Amebiasis

Etiology

The genus Entamoeba includes unicellular, anaerobic, and parasitic organisms, which infect humans, nonhuman primates, and other vertebrate and invertebrate species worldwide. To date, this group includes at least 9 species that infect the human intestinal lumen: E histolytica, E coli, E dispar, E moshkovskii, E bangladeshi, E hartmanni, E polecki, E nana, and I buetschlii. Entamoeba histolytica is the only species well recognized as pathogenic, with the other species considered nonpathogenic or rare causes of clinical disease.[2] Entamoeba histolytica is a member of the Sarcodina, with 20 to 40 μm in diameter. Like Giardia, it has a cyst with a chitin-containing cyst wall. Immature cysts have a single nucleus. As the cyst matures, the nucleus divides twice to form a cyst with 4 nuclei, the infectious form.[Anderson, D et al. Nester's Microbiology: A Human Perspective. 2005]

Epidemiology

Entamoeba histolytica is transmitted by the ingestion of amebic cysts through fecal-oral contact, usually through contaminated food or water sources, but also through sexual intercourse.[3][4] This pathogen continues to be an important global health issue, being the third leading cause of death from parasitic infections.[5] Although 90% of Entamoeba histolytica infections are asymptomatic, nearly 50 million people become symptomatic, with up to 100,000 deaths yearly.[6] Those infected by the Entamoeba species are primarily colonized by either E histolytica or E dispar. While Entamoeba histolytica is a pathogenic organism that can cause amoebic colitis and extraintestinal amoebiasis, E dispar is considered nonpathogenic and causes no signs of disease.[7] A fundamental limitation pervading all amebiasis epidemiology, especially in older studies, is the overestimation of true Entamoeba histolytica prevalence due to microscopy's inability to distinguish Entamoeba histolytica from nonpathogenic amebae. 

Infection occurs worldwide, with higher prevalence in tropical countries and in areas of low socioeconomic status and poor public health. Areas with high rates of infection include India, Africa, Mexico, and Central and South America. Risk factors for infection are primarily related to fecal-oral transmission and include poor hand hygiene, defecation into water sources, eg, rivers, lack of urban services, untreated drinking water, proximity to animals, and alcohol consumption.[8][9]

India remains one of the countries with the highest burden. Using a polymerase chain reaction (PCR) assay performed on microscopy-, culture-, and DNA dot-blot-positive samples, studies have shown that Entamoeba histolytica's overall prevalence was 11.1%, 8.0%, and 13.7%, respectively.[9] In Bangladesh, 2.2% of dysentery cases were caused by E histolytica in preschool children.[10] In Thailand, a systematic review and meta-analysis found an overall prevalence of 1.30%. High-risk groups included dam personnel, individuals with intellectual disabilities, and orphaned children.[11] In Mexico, seroprevalence of E histolytica in rural areas has been reported to be as high as 42%. In a national surveillance data (2014–2020), the incidence of amoebic liver abscess remained unchanged over that period, with young males most affected.[12][8]

The 2019 Global Burden of Disease (GBD) study provides the most comprehensive recent data. In 2019, approximately 2.54 million DALYs (95% UI: 0.85–6.19 million) were attributable to Entamoeba infection globally. A critical and counterintuitive finding from the GBD analysis is that age-standardized DALY rates in high-income North America and Australia showed a statistically significant increasing trend. Specific populations drive this rising burden in developed countries, including immigrants, returning travelers, men who have sex with men, and people who live in facilities with poor sanitary conditions.[1][CDC. Amebiasis. 2025] 

A Japanese study provided more data on Entamoeba histolytica cases in developed countries, including on sexual transmission. National surveillance during 2000 to 2013 documented 7,403 domestically acquired amebiasis cases, with the proportion of domestic (nonimported) cases rising from 63% to 85% over the period. The majority were middle-aged men, and from 2008 onward, cases attributed to heterosexual contact surpassed those from homosexual contact. The recent rise was also associated with increased detection via colonoscopy and in asymptomatic persons.[13] In a retrospective study with 50 patients diagnosed with amoebic colitis in Barcelona, 13 (26%) of the patients had HIV infection, all of them were men who have sex with men, of whom 4 (8%) lacked antecedents of travel abroad.[14]

In tropical areas, Entamoeba histolytica is the most common cause of liver abscess, accounting for up to 87.5% of the cases in countries such as India.[15]

Pathophysiology

Entamoeba histolytica exists as an environmentally resistant cyst and a trophozoite, the active form responsible for colonization and disease. Infection occurs through ingestion of mature cysts, which release trophozoites in the intestine. These colonize the colon and interact closely with the gut microbiota, which influences parasite survival and virulence. 

