Written by Dr Adrian Hyzler, Chief Medical Officer and Head of Medical Communications
Updated: June 2019
Dysentery is a general term that refers to bloody diarrhoea. It is most often caused by either the Shigella bacteria (‘bacillary dysentery’) or by the Entamoeba parasite. This briefing will concentrate on the latter, known as either Amoebic Dysentery or Amoebiasis.
There are at least six species of parasites in the Entamoeba group but symptoms are most common with the Entamoeba histolytica parasite. The amoebae are found in water, soil and other damp environments. They also live in the intestines of infected people and can be passed out in the stools of infected people, leading to contamination of drinking water or food supplies in countries with poor sanitation.
The majority of infected people develop no symptoms. In those 10% who do develop symptoms, diarrhoea is most common. Other symptoms may include abdominal pain and bloody diarrhoea, but sometimes a severe colitis can occur that can result in bowel perforation. People affected may also develop anaemia due to loss of blood.
Amoebic dysentery is often confused with ‘traveller’s diarrhoea’ because of its prevalence in developing nations. In fact, most traveller’s diarrhoea is bacterial or viral in origin.
It is estimated that more than 500 million people are infected with E. histolytica worldwide. Between 40,000 and 100,000 will die each year, placing this infection second only to malaria in mortality caused by protozoan parasites. Those most at risk are travellers to areas where amoebic dysentery is common, usually in countries with poor sanitation.
Amoebiasis occurs worldwide but is particularly common in tropical areas of Central and South America, western and southern Africa, and South Asia.
- Amoebic dysentery is an intestinal infection caused by a protozoan parasite called Entamoeba histolytica.
- Infection by Entamoeba histolytica may cause no symptoms.
- The symptoms of amoebic dysentery include fever, chills, diarrhoea, abdominal pain, and passing stool with blood and/or mucus.
- Entamoeba histolytica may invade the liver to form an abscess.
- Transmission of amoebic dysentery occurs mainly through the faecal-oral route (poor toilet hygiene), including ingestion of contaminated food or water.
- The incubation period (time from infection to symptoms developing) is variable, and may range from a few days to several months. It is usually 2 – 4 weeks.
- Treatment may include appropriate use of antibiotics as well as an anti-protozoal medication.
- Prevention: Maintain good personal food and environmental hygiene. Adopt the 5 Keys to Food Safety in handling food:
- Choose – Choose safe raw materials
- Clean – Keep hands and utensils clean
- Separate – Separate raw and cooked food
- Cook – Cook thoroughly
- Safe Temperature – Keep food at a safe temperature to prevent food borne diseases.
TRANSMISSION | How do you get Amoebic Dysentery?
Transmission occurs via the faecal–oral route (poor toilet hygiene), either directly by person-to-person contact or indirectly by eating or drinking faecal contaminated food or water.
E. histolytica is a protozoa – a single celled organism – that lives in the intestines of infected people and is passed out in their stools. The parasite can survive for months in damp environments such as soils, fertiliser or water. If someone then drinks contaminated water or eats contaminated food, they can become infected.
The parasite can also be passed to others by a handshake if it is present on the hands of an infected person, as a result of poor personal hygiene practices. An even more likely route of transmission occurs in the case of infected food handlers.
Sexual transmission of E. histolytica is also possible.
Low standards of hygiene and sanitation, particularly those related to over-crowding, tropical climate, contamination of food and water with faeces, and inadequate disposal of faeces, all account for the high rates of infection seen in developing countries. It has been suggested that some animals, such as dogs, pigs, and monkeys, may act as reservoir hosts to the protozoa, but this has not been proven. In resource rich countries, risk factors include communal living, oral and anal sex, compromised immune system, and migration or travel from endemic areas.
SYMPTOMS | What are the symptoms of Amoebic Dysentery?
Nine out of ten infected people have no symptoms but it should be remembered that the parasites could still be present in the stools of infected people who have no symptoms. Infections can sometimes last for years.
Symptoms of ‘amoebic colitis’ take from a few days to a few weeks to develop and include the following:
- Abdominal pain
- Diarrhoea that can contain blood and mucus – occasionally it can present with bleeding from the back passage without diarrhoea
- Fever may be present, but is not common
- Loss of appetite and weight loss are common in chronic infections.
Severe amoebiasis is known as ‘necrotising colitis’ and occurs in about 10% of symptomatic cases. It causes much more debilitating symptoms, with severe abdominal pain and profuse diarrhoea, often with a tender, swollen abdomen. Fever and chills are usually present.
Occasionally the bowel can ulcerate and develop a hole (‘perforate’) leading to severe infection in the abdomen and acute shock that needs urgent hospitalisation, and may lead to death.
Those most at risk of severe complications are the very young, the malnourished and pregnant women.
