When your gut (intestines) becomes infected with the parasite Entamoeba histolytica (often shortened to E. histolytica), the condition is known as amoebiasis.
E. histolytica lives in the intestines of infected people. It can be passed out in their stools (faeces). Infection often occurs after someone drinks water contaminated by infected faeces or eats food prepared or washed using contaminated water. 9 out of 10 people with amoebiasis do not develop any symptoms. In those who do develop symptoms, diarrhoea, which can be bloody, is the most common symptom. Those most at risk of amoebiasis include travellers to areas where amoebiasis is common (mostly countries with poor sanitation).
Treatment includes medication to kill the parasite and drinking plenty of fluids to prevent a lack of fluid in the body (dehydration). If you develop diarrhoea after travelling abroad to places where E. histolytica is common, you should see your doctor so that amoebiasis or other infections can be excluded.
What is a parasite?
A parasite is a general term for any living thing that lives in, or on, another living organism. It may feed on its host, or obtain shelter using its host but it contributes nothing to its host's well-being or welfare. Human parasites include fungi, protozoa and worms.
What is amoebiasis?
Amoebiasis is a condition in which your gut (intestines) becomes infected with the parasite E. histolytica. Entamoebae are a group of single-celled parasites (living things that live in, or on, other living organisms) that can infect both humans and some animals. There are at least six species of entamoeba that can infect the human gut but only E. histolytica causes disease.
E. histolytica is an amoeba. An amoeba is the name given to any single-celled microscopic animal with a jelly-like consistency and an irregular, constantly changing shape. Amoebae are found in water, soil and other damp environments. They move and feed by means of flowing extensions of their body, called pseudopodia. Amoebae are types of germs (protozoa). Protozoa is a more general name for microscopic, single-celled organisms. Some protozoa, including E. histolytica, are important parasites of humans.
How do you develop amoebiasis?
E. histolytica lives in the gut (intestines) of infected people. It can be passed out in their stools (faeces). The parasite - a living thing that lives in, or on, another living organism - can actually survive for weeks, or even months, in soil, fertiliser, or water that is contaminated with infected faeces. If another person then drinks the contaminated water or eats contaminated food, they too can become infected.
E. histolytica can also be present on the hands of an infected person if they follow poor hygiene practices (for example, not washing and drying their hands properly after going to the toilet). If they are then in contact with another person, or prepare food for others, the parasite can enter another person's mouth and start to multiply in their gut, and therefore amoebiasis can be passed on. Sexual transmission of E. histolytica is also possible, usually in men who have sex with men.
E. histolytica is more likely to infect people who live in developing countries where sanitation and hygiene are poor. In the UK, most people with E. histolytica infection have caught it whilst travelling abroad.
Most people who become infected with E. histolytica do not develop any symptoms. However, symptoms may develop if the parasite causes inflammation of the lining of your gut. In some people, E. histolytica can also get into the bloodstream from the gut and spread around the body to the liver, lungs and sometimes other organs.
Note: the parasites can still be present in the stools of infected people who have no symptoms.
How common is amoebiasis?
It is estimated that around 1 in 10 people in the world are infected with E. histolytica. As mentioned above, not everyone develops symptoms. It is thought that, worldwide, about 40 to 50 million people infected with E. histolytica develop amoebic colitis or abscesses, causing up to 100,000 deaths per year.
E. histolytica infection is particularly common in areas of Central and South America, Africa, and Asia. If you travel to such areas you may be at risk of infection. Ways to prevent infection are described below.
What are the symptoms of amoebiasis?
About 9 out of 10 people who are infected with E. histolytica have no symptoms. If infection does cause symptoms, they may be due to:
The E. histolytica parasite can cause inflammation of the lining of your gut (intestines). This condition is known as amoebic colitis. 'Colitis' is a general term used for inflammation of the lining of the large intestine (the colon). 'Amoebic' refers to the fact that the colitis is caused by the amoeba E. histolytica. The disease is often mild and can just lead to tummy (abdominal) pain and diarrhoea. However, more severe inflammation with ulceration of the intestinal lining can occur in some people and so-called 'amoebic dysentery' can develop. (Dysentery is any infection of the intestines, causing severe diarrhoea with blood and mucus.)