Approximately 90% of cases are asymptomatic and self-limiting. Symptomatic disease occurs when trophozoites invade the mucosa and submucosa using secretory enzymes, and some enter the portal circulation, dispersing to the liver and other soft organs. Disease of the right colon is common and is associated with the following serious complications: strictures, rectovaginal fistulas, bowel obstruction, toxic megacolon, perforation, peritonitis, and death. Only 1% of clinical cases of amebiasis involve the liver, with the right lobe of the liver more likely to be involved. 

A key question in amebiasis pathogenesis is why only about 10% of infected individuals develop invasive disease while most remain asymptomatic. Disease progression also appears to result from interactions among parasite virulence factors, host immune responses, and the gut microbiota. In invasive disease, trophozoites penetrate the mucus layer and intestinal epithelium using adhesion molecules, particularly the Gal/GalNAc lectin, causing tissue damage through apoptosis, necrosis, and trogocytosis. This leads to inflammation, ulceration, and dysentery. In some cases, parasites disseminate through the portal circulation to the liver, resulting in amoebic liver abscesses. The molecular mechanisms regulating virulence activation remain incompletely understood and continue to be an active area of research.[16][17] 

Histopathology

Early amebic lesions are characterized by mucosal thickening caused by glandular hyperplasia and stromal edema, with preservation of the surface epithelium. Histologically, a mild-to-moderate neutrophilic infiltrate, reactive lymphoid hyperplasia, and small numbers of trophozoites within the surface exudate are noted. As the disease progresses, mucosal destruction becomes evident, with depletion of mucin, epithelial injury, and a more intense inflammatory infiltrate composed of neutrophils, plasma cells, eosinophils, macrophages, and lymphocytes.

Trophozoites become more numerous, particularly at sites of epithelial lysis. Advanced invasive disease is characterized by the classic flask-shaped ulcer, in which a relatively small mucosal defect extends deeply into the submucosa. Extensive tissue lysis and microhemorrhages occur, and trophozoites containing ingested erythrocytes (hematophagous amebas), a hallmark of invasive amebiasis, can often be identified in stool, rectal scrapings, or ulcer exudates. The ulcer base is typically covered by a proteinaceous and fibrin-rich exudate containing red blood cells and abundant trophozoites.[18]

History and Physical

Patients with amoebic colitis typically present with a several-week history of cramping abdominal pain, weight loss, and watery or bloody diarrhea. Most patients with symptoms have a clinical course similar to chronic colitis, but some present symptoms of acute colitis even months to years after exposure. In a retrospective observational study including 50 patients diagnosed with amoebic colitis, diarrhea, abdominal pain, and dysentery were the most frequent symptoms. In industrialized countries, where amebiasis is less frequently considered, intestinal amoebiasis may be misdiagnosed as inflammatory bowel disease (IBD), since clinical presentations can overlap and characteristic histopathological features may be absent or not identified on biopsy, exposing patients to the risk of inappropriate immunosuppressive therapy and worsening of the underlying infection.[14][17] 

Amebic liver abscess (ALA) usually presents with a short history of fever and abdominal pain in the right upper quadrant, 2 to 4 weeks after exposure, with less than one-third of the patients having diarrhea at diagnosis. About 10% of patients have jaundice, and 5% to 14% of cases have pulmonary symptoms like chest pain and shortness of breath. ALA is the most common extraintestinal complication, and is still a common problem in the tropical world, being the cause of over three-quarters of all liver abscesses. Young male adults and people with alcohol consumption are at the greatest risk. This nonsuppurative complication is predominant on the right lobe of the liver, with the lesions containing dead hepatocytes and cellular debris.[19] Physical examination usually demonstrates tenderness over the liver and hepatomegaly in about 50% of cases.[20]

Pleuropulmonary involvement is a rare presentation, including atelectasis and transudative pleural effusions, almost always by direct extension through the diaphragm from a right-lobe liver abscess. In rare cases, an amoebic liver abscess can rupture into the pleural space with empyema or hepato-bronchial fistula, causing fevers, coughing, and respiratory distress. Cardiac infection is an even less common complication than pleuropulmonary disease and occurs when a liver abscess ruptures into the pericardium and presents with symptoms of pericarditis or cardiac tamponade. Amoebic brain abscesses are very rare, with sudden onset symptoms, eg, headache, vomiting, and mental status changes with rapid progression to death.[7][10] 

