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Shigella infections and bacillary dysentery

What is dysentery?

Dysentery is an infection of the bowel caused either by a bacterium called Shigella (bacillary dysentery) or, less commonly, by an amoeba (amoebic dysentery). This leaflet deals only with bacterial dysentery and Shigella infection.

Dysentery in its severe form has been known to man since ancient times but most cases acquired in the UK today are mild. In its classical form it leads to symptoms of cramping abdominal pain, diarrhoea, and blood and mucus in the faeces. Initially the diarrhoea may be copious but it soon becomes frequent and of small volume; the patient often complains of painful defaecation. It is sometimes accompanied by other symptoms such as vomiting and fever. The infection rarely involves other parts of the body.

In the UK, most cases of Shigella infection do not present with the classical symptoms. Most patients have only mild abdominal pain and diarrhoea, often without blood in the faeces. Many affected people probably never visit their GP, and the condition usually settles within a day or two without treatment. However, more severe forms of infection can occur, particularly in travellers abroad. In these cases the patient may become very ill and dehydrated, and in the absence of medical treatment dysentery can be fatal.

What is Shigella?

There are four different species of Shigella: S. dysenteriae, S. flexneri, S. boydii, and S. sonnei. In general, S. dysenteriae tends to produce the most severe disease while S. sonnei produces the mildest.

The organism invades the cells lining the large bowel and multiplies there, killing the cell; this is the cause of the symptoms produced. However, it occasionally invades the bowel beyond the surface lining. S. dysenteriae also produces a toxin similar to that produced by E. coli O157, which causes additional tissue damage, and may lead to the haemolytic-uraemic syndrome and kidney failure.

In the UK and other developed countries most cases of infection (>90%) are due to S. sonnei and to a much lesser extent S. flexneri, while in developing countries S. dysenteriae and S. flexneri are the commonest species. This explains why most cases acquired in the UK are relatively mild, whereas travellers who acquire infection abroad are more likely to suffer from a more severe illness.

How is it acquired?

Shigella needs fewer organisms to cause illness than Salmonella and is thus more easily spread from person to person. Although transmission via food is much less common than with Salmonella, contamination of food with human sewage (either directly or via contaminated water) has led to outbreaks, especially with cold uncooked foods such as salads. Shigella is often transmitted directly from one person to another. This can happen if a person with diarrhoea fails to wash their hands after visiting the lavatory, and then comes into contact with another person.

Who gets Shigella infection?

In developed countries, most cases are seen in young children. This is explained by the relatively poor personal hygiene of small children and their attendance at schools and day nurseries where they come into close contact with other children. If sanitary facilities in schools are inadequate, this will contribute to the spread of infection, and contamination of the environment by faeces may also promote the spread of the organism. In addition, family members frequently acquire infection from infected people in the same household, so that infection can spread from children to adults and vice versa.

Many adults probably also acquire Shigella infection but because it is usually mild most of these cases never come to medical attention. Outbreaks of infection have been described in other groups of people in close contact such as in military bases and among cruise ship passengers. Shigella is an important cause of diarrhoea in travellers to developing countries.

In developing countries, Shigella is a common infection because of inadequate sewage disposal and lack of effectively treated water supplies. It is a cause of severe, potentially fatal, infection in children. Shigella is of major importance in refugee camps or following natural disasters, when once again disposal of sewage and the provision of clean water may be extremely difficult. It has been suggested that in developing countries flies may spread the infection from person to person, as the disease is commonest at the time of year when the fly population is highest.

In the UK, the incidence varies quite markedly between different years. In the past decade, the number of notifications of dysentery has ranged from less than 2,000 to over 10,000 per year. Outbreaks of infection are common and usually occur in schools and institutions where people are in close contact. Almost certainly, many more cases occur which are never detected.

How is it diagnosed?

Shigella infection is diagnosed by collecting a sample of faeces from the patient and growing the organism in the laboratory.

How is it treated?

The most important complication of bacterial dysentery is dehydration due to loss of fluid through diarrhoea and vomiting. The mainstay of treatment is therefore to replace the fluid which has been lost. In most cases the fluid can be given by mouth but in severely ill patients intravenous replacement may be required.

Since most cases originating in the UK are mild and short-lived, treatment with antibiotics is rarely helpful. However, a patient with more severe dysentery which is not resolving of its own accord may benefit from antibiotics which can shorten the duration of the illness and reduce the period of excretion of the organism. This may be important in reducing the likelihood of spread of the infection to other people.

How can it be prevented?

The most important preventive measure is the provision of safe water supplies and effective disposal of sewage. In developed nations possessing such facilities, such as the UK, the spread of infection is controlled by emphasising good personal hygiene and the provision of adequate toilet and hand washing facilities. The use of soap when washing hands is important, but disinfectant solutions are not usually recommended. In addition, since towels for communal use may become contaminated with the organism, disposable paper towels are preferred.

Since many outbreaks occur in schools and nurseries, regular cleaning of toilets and other communal areas is essential. In the past, a strict policy of exclusion from school or work was recommended until repeat faeces specimens showed that the organism was no longer being excreted by the patient. This is no longer the case, except in the rare cases of S. dysenteriae infection, and it is recommended that children should be excluded from school only until the diarrhoea has ceased and the child is passing normal bowel motions again. Adults are only excluded from work if they are food handlers or work in health-care premises. In the case of food handlers, it is recommended that the worker is excluded until 48 hours after the patient passes a formed motion.