In contrast to enteric fever which is relatively uncommon and cholera which has been almost unknown in Britain for the last 100 years, there is an increase in the reported incidence of food poisoning, of which that of bacterial origin is by far the most common type.
Food poisoning may also be due to intestinal allergy, e.g. to shellfish, or to children eating unripe fruit or other unsuitable foods. Rarely a poisonous substance may be eaten, e.g. Amanita phalloides, in mistake for a mushroom or a chemical poison in food may be unwittingly consumed. Examples of the latter range from barium carbonate used in baking in mistake for flour, to arsenic or powdered glass administered murderously in the tradition of the Borgias.
Food which has been placed in a container previously used for holding a chemical poison may be contaminated. Placing acid fruit juices in cheap enamel or zinc vessels may result in the liberation of antimony or zinc. Home-made wine kept in glazed earthenware containers may be the source of chronic lead poisoning.
Aetiology. Bacterial food poisoning is usually divided into the infection and toxin types.
INFECTION TYPE: The organisms mainly responsible belong to the Salmonella group whose source is certain birds, cattle and reptiles, such as pet tortoises. The domestic fowl is one of the commonest sources of salmonellae and modern methods of poultry husbandry involving battery-rearing and deep-freezing of carcasses encourage the spread and transmission of infection, the organisms surviving in the frozen birds.
Salmonella typhimurium causes at least three-quarters of the cases of food poisoning of the ‘infection’ type in Britain. Food may be contaminated with infected excreta of mice ot rats, or infection may be transferred by flies or by human carriers employed in the handling of food.
The size of the infecting dose of bacteria bears a close relationship to the speed of onset of symptoms and to the severity of the illness. This indicates the dangers of bacterial multiplication which may take place when food is contaminated and thereafter remains warm for many hours or days.
The types of food which are particularly likely to be affected are twice-cooked meat dishes, stews, gravies, soups, custards, milk and synthetic cream.
The danger of food poisoning is greatly reduced if such foods are kept in a refrigerator.
Ducks tend to be carriers of salmonella organisms in the oviduct and alimentary tract, and duck’s eggs are not suitable for the preparation of lightly cooked foods.
Hen’s eggs are rarely affected.
TOXIN TYPE: Such poisoning is most commonly caused by the enterotoxin produced by Staph. aureus. This frequently originates from a food handler who may be a carrier or suffering from a septic lesion and so contaminates food.
Incubation under suitable conditions of temperature leads to growth of the organism and production of toxin which is relatively heat resistant and may not be destroyed by cooking. Strains of clostridium welchii, many of them relatively resistant to heat, may contaminate certain foods, particularly meat.
Pre-cooking of stews and pies may not destroy all the spores and the keeping of such food, even when followed by heating before consumption, will lead to the formation of toxins which can give rise to gastroenteritis, sometimes severe. Other bacteria (e.g. Streptococcus and Bacillus) may contaminate food without obvious spoilage and may also cause mild gastroenteritis.
Outbreaks of food poisoning affecting large numbers of persons occur in canteens, restaurants, hospitals and other institutions.
Clinical Features.(Bacterial food Poisoning)
The simultaneous occurance of symptoms in more than one member of a household or institution often simplifies diagnosis. The incubation period is a useful pointer to the aetiology.
If vomiting starts within 30 minutes of the ingestion of suspected food, it is likely to be due to a chemical poison; if it arises 12 to 48 hours later, it is probably due to a Salmonella infection. The incubation periods of staphylococcal and clostridial food poisoning are usually intermediate between these extremes being from one to 12 hours.
The symptoms in any single outbreak vary in severity depending on the type and amount of the poisonous substance ingested. The principal symptoms are nausea, vomiting, diarrhea and central abdominal colic. Staphylococcal food poisoning may be associated only with vomiting while diarrhea and abdominal pain are more prominent with Cl. Welchii toxins.
In severe cases there may be prostration, collapse and signs of dehyderation. In the chemical and toxin types of food poisoning the onset tends to be sudden and severe and the patient may rapidly become shocked. Recovery however usually occurs within 24 hours. In the infection type, symptoms develop more slowly and there is usually pyrexia and toxicity. The patient may be ill for several days. The stools are watery and offensive, and may contain blood and some mucus, in contrast to bacillary dysentery where there is also pus.
A rare cause of bacterial food poisoning is the ingestion of one of the most potent poisons known to man, namely the toxin produced by Cl. botulinum. Imperfectly treated tinned food or preserved fish may be contaminated with the organism and be the source of the toxin. The clinical features differ from all other types of bacterial food poisoning and consist chiefly of vomiting, constipation, thirst and the secretion of viscid saliva and of ocular and pharyngeal pareses and aphonia.
Diagnosis. (Bacterial food Poisoning)
A specimen of the patient’s stool or vomit together with the suspected food, if available, should be sent for culture. Organisms of the Salmonella group can usually be readily isolated. In more severe cases blood should be sent for culture. Notification of Salmonella infection and other types of food poisoning is compulsory in Britain.
Food poisoning must be differentiated from acute surgical abdominal emergencies especially in children.
Treatment. Most cases are miled and can be treated at home. Solid food should be withheld and the patient instructed to take fluids only. A quarter teaspoonful of table salt added to one pint of water flavoured with a small quantity of fruit juice provides a satisfactory oral replacement solution. Patients who are severely ill, collapsed or dehydrated require intravenous fluid therapy.
Symptoms normally pass off spontaneously in a day or two. When acute symptoms cease, semi-fluid low-roughage diet may be taken containing bread, butter, eggs, fish, softpuddings and jellies. To control diarrhea, kaolin mixture may be given in 10 ml doses every two to four hours. Codeine phosphate 30 mg six-hourly is also useful.
Antibiotics should not be given for acute diarrhea and vomiting as they are ineffective and frequently exacerbate symptoms. If salmonella bacteraemia is suspected or confirmed, ampicillin, 1 g every six hours should be given by intramuscular injection.
If the poisoning is thought to be due to a chemical or a poisonous food, the patient’s stomach should be washed out with tepid water, using the technique described onward.
Prevention: In salmonella food poisoning the carrier state persists on the average for about 14 days after infection but may be much longer, and the patient must not be allowed to handle food until he has stopped excreting the organism. A reduction in the high incidence of food poisoning can best be achieved by improving the standards of personal hygiene, especially in those handling food, and by stressing the importance of hand-washing after using the lavatory. Increasing facilities for low temperature storage of food which has to be kept for some hours or days before being consumed is of the greatest importance. It is essential to keep frozen poultry at room temperature for at least eight hours before cooking or pathogens at the centre may survive unharmed.
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