Overview
Entamoeba histolytica is a single-celled parasite that causes amebiasis (also spelled amoebiasis). It is a potent pathogen: its active form can secrete enzymes that dissolve host tissue, kill host cells on contact, and engulf red blood cells. When it invades the intestinal lining, it causes amebic colitis. In some cases it travels through the bloodstream to the liver and causes an amebic liver abscess.[1] Amebiasis has been described as the second leading cause of death from parasitic disease worldwide.[1]
Yet most infections cause no symptoms at all.[2] Why some people get sick and others do not is an active research question, and the gut microbiome appears to be part of the answer.
Classification
- Phylum: Amoebozoa
- Family: Entamoebidae
- Genus: Entamoeba
- Species: E. histolytica
One of six Entamoeba species in the human gut
Six Entamoeba species live in the human intestine: E. histolytica, E. dispar, E. moshkovskii, E. polecki, E. coli, and E. hartmanni.[3] This matters for anyone reading a stool test result:
- E. histolytica and E. dispar look identical under the microscope but are genetically distinct species. E. histolytica is the pathogen; E. dispar is considered a commensal. Recognizing that these are two species overturned earlier thinking about how common amebiasis is and how it should be diagnosed.[1]
- No convincing causal link has been shown between E. dispar or E. moshkovskii and symptoms, even though they are sometimes found in people with digestive complaints.[3]
- Entamoeba coli is not Escherichia coli. Despite the similar name, the amoeba E. coli is unrelated to the bacterium Escherichia coli.
How Amebiasis Develops
Infection follows the fecal-oral route, typically by swallowing cysts in contaminated food or water. In the gut, the parasite can live without causing harm. Disease occurs when trophozoites invade the intestinal mucosa, causing colitis. Sometimes they breach the mucosal barrier and travel through the portal circulation to the liver, where they form abscesses of dead and dying liver cells.[1]
Amebic liver abscesses grow relentlessly and were once almost always fatal. Today, even large abscesses can usually be cured with antibiotic treatment.[1]
The Microbiome Connection
A prospective study of infants in an urban slum of Dhaka, Bangladesh, followed E. histolytica from colonization to disease:[2]
- About 80% of children were infected with E. histolytica by age 2.
- Most infections were asymptomatic.
- Diarrhea was associated with a high parasite burden and with expansion of Prevotella copri in the gut microbiota.
- Intestinal IgA antibodies against a parasite surface protein (the Gal/GalNAc lectin) were associated with protection from reinfection.
These findings suggest that disease depends on both the amount of parasite and the composition of the microbiota. A review of parasitic protozoa described an emerging body of work suggesting that the intestinal microbiota may help explain why the same infection causes severe disease in one person and no symptoms in another.[4] Learn more about Prevotella copri, a common gut bacterium whose effects appear to depend strongly on context.
Diagnosis
Because microscopy cannot distinguish E. histolytica from harmless E. dispar, modern diagnosis relies on tests that detect E. histolytica-specific antigen or DNA in stool and other samples. Conventional and real-time PCR can detect and differentiate E. histolytica, E. dispar, and E. moshkovskii. Experts recommend specific diagnosis of E. histolytica rather than treating simply because Entamoeba was seen in a stool sample, to avoid unnecessary treatment.[3]
Liver abscess is typically evaluated with imaging and blood tests, and a clinician will interpret results alongside travel and exposure history.
Treatment
Amebiasis is treated with prescription antiparasitic medication. Treatment usually targets both the invasive parasites in tissue and those remaining in the intestine, so a clinician may prescribe more than one medication in sequence. The drugs, doses, and duration depend on whether the infection is asymptomatic, intestinal, or has spread to the liver, so treatment should always be directed by a clinician. Probiotics have not been established as a treatment for amebiasis.
Prevention
- Drink safe water. In areas with limited sanitation, use boiled, filtered, or bottled water, and avoid ice made from untreated water.
- Be careful with raw food such as salads and unpeeled fruit when traveling.
- Wash hands with soap and water after using the toilet and before handling food.
For related waterborne parasites, see Giardia lamblia and Cryptosporidium, and our page on travelers' diarrhea.
When to See a Clinician
Seek care promptly for bloody or mucus-filled diarrhea, persistent diarrhea, or fever with pain in the upper right abdomen, especially if you have lived in or traveled to an area with limited sanitation, even months earlier. Seek urgent care for severe abdominal pain, high fever, or signs of dehydration.
This page is for educational purposes and is not medical advice. Consult a qualified healthcare provider about diagnosis and treatment of parasitic infections.