So, symptoms of amoebic dysentery include severe abdominal pain and diarrhoea which can contain blood and mucus. High temperature (fever) may be another symptom but this is not common. You may also experience loss of appetite and weight loss. Symptoms can last for several weeks. Some people with amoebic colitis may just develop bleeding from their back passage (rectal bleeding) with no diarrhoea.
Severe amoebic colitis is known as 'fulminant' or 'necrotising' colitis. The person is very unwell with very severe bloody diarrhoea, very severe abdominal pain and a swollen (distended) abdomen with tenderness when their abdomen is examined by a doctor. Fever is also present. Occasionally, a hole (perforation) in the intestine may occur. This severe infection seems to be more common in certain groups of people, including the very young, pregnant women and those with underlying poor nutrition.
In a few people with amoebic colitis, an 'amoeboma' can develop. This is essentially a lump (mass) of tissue that builds up or forms in the wall of the intestine, due to the inflammation. It may cause a blockage of the intestine. Anaemia is another complication of amoebic colitis (due to blood loss in the bloody diarrhoea).
Note: if you develop diarrhoea after travelling abroad to places where E. histolytica is common, you should see your doctor so that amoebiasis or other infections can be excluded.
Amoebic liver abscess
The E. histolytica parasite can invade right through your gut wall, get into your bloodstream, and pass in your bloodstream to your liver. Once in your liver, it can cause an amoebic liver abscess to form. (An abscess is any localised collection of pus in the body that is surrounded and walled off by damaged and inflamed tissues.) An amoebic liver abscess contains pus and liquified, dying liver tissue.
Symptoms include fever and right upper abdominal pain with tenderness in this area when a doctor examines you. You may notice that your skin and the whites of your eyes become yellow (jaundiced) and your liver may also become enlarged (again, a doctor may be able to tell this when they examine your abdomen).
Only 3-4 out of 10 people with an amoebic liver abscess have symptoms of amoebic colitis at the same time. However, many people with a liver abscess may recall an episode of bloody diarrhoea within the previous year. So, an amoebic liver abscess can develop some time after initial infection with E. histolytica. It can be many years in some people.
Spread from an amoebic liver abscess
Rarely, an amoebic liver abscess can burst (rupture) and lead to damage to your diaphragm. The diaphragm is the thin muscle that separates your chest cavity from your abdominal cavity. This can allow spread of the abscess into your chest cavity, affecting your lungs and your pleura - the membrane that covers your lungs. Symptoms of such a complication include cough, difficulty breathing and pain in your chest when you breathe in.
Also, very rarely, in someone with an amoebic liver abscess, infection can spread to their brain and central nervous system. This can be very serious and it needs quick treatment. Symptoms include headache, feeling sick (nausea), being sick (vomiting) and confusion.
How is amoebiasis diagnosed?
Amoebiasis may be diagnosed when E. histolytica is seen in your stools (faeces) after a stool sample is sent to the laboratory and examined under a microscope. Ideally, three stool specimens from different days should be examined. However, in many people with an amoebic liver abscess, E. histolytica may not be seen in their stools. Sometimes the parasite can be seen when fluid is drawn out of a liver abscess and examined under a microscope. A needle is passed through your skin into the abscess, usually using an ultrasound scan to guide the person performing the procedure.
Other methods to detect E. histolytica in your stools have also been developed, including stool antigen detection (looking for E. histolytica proteins in your stools). The infection may also be diagnosed using a blood test that looks for evidence of E. histolytica infection in your blood. (If you have infection, antibodies to E. histolytica are usually found in your blood. These are another type of protein made in response to E. histolytica antigen.)
