CI. perfringens) alpha toxins and diphtheria toxins

(ii) Neurotoxic e.g. tetanus and botulism toxins

(iii) Erythrogenic toxins (Dick toxins) e.g. streptococcus toxins

(iv) Staphylococcal enterotoxin which is thermo-stable, withstanding heating to 100°C for 15 min. It is responsible for food poisoning and

(v) Lethal toxins e.g. toxins produced by CI. welchii.

Endotoxins are intracellular substance released after bacterial degradation. They are non specific in action and induce general symptoms e.g. fever etc. They are weakly toxic, less antigenic and thermo-stable.

Endotoxins are found on bacterial cell surface identical to ‘O’ antigen (lipid A) composed of polysaccharide phospholipids and protein. They are found in Gram- negative bacteria. Toxiod can not be produced. Enterotoxaemia, tetanus and botulism are common diseases of animals and poultry caused by exotoxins.

Enterotoxaemias:

Toxins produced by Clostridium perfringens cause enterotoxaemias. There are six types of the organism, e.g. A, B, C, D, E and F. Type B and C toxins cause severe enteritis, dysentery, toxaemia and high mortality in young lambs, calves, pigs and foal. Type C causes enterotoxaemia in adult cattle, sheep and goats and in piglets during first few days of life.

In adult sheep the disease is known as struck. Foal enterotoxaemia is due to B type whereas type B and C cause calf enterotoxaemia. Type D enterotoxaemia (pulpy kidney disease) is noticed primarily in young sheep. Central nervous system is affected. Incoordination, convulsions and sudden death are quite common symptoms.

Enterotoxaemia may be confirmed by detection of toxins in the intestinal content. Specific hyper immune serum and antimicrobials are used for treatment. Immunization of pregnant animals by toxoid is advocated for prevention of disease.

Tetanus:

It is a toxico-infectious disease. Toxaemia is caused by a neurotoxin (tetanospasmin) released by Clostridium tetani growing in infected punctured wound and necrotic tissues. Almost all mammals are prone to tetanus, horses being most susceptible. Birds are, however, resistant.

The disease may be associated with shearing, docking and castration. Bacteria remain localized at the original site of infection but disease producing toxin spreads from wound along motor nerves to CNS. Tetanus toxin acts on nerve cells in the cerebrospinal axis. Toxin causes spasmodic tonic contractions of the voluntary muscles by nerve cell irritation.

It is heat labile (5 min at 65 °C) protein in nature. Its molecular weight is 60000-67000. Acid and digestive enzymes destroy the toxin. Its 0.09 x 10³ µg is lethal to a mouse. One ml culture filtrate can destroy 50,000 to 75,000 guinea pigs. One mg of the toxin can kill 50-75 million mice. After an incubation period of 1 to several weeks local stiffness involving massator muscle and muscles of neck, hind limb and wound area is recorded which is followed by general stiffness one day later.

Spasm of head muscles poses difficulty in prehension and mastication of food leading to lock jaw. Prolapse of third eye lid, dilatation of nostrils and high response to normal stimuli are other symptoms. General spasm causes increased action, rapid breathing and congestion of mucous membranes. Sheep, goat and swine often fall to the ground and develop opisthotonus. Temperature may rise to 42-43°C. Death occurs in 15-10 days. Mortality may be up to 80%.

Treatment and Control:

In early phase of symptoms, animals are successfully treated with antitoxin (anti-tetanus serum). In horses tranquilizers or barabiturates in conjunction with 300,000 units of tetanus antitoxin every 12 hour intervals are found quite effective in treatment. Penicillin and broadspectrum antibiotics are also advocated for elimination of organism. Animals should be housed in quiet dark stall.

Chlorpromazine is injected (0.4 mg/kg body wt. i.v. or 1 mg/kg body wt. IM) twice daily for 8-10 days in cattle and horses for relaxation of muscle tetany. Disease is prevented by active immunization with tetanus toxoid. Mares should be vaccinated during the last 6 weeks of pregnancy and foals at 5-8 weeks of age. Oxidising disinfectants such as iodine or chlorine is used for post-operative dressing.

Botulism:

It is an example of intoxication. The disease results from ingestion of preformed botulinus toxin produced as a result of the growth of casual bacterium Clostridium botulinum in decomposing animal tissues and plant materials. A rapid flaccid motor paralysis of skeletal muscles causes death of animals in 2-4 day.

There are 8 types of organisms. Types A, B and E cause human botulism, C-alpha in ducks and chickens, C- beta and D in cattle and C-beta in horses. Common sources of toxin are decaying carcasses of domestic and wild animals and water fowls and vegetable materials such as decaying grass, hay, grain and silage. Dogs, cats and swine are resistant.

It is most common in birds. Toxic infectious botulism is the name given to the disease in which organism grows in tissue of a living animal producing toxin and disease. This is common in foals and horses. Toxin is highly potent, i.e. 1 mg toxin causes death of 40 million mice. It is protein with molecular weight of 10, 00,000 (Type A toxin).

It is destroyed by boiling (100°C) for 10 min. but not by gastric juice. Being neurotoxic, it acts on peripheral, somatic and autonomic nerve fibres. Passage of impulses from nerve to motor end plates are affected. Acetylcholine secretion is inhibited.

Symptoms are muscle paralysis, disturbed vision, difficulty in chewing and swallowing and death due to cardiac and respiratory paralysis. In cattle, disease is known as Lamsiekte in South Africa. It is caused by type D toxin. Cattle suffer from disease due to eating of decomposed carcasses contaminated with toxins. In cattle normal skin sensation, sternal to lateral recumbency, drooling of saliva, inability to urinate and dysphagia are common symptoms. In poultry the disease is known as limber neck. It occurs due to paralysis of neck muscle.

Disease is diagnosed by eliminating other causes of motor paralysis and detecting toxin in suspected animal tissues or feed. Filtrates of stomach and intestinal contents are tested for toxin in experimental animals. In toxic infectious bolulism, the organism is cultured from tissues of affected animals.

Control:

Correction of diet and disposal of carcasses, removal of decaying grass and spoiled silage from the diet of animals, use of toxoid for immunization of cattle and antitoxin for ducks are advocated.