Amoebiasis

What is amoebiasis?

Amoebiasis is a condition in which the 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 amoebiasis disease.

Amoeba histolytica

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.

• Other damp environments.

They move and feed by means of temporary arm-like extensions of their body called pseudopodia.

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, such as amoeba.

• Worms.

How do you develop amoebiasis?

• E. histolytica lives in the intestine of infected people. It can be passed out in their stools (faeces).

• The amoebiasis parasite 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 or in tropical areas. 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. 1 in 10 people with amoebiasis will develop symptoms.

• 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.

How common is amoebiasis?

It is estimated that around 1 in 10 people in the world are infected with E. histolytica. As mentioned above, only about 1 in 10 of these will develop symptoms.

It is thought that, worldwide, about 40 to 50 million people infected with E. histolytica develop amoebic dysentery, colitis or abscesses, causing up to 100,000 deaths per year. It is the third most common death from parasites worldwide.

E. histolytica infection is particularly common in areas of Central and South America, Africa, and Asia.

Amoebiasis symptoms

Amoebiasis symptoms may be due to:

Amoebic colitis

The E. histolytica parasite can cause inflammation of the lining of the 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:

• Abdominal pain.

• Diarrhoea.

Amoebic dysentery

More severe inflammation with ulceration of the intestinal lining can occur in some people and this is called amoebic dysentery. (Dysentery is any infection of the intestines, causing severe bloody diarrhoea.)

Symptoms of amoebic dysentery include:

• Severe abdominal pain.

• Diarrhoea which often contains blood and mucus.

• High temperature (fever) may be another symptom but this is not common.

• 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.

Anaemia is a complication of amoebic dysentery (due to blood loss in the bloody diarrhoea).

Necrotising' colitis

Severe amoebic colitis is known as 'fulminant' or 'necrotising' colitis. The person is very unwell with:

• Very severe bloody diarrhoea,

• Very severe abdominal pain.

• A swollen (distended) abdomen with tenderness when their abdomen is examined.

• Fever is also present.

• Occasionally, a hole (perforation) through the wall of the intestine may occur.

This severe infection seems to be more common in certain groups of people, including:

• Children, particularly babies and toddlers.

• Pregnant women.

• People with underlying poor nutrition.

• People drinking over the recommended levels of alcohol.

• People with a diagnosis of cancer.

• People taking steroid medication.

Amoeboma

In a few people with amoebic colitis, an 'amoeboma' can develop. This is essentially a 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.

Amoebic liver abscess

The E. histolytica parasite can invade right through the gut wall, get into the bloodstream, and pass from the bloodstream into the liver.

Once in the 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.

• Right upper abdominal pain with tenderness in this area when examined.

• The skin and the whites of the eyes may become yellow (jaundice).

• The liver may also become enlarged (a doctor may be able to tell this when they examine the 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. In some people this can take years.

Spread from an amoebic liver abscess

Rarely, an amoebic liver abscess can burst (rupture) and lead to damage to the diaphragm (the thin muscle that separates the chest cavity from the abdominal cavity).

This can allow spread of the abscess into the chest cavity, affecting the lungs and the pleura (the membrane that covers the lungs).

Symptoms of such a complication include:

• Cough.

• Difficulty breathing.

• Pain in the chest when breathing in.

Very rarely, in someone with an amoebic liver abscess, amoebiasis infection can spread to the brain and central nervous system. This is very serious and needs fast treatment. Symptoms include:

• Headache.

• Feeling sick (nausea).

• Being sick (vomiting).

• Confusion.

How is amoebiasis diagnosed?

Stool microscopy

Amoebiasis may be diagnosed when E. histolytica is seen in the 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 as the test is not very sensitive.

However, in many people with an amoebic liver abscess, E. histolytica may not be seen in their stools.

Abscess fluid extraction under ultrasound scan guidance

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 the skin into the abscess, usually using an ultrasound scan to guide the person performing the procedure.

Stool antigen test

Other methods to detect E. histolytica in the stools have also been developed, including stool antigen detection (looking for E. histolytica proteins in the stools).

Blood test

The infection may also be diagnosed using a blood test that looks for evidence of E. histolytica infection in the blood. It takes a week or so after infection for a blood test to show the infection and the blood test remains positive for years after an infection. Therefore this test is not always useful as it can be difficult to assess whether this is a new infection.

Other blood tests may also be carried out where there is amoebic colitis or an amoebic liver abscess, for example, blood tests to look at the liver function and to check for anaemia.

Colonoscopy

Sometimes a colonoscopy is performed if bloody diarrhoea persists and other tests have been negative. A colonoscopy is a procedure which uses a thin, flexible telescope passed through the back passage (anus) into the colon to allow examination of the 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 the intestinal lining.

Liver ultrasound scan

A liver ultrasound scan or a CT scan of your liver can show a liver abscess.

Amoebiasis treatment

Someone with E. histolytica in a stool (faeces) specimen, even without symptoms, is usually advised to be treated with medication to kill the parasite. This is because the infection can still be passed to others, even without symptoms. Symptoms may also develop at a later stage.

Metronidazole or tinidazole (antibiotics) are the first-line treatment. After this, a medicine called a luminal amoebicide (a medication specially designed to kill amoebae) is often used. Examples of these include diloxanide furoate, iodoquinol and paramomycin.

The treatment of symptomatic amoebiasis then depends on the symptoms.

Amoebic colitis

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 luminal amoebicide, usually diloxanide furoate or paromomycin, is then used to get rid of any parasites that may still be living in the gut.

After treatment is completed, testing of a follow-up stool sample is advised to ensure that the parasites have been cleared.

Risk of dehydration

People with amoebic colitis are at risk of becoming dehydrated. It can occur if the water and salts that are lost in the stools are not replaced by drinking adequate fluids. Therefore, drinking plenty of fluids is very important when people have amoebic colitis.

People with severe dehydration may need admission to hospital so that they can be given fluids through a drip (intravenously).

Surgery

Occasionally, someone who develops fulminant colitis or a 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 in clearing the abscess in most people.

Again, a luminal amoebicide, usually diloxanide furoate, is then used to get rid of any parasites that may still be living in the gut.

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

Simple hygiene measures can reduce the risk of becoming infected with amoebiasis and of passing the infection on to others. Other preventative measures in high-risk areas include:

• Avoiding eating raw fruit and vegetables.

• Not drinking tap water.

• Avoiding eating from street vendors.

• Using protection (a condom) when having oral or anal sex.

You can find out about these other steps in the separate leaflet called Gastroenteritis.

Vaccine development for preventing amoebiasis will be very beneficial worldwide. Although deaths are rare in travellers returning to the Western world from high-risk areas, a vaccine could prevent a significant proportion of the 100,00 deaths worldwide each year. Research is taking place.

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 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 high-risk areas, re-infection can occur.

For more details, see the separate leaflet called Traveller's diarrhoea.