If the E. histolytica parasite gets into the bloodstream it can travel to the liver where it can cause an ‘amoebic abscess’- a localised collection of pus that is walled off by inflamed tissues. Symptoms include fever, often with night sweats, and pain under the rib cage on the upper right side of the abdomen. Gradually the skin and whites of the eyes may become yellow, as the liver enlarges and jaundice develops. Often the episode of dysentery will have resolved months prior to these symptoms developing.
Rarely the abscess can invade the lungs causing symptoms that include cough, difficulty breathing, and pain in the chest on inspiration.
Also, very rarely, in someone with an amoebic liver abscess, infection can spread to the brain and central nervous system. This can be very serious, requiring prompt treatment. Symptoms include headache, nausea, vomiting and confusion.
Entamoeba histolytica infection is associated with malnutrition and stunting of growth, especially in chronic infection in childhood.
DIAGNOSIS | How is it diagnosed?
A diagnosis of amoebiasis is confirmed by demonstrating the presence of E. histolytica following lab analysis of a minimum of three stool samples from different days – this is necessary because cysts are shed intermittently and may not show up in every sample.
The infection can also be detected by more advanced tests that identify antibodies in blood samples, but these only become positive after about two weeks and may remain positive after treatment – meaning that a positive result may relate to an old, previously treated infection.
If all tests are negative it may be necessary in cases of severe diarrhoea, to perform a colonoscopy to take direct tissue samples from the infected intestine for microscopic examination.
If a liver abscess is suspected, a liver ultrasound or a CT scan may confirm the presence of the abscess, and fluid will then need to be drawn directly from the liver by needle aspiration for identification of the protozoa.
More recent developments include a kit that detects the presence of amoeba proteins, or DNA, in the faeces. These tests are not in widespread use due to their expense.
TREATMENT | How is Amoebic Dysentery treated?
The drug ‘paromomycin’ is commonly used to eradicate the parasite. Treatment is advised in all confirmed cases, even in the absence of symptoms, because even if asymptomatic, it is still possible to pass on the infection to others: the amoebae will still pass out in the stools and the infected person may still develop symptoms at a later stage.
In amoebic colitis, antibiotics are added to treat the associated bacterial infection. Successful treatment is confirmed by negative stool analysis. It is important to maintain good hydration by drinking plenty of fluids. In cases of severe dehydration, admission to hospital may be necessary to administer intravenous fluids.
In cases of perforation, emergency surgery may be necessary.
Similar antibiotics are also needed to treat an amoebic liver abscess, again using paromomycin to treat any parasites persisting in the intestine. Large abscesses are likely to require surgical drainage.
Treatment of other Entamoeba infections apart from E. histolytica is not needed.
Most people who develop amoebic colitis or an amoebic liver abscess, can be successfully treated with medication to eradicate the parasite from the intestine. Occasionally, treatment does not effectively remove the parasites completely and symptoms can recur.
In rare cases, ‘necrotising colitis’ can develop and this causes severe illness and carries a worse outlook. If an amoebic abscess ruptures, or infection spreads to the central nervous system, this also has a bleak prognosis. Necrotising amoebic dysentery is reported to have 55-88% mortality.
PREVENTION | What precautions should I take?
The most important principle of prevention is strict personal hygiene, in the form of hand washing. Children also need to be taught the importance of thorough hand washing.
The following guidelines are recommended to prevent the spread of infection to others when infected with amoebic dysentery or any serious diarrhoea:
- Pay close attention to personal hygiene – wash hands thoroughly with soap and hot running water after going to the toilet. It is also important to dry hands properly after washing.
- If a symptomatic child wears nappies or uses a potty, be especially careful to use gloves and wash your hands as above, particularly before preparing, serving, or eating food.
- Do not share towels and flannels.
- Do not prepare or serve food for others.
- All soiled clothing or bedding must be cleaned into the toilet and then washed at high temperature in a separate wash.
- Disinfect the toilets regularly with hot water and detergent, wiping the flush handle, toilet seat, bathroom taps, surfaces and door handles at least once a day – use a disposable cloth.
- Do not go to work, school, college, etc., while you have diarrhoea. Your doctor will advise you when it is safe to return. Avoid contact with other people as far as possible during this time.
Food handlers who develop diarrhoea or vomiting must inform their employer and immediately leave the food-handling area. If amoebiasis is confirmed, the employer must be informed and the food handler must stay away from work until the doctor advises it is safe to return.
Carers who work with vulnerable groups such as the elderly, unwell or the young should inform their employer and stay away from work as above.
In areas of poor sanitation, avoid:
- Tap water – instead boil water for more than 1 minute before using
- Watered down fruit juices
- Ice cream
- Ice cubes
- Raw vegetables
- Raw or under-cooked meat
- Fruit that has been peeled other than by yourself
- Mayonnaise or sauces
No vaccine is available to prevent amoebic dysentery.
Good sanitary practice, as well as responsible sewage disposal or treatment, are necessary for the prevention of E. histolytica infection on an endemic level. E. histolytica cysts are usually resistant to chlorination, therefore sedimentation and filtration of water supplies are necessary to reduce the incidence of infection.