Overview
Amoebiasis is an infection of the gut caused by a microscopic parasite swallowed in contaminated food or water. CDC reports that only 10% to 20% of infected people become ill at all.
When it does cause illness, the parasite invades the wall of the large intestine, producing bloody diarrhoea. It can also travel in the blood to the liver and form an abscess there, which presents quite differently, with fever and pain under the right ribs but often no diarrhoea at all.
Disease Summary
- Category
- Infectious Disease
- Cause
- Entamoeba histolytica
- Transmission
- Contaminated food and water, and the faecal-oral route
- Body System
- Digestive System
- Preventable
- Yes
- Curable
- Yes
Symptoms
Common Symptoms
Most people with amoebiasis experience one or more of the following symptoms.
Often No Symptoms at All
CDC reports that only 10% to 20% of infected people experience illness, while the rest carry and pass on the parasite without knowing.
Loose Stools or Diarrhoea
The commonest symptom when illness does occur, often lasting longer than an ordinary episode of diarrhoea.
Abdominal Pain and Cramping
Listed by CDC among the typical symptoms.
Blood in the Stool
CDC describes severe infection, known as amoebic dysentery, as presenting with bloody stools.
Fever
CDC lists fever with the severe, dysenteric form rather than with mild illness.
Weight Loss
Listed by CDC with both the intestinal illness and with liver abscess.
Tiredness and Wind
Commonly reported in milder cases alongside the change in bowel habit.
Pain Under the Right Ribs
CDC describes pain in the upper right abdomen with fever and weight loss, usually without diarrhoea, as the picture of a liver abscess.
Danger Signs
Seek urgent medical attention immediately if any of the following symptoms develop.
Bloody Stools With Fever
CDC states this combination defines the severe form and separates it from mild illness.
Pain Under the Right Ribs With Fever and No Diarrhoea
CDC gives exactly this combination as the picture of an amoebic liver abscess.
Severe or Very Frequent Diarrhoea With Dehydration
Passing many stools a day with fever and vomiting needs urgent rehydration.
Severe Abdominal Pain With a Rigid, Tender Abdomen
This can indicate perforation of the bowel, which is a surgical emergency.
Breathlessness, Chest Pain or Confusion
CDC notes that in a small number of cases the parasite spreads to the lungs or the brain.
Yellowing of the Skin or Eyes
Jaundice can develop with a liver abscess and needs urgent assessment.
Diagnosis
Stool Examination, Repeated
CDC advises collecting several stool samples across different days, noting that three consecutive samples within ten days improves the chance of detection.
Identifying the Species, Not Just the Parasite
CDC states that microscopy cannot distinguish the parasite that causes disease from harmless look-alikes, and that a specific antigen or molecular test is needed to confirm it.
Blood Antibody Test for Suspected Liver Abscess
CDC states that antibody testing detects infection in around 95% of cases outside the gut, making it the key test when a liver abscess is suspected.
Imaging of the Liver
Ultrasound or a scan identifies an abscess, which CDC describes as the commonest form of disease outside the intestine.
Exposure Assessment
Travel or residence in areas with poor sanitation, living in institutional settings, and certain sexual practices all raise the likelihood.
Laboratory Tests
Stool Examination for the Parasite
Finds the parasite, but CDC cautions that it cannot by itself confirm the disease-causing species, because harmless relatives look identical.
Stool Antigen Test
Detects proteins specific to the disease-causing species and so distinguishes it from the harmless look-alikes.
Molecular Stool Test
CDC calls this the method of choice for telling the disease-causing species apart from its harmless relative.
Blood Antibody Test
CDC reports detection in around 95% of cases outside the gut and 70% of active intestinal infection, but only 10% of symptom-free carriers.
Liver Ultrasound or Scan
Identifies a liver abscess. CDC states that abscesses larger than 5 centimetres should be drained alongside medicine.
Treatment
Metronidazole
CDC names this as the first-line medicine. It kills the parasites that have invaded the bowel wall or the liver.
Tinidazole
CDC names this as the alternative first-line option, working in the same way over a shorter course.
A Second Medicine to Clear the Gut
CDC names paromomycin and iodoquinol as medicines that clear the remaining parasites from inside the bowel. This second step prevents relapse and stops onward transmission, and is often missed.
Rehydration
Replacing fluid and salts lost through diarrhoea matters as much as killing the parasite, particularly in children.
Drainage of a Large Liver Abscess
CDC states that abscesses larger than 5 centimetres should be drained as an addition to medicine.
Prevention
Drink safe water
CDC advises avoiding untreated water in areas with poor sanitation. Boiling or properly treating water removes the parasite.
Wash hands after the toilet and before preparing food
CDC describes this as essential, because the parasite passes by the faecal-oral route.
Peel or cook fruit and vegetables
CDC advises avoiding unpeeled produce where sanitation is poor, and washing, peeling or cooking what you eat.
Avoid unpasteurised dairy and food from uncertain sources
CDC names unpasteurised dairy and street vendor food among the exposures to avoid in areas with poor sanitation.