Sometimes a colonoscopy is performed if you have bloody diarrhoea and other tests have been negative. A colonoscopy is a procedure which uses a thin, flexible telescope passed through your back passage (anus) into your colon to allow examination of your colon. A tissue sample (biopsy) taken at colonoscopy and examined under the microscope can show the parasites - living things that live in, or on, other living organisms - in your intestinal lining. A liver ultrasound scan or a CT scan of your liver can show a liver abscess. Other tests may also be carried out if you have amoebic colitis or an amoebic liver abscess; for example, blood tests to look at your liver function, blood tests to look for anaemia, etc.
What is the treatment for amoebiasis?
If you have no symptoms but are found to have E. histolytica in a stool (faeces) specimen, it is usually advised that you should be treated with medication to kill the parasite. The medicine diloxanide furoate is commonly used. Treatment is advised because you can still pass on the infection to others even if you have no symptoms. The amoebae will still pass out in your stools. Also, you may still develop symptoms at a later stage.
The treatment of symptomatic amoebiasis then depends on your symptoms.
Medicines called antibiotics are needed to treat amoebic colitis. Metronidazole is the usual antibiotic that is used but tinidazole may be a good alternative. A second medicine, usually diloxanide furoate, is then used to get rid of any parasites that may still be living in your gut. After treatment is completed, testing of a follow-up stool sample is advised to ensure that the parasites have been cleared.
If you have amoebic colitis, you are at risk of becoming dehydrated. Dehydration means there is a lack of fluid in your body. It can occur if the water and salts that are lost in your stools are not replaced by you drinking adequate fluids. Therefore, drinking plenty of fluids is very important if you have amoebic colitis. Your doctor will advise you about how much fluid and what type of fluids you should drink.
You may be given special rehydration drinks.
If you do become severely dehydrated, you may need admission to hospital so that you can be given fluids through a vein (intravenously).
Occasionally, someone who develops fulminant colitis or a hole (perforation) in their bowel may need surgery to remove part of their intestine.
Amoebic liver abscess
Antibiotics are also needed to treat an amoebic liver abscess. The same antibiotics are used and are usually very effective, clearing the abscess in most people. Again, a second medicine, usually diloxanide furoate, is then used to get rid of any parasites that may still be living in your gut. If you also have colitis symptoms, drinking plenty of fluids is essential, as described above.
If a very large amoebic liver abscess develops, or antibiotic treatment is not successful, surgery may be needed to drain the abscess. Surgery may also be needed if the liver abscess bursts (ruptures).
Preventing amoebiasis and preventing spread to others
Simple hygiene measures can reduce the chance of becoming infected with amoebiasis and of passing the infection on to others. You can find out about these steps in the separate leaflet called Gastroenteritis.
What is the outlook (prognosis) for amoebiasis?
Most people who develop amoebic colitis or an amoebic liver abscess can be successfully treated with medicines to kill the parasite and eliminate it from the gut (intestines). Occasionally, treatment with medicines does not get rid of the parasite completely and symptoms can come back (recur).
In rare cases, fulminant 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, again this has a worse prognosis.
If strict hygiene measures are not undertaken and precautions are not used when travelling to 'at-risk' areas, re-infection can occur.
Further reading and references
; NICE Evidence Summary, March 2013
; Status of vaccine research and development for enterotoxigenic Escherichia coli. Vaccine. 2016 Mar 15. pii: S0264-410X(16)00287-5. doi: 10.1016/j.vaccine.2016.02.076.
; Guidelines for the prevention and treatment of travelers' diarrhea: a graded expert panel report. J Travel Med. 2017 Apr 124(suppl_1):S57-S74. doi: 10.1093/jtm/tax026.
; Traveler's Diarrhea. Med Clin North Am. 2016 Mar100(2):317-30. doi: 10.1016/j.mcna.2015.08.017.
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