Causes of dysentery 

Bacillary dysentery (shigellosis) is caused by shigella bacteria. Amoebic dysentery is caused by an amoeba (a single-cell parasite) usually found in tropical areas.

Bacillary dysentery

There are four types of shigella:

  • Shigella sonnei: this is the most common type in the UK and produces the mildest symptoms
  • Shigella flexneri
  • Shigella boydii
  • Shigella dysenteriae: this produces the most severe symptoms

The shigella bacteria are found in faeces and are spread through poor hygiene; for example, by not washing your hands after having diarrhoea.

If you do not wash your hands, you can transfer the bacteria to other surfaces. The bacteria can then infect someone else if they touch the surface and transfer the bacteria to their mouth. The bacteria will travel from the mouth to the bowel, invading the cells that line the large bowel. The bacteria multiply, killing the cells and producing the symptoms of dysentery.

In the UK, most cases of bacillary dysentery are spread within families and in places where people are in close contact with one another, such as in schools, nurseries, military bases and day centres. The condition can be spread for up to four weeks after a person has become infected.

Dysentery is also spread through food that has been contaminated with human faeces (stools), particularly cold, uncooked food, such as salad. This is more likely to happen in countries where:

  • there is poor sanitation
  • water supplies and sewage disposal are inadequate
  • human faeces are used as fertiliser

Severe dysentery is more common in developing countries.

The time between coming into contact with the bacteria and the symptoms starting (the incubation period) is usually one to seven days.

Amoebic dysentery

Amoebic dysentery (amoebiasis) is caused by an amoeba (a single-celled parasite) called Entamoeba histolytica. It is mainly found in tropical areas so it is usually picked up abroad.

When the amoebas inside the bowel of an infected person are ready to leave the body, they group together and a shell surrounds and protects them. This group of amoebas is known as a cyst.

The cyst passes out of the person's body in their faeces and is able to survive outside the body. If hygiene standards are poor; for example, if the person does not dispose of their faeces hygienically, it can contaminate the surroundings, such as nearby food and water.

If another person then eats or drinks food or water that has been contaminated with faeces containing the cyst, they will also become infected with the amoeba. Amoebic dysentery is particularly common in parts of the world where human faeces are used as fertiliser.

After entering the person's body through their mouth, the cyst will travel down into their stomach. The amoebas inside the cyst are protected from the stomach's digestive acid. From the stomach, the cyst will travel to the intestines where it will break open and release the amoebas, causing the infection. The amoebas are able to burrow into the walls of the intestines and cause small abscesses and ulcers to form. The cycle then begins again.

The amoebas that cause dysentery can also be sexually transmitted during mouth-to-anus contact.


Page last reviewed: 26/03/2013

Next review due: 26/03/2015