Treat symptom-free carriers
CDC advises treating even symptom-free infection with the disease-causing species, because a proportion develop illness within a year.
Safer sexual practices
The parasite can pass through oral-anal contact, so barrier protection and hygiene reduce this route.
Risk Factors
People in Areas With Poor Sanitation
CDC describes the distribution as worldwide, especially in tropical countries with poor sanitation.
- Communities without safe drinking water
- Communities without proper sewage disposal
- Crowded living conditions
- Households where someone is an unrecognised carrier
Travellers and People Living Abroad
CDC names several groups who acquire the infection away from home.
- Humanitarian and aid workers
- Long-term travellers staying more than six months
- People visiting family in regions where it is common
- Immigrants and refugees from high-prevalence areas
People in Institutional Settings and Other Higher-Risk Groups
CDC identifies specific settings and practices that raise the risk.
- People living in facilities with poor sanitation
- Men who have sex with men, through oral-anal contact
- People whose immune system is suppressed
- Anyone caring for someone with the infection
Complications
Amoebic Dysentery
CDC describes the severe form with bloody stools and fever, which causes significant fluid loss and can be dangerous in children and older adults.
Liver Abscess
CDC states that this is the commonest form of disease outside the intestine. It presents with fever and right upper abdominal pain, usually without diarrhoea.
Perforation of the Bowel
Invasive disease can progress to perforation and inflammation of the abdominal cavity, which is a surgical emergency.
Spread to the Lungs
CDC notes that in a small number of cases the parasite spreads to the lungs, causing chest pain and breathlessness.
Spread to the Brain
CDC names brain abscess as a rare form of disease outside the intestine.
Jaundice
Yellowing of the skin and eyes can develop with a liver abscess.
When to Seek Medical Care
Diarrhoea With Blood
Bloody diarrhoea is never normal and always needs assessment, whatever the cause.
Fever With Pain Under the Right Ribs
Especially after being in an area with poor sanitation, and especially without diarrhoea.
Diarrhoea That Lasts More Than a Few Days
Prolonged diarrhoea deserves a stool test rather than repeated courses of general treatment.
Severe Abdominal Pain or a Rigid Abdomen
This may indicate perforation and is a surgical emergency.
Signs of Dehydration
Sunken eyes, little urine, drowsiness or, in a child, no tears when crying.
Prognosis
Most Infections Cause No Illness
CDC reports that only 10% to 20% of infected people become ill.
Treatment Is Effective
CDC states that antibiotics effectively treat amoebiasis, using one or two medicines depending on how severe the illness is.
Liver Abscesses Respond Well
Most respond to medicine, with drainage reserved for larger abscesses.
Untreated Carriers Can Become Ill Later
CDC states that 4% to 10% of untreated symptom-free carriers develop illness within a year, which is why they are treated.
Differential Diagnosis
Shigellosis and Other Bacterial Dysentery
Also cause bloody diarrhoea with fever, and are separated by stool testing. The treatments differ entirely.
Giardiasis
Causes prolonged diarrhoea and bloating but not usually blood in the stool, and is found on the same stool test.
Inflammatory Bowel Disease
Causes bloody diarrhoea over months and is separated by endoscopy. Treating it as amoebiasis, or the reverse, delays proper care.
Bacterial Liver Abscess
Causes the same fever and right upper abdominal pain, and is separated by blood culture and by antibody testing.
Gallstones or Gallbladder Infection
Cause pain under the right ribs with fever, and are separated by ultrasound.
References
CDC: About Amebiasis
How the infection spreads, who becomes ill, symptoms, testing and treatment.
CDC: DPDx, Amebiasis
Life cycle, laboratory diagnosis and the distinction between the disease-causing species and its harmless relatives.
CDC Yellow Book: Amebiasis
Clinical presentation, higher-risk groups, diagnostic testing and treatment.
MedlinePlus: Amebiasis
Plain-language overview used here for the geographic distribution of the infection.
Frequently Asked Questions
Can you have amoebiasis without knowing?
Yes, and most people do. CDC reports that only 10% to 20% of infected people become ill. The rest carry the parasite without symptoms and can still pass it on, which is why CDC advises treating even symptom-free infection.
How is it caught?
By swallowing the parasite, usually in contaminated food or water, or through contact with faeces from an infected person. CDC names contaminated water and food, contaminated surfaces and fingers, nappy changing and sexual contact among the routes.
Why does it need two different medicines?
Because they do different jobs. The first kills the parasites that have invaded the bowel wall or the liver. The second clears the parasites remaining inside the bowel, which prevents relapse and stops the person infecting others. Missing the second step is a common reason the infection returns.
Can it affect the liver?
Yes. CDC describes liver abscess as the commonest form of disease outside the intestine. It causes fever and pain under the right ribs with weight loss, usually without diarrhoea, so the link to a gut infection is easily missed.
How is it different from other causes of bloody diarrhoea?
It cannot be told apart by symptoms alone. Bacterial causes such as shigellosis produce a very similar illness and need different treatment, so a stool test is what separates them.