Evaluation

Classically, amoebic infections were diagnosed by stool microscopy, identifying trophozoites; however, this method could not distinguish E histolytica from nonpathogenic amoebic infections. Now, several tools are used for diagnosis, including stool PCR, which has the highest sensitivity in distinguishing E histolytica from E dispar. Other diagnostic modalities available include stool microscopy, stool antigen detection, serology, stool molecular studies, and colonoscopy with a histologic examination.[7]

Laboratory Studies

Serology has high sensitivity and specificity and a quick turnaround time, but is not helpful in distinguishing between acute and prior infection. Antibodies are not detectable in an active infection until at least a week into the infection and will remain detectable in individuals for years. Stool microscopy is widely available, requires minimal equipment, and can be used to screen for other parasites. Disadvantages include poor sensitivity (reported at less than 60%), inability to differentiate from other Entamoeba species, and reliance on the observer's experience or skills.[8][21] This technique is still the most commonly used laboratory diagnosis of intestinal amoebiasis in developing countries.[21]

Stool antigen detection is simple, has a quick turnaround time, and can differentiate between E histolytica and E dispar. A wide range of antigen detection kits using enzyme-linked immunosorbent assay (ELISA), radioimmunoassay, or immunofluorescence are commercially available, with sensitivities up to 88%.[8][21][22] Popular mAbs for detecting E histolytica in stool samples use antibodies against the Gal/Lectin GalNAc, with sensitivities and specificities of around 80% to 99% and 86% to 98%, respectively.[23]

The consistent genetic diversity detected in the small rRNA subunit of E histolytica and E dispar prompted the use of this target for differentiating the 2 species. PCR techniques are 100 times more sensitive than the current best available enzyme immunoassay (ELISA) kit. Stool molecular studies are considered the gold standard, with a sensitivity of 92% to 100% and a specificity of 89% to 100%. Testing is becoming more widely available but is considered more expensive, requiring appropriate equipment and kits (eg, for DNA extraction) and a skilled technician.[23] 

Other Diagnostic Studies

Colonoscopy with histologic examination is not used for routine diagnosis, but can be used to look for other causes. Typical intestinal amebic ulcers are found in the colon (primarily in the cecum), sigmoid colon, and rectum. Two types of ulcers, nodular and irregular, have been described. Nodular lesions appear as small (0.1- to 0.5-cm), rounded, slightly elevated areas of the mucosa with irregular necrotic centers. Irregular or serpiginous ulcers 1 to 5 cm in length are usually found in the cecum and ascending colon.[8][18]

Computed tomography (CT) may help identify features of amoebic colitis, including deep ulceration, patchy distribution, and omental wrapping. CT alone should not be used to diagnose amoebic colitis since features may be indistinguishable from other entities such as IBD. In ALA, thoracic x-ray data may be useful in diagnosis, as in other types of hepatic abscesses, showing elevation of the right hemidiaphragm, atelectasis, or pleural effusion. An ultrasound is the gold standard technique for the diagnosis of amoebic liver abscesses and is of tremendous importance in countries with limited resources. An abscess is most commonly unilocular, unlike a pyogenic abscess. On ultrasound imaging, ALA will appear as a cystic intrahepatic hypoechoic lesion with thick walls, often in the right hepatic lobe near the capsule. CT can also depict a nonenhancing center with an inflammatory ring after contrast. Early lesions due to the amoebic invasion of hepatic parenchyma are multifocal (microabscesses) because of tissue destruction and necrosis by proteases from E histolytica, and neutrophil recruitment to the site of infection. CT and magnetic resonance imaging can detect small abscesses, and Gallium scans can help differentiate amoebic (cold images) from pyogenic abscesses (hot images).[23][24]

Treatment / Management

Due to the risk of spread and the risk of extraintestinal manifestations, all E histolytica infections should be treated. The goal of therapy is to eliminate invading trophozoites and eradicate intestinal carriage. Treatment consists of 2 agents, a systemically absorbed tissue amoebicide and a luminal amoebicide, to eliminate cysts from the intestinal lumen. Tissue amoebicidal agents are nitroimidazole antibiotics, including metronidazole and tinidazole, with a Cochrane review finding tinidazole more effective and associated with fewer adverse events (however, tinidazole is not available in every country). Luminal amoebicides include paromomycin, diloxanide furoate, iodoquinol, and nitazoxanide.[24] Combination therapy has been shown to prevent disease recurrence; therefore, while asymptomatic amoebic cyst passage is treated exclusively with a luminal amoebicide, symptomatic amoebiasis requires a tissue amoebicide followed by a luminal amoebicide.(B3)

The recommended dosage for metronidazole is 500 to 800 mg orally 3 times a day for 5 days to treat intestinal amebiasis, or 5 to 10 days to treat ALA. Metronidazole can be used in children at a dosage of 35 mg/kg to 50 mg/kg per day, divided into 3 doses. Tinidazole is used at 2 g once daily for 3 days to treat intestinal amebiasis, or for 5 days in cases of ALA. In children, the tinidazole dose is 50 mg/kg once daily for 3 days. This initial treatment should be followed by a luminal amoebicide. Luminal agents include paromomycin and diloxanide furoate. The dosage for paromomycin is 25 mg/kg to 35 mg/kg per day, divided into 3 doses, or 500 mg 3 times daily, for 7 days; and diloxanide furoate is 500 mg orally 3 times a day for 10 days.[7][24][MSF. Amoebiasis. 2025](B3)

In patients with fulminant amoebic colitis or signs of peritonitis, broad-spectrum antibiotics should be started. Surgical intervention may be required with bowel perforation or toxic megacolon. In uncomplicated cases of amebic liver abscess, no evidence of the benefit of drainage in addition to medical therapy has been shown.[20] In situations where clinical response to antibiotic therapy is lacking, aspiration or catheter drainage may be necessary.[7] Others suggest considering drainage of ALA in cases of imminent rupture, no clinical response after 3 or 4 days of medical treatment, an abscess diameter greater than 10 cm, or ALA in the left lobe of the liver.[24](A1)

Differential Diagnosis

The differential diagnosis for E histolytica intestinal amebiasis includes:

• Other pathogens causing disentery, eg, Shigella, Escherichia coli, Salmonella, Campylobacter, Clostridioides difficile
• Other parasites, eg, Giardia, Cryptosporidium, Cyclospora, Schistosoma
• Inflammatory bowel disease
• Ischemic bowel disease

The differential diagnosis for E histolytica ALA includes:

• Pyogenic liver abscess
• Echinococcal disease
• Malignancy

Please see StatPearls' companion resources, "Diarrhea," "Chronic Diarrhea," "Colitis," and "Liver Abscess" for further information on differential diagnoses. 

Prognosis

Uncomplicated infections and early treatment have a mortality rate of less than 1%. Risk factors for complicated infections and increased mortality include the following: 

• Young age 
• Pregnancy 
• Corticosteroid treatment
• Malignancy 
• Malnutrition 
• Alcoholism [7][25]

Fulminant amoebic colitis is associated with 40% mortality. Amebic liver abscess with prompt medical treatment has been highly effective, with mortality rates between 1% and 3%. Pleuropulmonary amebiasis is associated with mortality up to 16%, while cardiac involvement has a mortality of up to 30%.[26]

Complications

Complications of intestinal amebiasis include:

• Fulminant/necrotizing colitis: Occurs in less than 0.5% of cases but carries more than 40% mortality. Presents with profuse bloody diarrhea, fever, pronounced leukocytosis, peritoneal signs, and often intestinal perforation. Risk factors include pregnancy, immunosuppression, corticosteroid use, diabetes, and alcohol use. 
• Toxic megacolon: Approximately 0.5% of cases; classically associated with inappropriate corticosteroid administration for misdiagnosed IBD. 
• Ameboma: A localized annular inflammatory mass of granulation tissue, usually in the cecum or ascending colon. Ameboma can cause obstructive symptoms and mimic colonic carcinoma on imaging and endoscopy. 
• Intestinal perforation and peritonitis: May present as an acute abdomen with tachycardia, hypotension, and generalized peritonitis requiring urgent surgical intervention. 
• Strictures, bowel obstruction, and rectovaginal fistulas: These conditions are reported as serious complications of right-sided colonic disease. 
• Perianal ulceration and fistula formation: These complications are unusual manifestations of amoebic colitis.[7][10]

While the majority of ALA patients have an uncomplicated course, a number of complications have been described, including rupture into abdomino-thoracic structures, biliary fistula, vascular thrombosis, bilio-vascular compression, and secondary bacterial infection. The incidence of ALA rupture to the intraperitoneal or pleuropulmonary spaces varies from 6% to 40%. Cardiac tamponade resulting from the rupture of the left lobe of ALA into the pericardium is one of the most serious complications of ALA.

Rupture of biliary ducts into the abscess cavity can result in a biliary fistula. Although rupture of ALA into the hollow viscus leading to the formation of hepatic fistulas (hepatobronchial, hepatogastric, hepatoenteric, and hepatocolonic fistula) is rare, such complications are often innocuous because of the spontaneous drainage of the abscess through the fistula. Thrombosis of major vessels, eg, the portal vein, hepatic vein, and inferior vena cava, has been reported in patients with ALA.[19][27]