Infection due to the protozoan, Entamoeba histolytica, can result in amebic colitis as well as complications such as an amebic liver abscess. Amebic liver abscess is the most common extraintestinal manifestation of amebiasis. Men between the ages of 18 and 50 are most commonly affected. Areas with high rates of amebic infection include India, Africa, Mexico, and Central and South America. Approximately 80% of patients with this disease will develop symptoms within 2 to 4 weeks, including fever and right upper quadrant abdominal pain with 10% to 35% of patients experiencing associated gastrointestinal symptoms. Diagnosis is based on clinical symptoms and relevant epidemiology coupled with radiographic studies and serologic tests. Optimal treatment includes the use of metronidazole followed by a luminal agent such as paromomycin. Rarely, therapeutic aspiration is indicated. ,
Amebic liver abscess is caused by the invasive enteric protozoan Entamoeba histolytica. Many Entamoeba species, namely E. dispar and E. moshkovskii infect humans, but only E. histolytica causes amebiasis. This organism is distributed throughout the world, posing a substantial risk in countries without adequate sanitation of municipal water supplies. 
Amebic liver abscess is uncommon in children and ten times more common in men than in women, particularly in individuals between the ages of 18 and 50. The reason for such a striking difference is not clear but thought to be due to factors such as hormonal effects and alcohol consumption. In the United States, most cases are found in immigrants from endemic areas, with people living in states bordering Mexico having the most disease. Worldwide, areas with high rates of infection include India, Africa, and Mexico as well as parts of Central and South America. Most individuals are infected by ingesting contaminated food or water although other modes of transmission include oral and anal sex, particularly among men who have sex with men. 
Upon ingestion of contaminated food and water, the infection starts with ingestion of the quadrinucleate cyst of E. histolytica. Excystation in the small intestinal lumen is followed by production of motile, potentially invasive trophozoites. In most infections, the trophozoites form new cysts and are limited to the intestinal mucin layer. In other cases, the trophozoites adhere to and lyse the colonic epithelium with subsequent invasion of the colon. Neutrophils respond, resulting in further cellular damage at the invasion site. Once the trophozoites invade the colonic epithelium, subsequent spread to extraintestinal sites such as the liver (by hematogenous spread through the portal circulation) and peritoneum can occur. 
Adherence of E. histolytica to colonic epithelial cells is thought to be through the galactose/N-acetylgalactosamine-specific lectin. E. histolytica carries cytolytic capabilities and also kills mammalian cells through programmed apoptosis. Once E. histolytica trophozoites reach the liver, they create abscesses consisting of well-circumscribed areas of cellular debris, dead hepatocytes, and liquefied cells. The lesion is surrounded by a rim of connective tissue with some inflammatory cells and amebic trophozoites. In humans, the small number of organisms compared to the actual dimensions of the abscess supports the concept that E. histolytica can destroy hepatocytes without contact with the cells.
Patients can present with amebic liver abscess months to years after travel to an endemic area, making a thorough travel history and knowledge of epidemiologic risk factors imperative. In the United States, the typical patient with an amebic liver abscess is an immigrant (usually Hispanic male) between the ages of 20 and 40. Eighty percent of patients will develop symptoms within 2 to 4 weeks of exposure, including fever, dull and aching right upper quadrant or epigastric abdominal pain, and cough. Patients who present subacutely will have weight loss and less frequent development of fever and abdominal pain. Ten percent to 35% of patients have gastrointestinal symptoms including nausea, vomiting, abdominal cramps, diarrhea, constipation, or abdominal distension. On exam, hepatomegaly with point tenderness either over the liver, below the ribs, or in the intercostal spaces is typical.
Patients with an amebic liver abscess will typically have evidence of leukocytosis, elevated serum transaminases, and alkaline phosphatase on laboratory evaluation. On imaging, most amebic liver abscesses will be found in the right lobe. Imaging modalities include ultrasound (round, hypoechoic mass), CT scan (low-density mass with peripheral enhancing rim), and MRI (low signal intensity on the T1-weighted image and high signal intensity on T2-weighted image), which are fairly sensitive but without absolute specificity for an amebic liver abscess. Travel to an endemic area coupled with typical signs and symptoms and visualization of lesions on imaging must be coupled with serologic testing. Serum antigen detection has a sensitivity of over 95% with serologic testing (indirect hemagglutination) having a sensitivity of 70% to 80% in acute disease and greater than 90% in convalescent disease. It should be noted that early in the disease course (the first 7 days) there may be false-negative serologic tests. Stool microscopy, on the other hand, has a sensitivity of only 10% to 40%. 
Treatment entails the use of a nitroimidazole, preferably metronidazole at a dose of 500 mg to 750 mg by mouth 3 times per day for 10 days. Alternatively, tinidazole 2 g by mouth daily for 5 days can be used. As parasites can persist in the intestine in 40% to 60% of patients, treatment with a nitroimidazole should be followed with a luminal agent such as paromomycin. Metronidazole and paromomycin should not be given at the same time because diarrhea, a common side effect of paromomycin, can make assessing response to therapy difficult. Therapeutic aspiration (usually through image-guided percutaneous needle aspiration or catheter drainage) is occasionally required and should be considered in patients with no clinical response to antibiotics within 5 to 7 days, in those with a high risk of abscess rupture (cavitary diameter of more than 5 cm or presence of lesions in the left lobe), or in cases of bacterial coinfection of amebic liver abscess.
For simple abscess, the prognosis is excellent.
Complex abscesses and large abscesses can rupture and may be associated with a high mortality.
Overall, most patients have a favorable outcome with prompt treatment.
An amebic liver abscess may rupture into the chest, abdomen, or pericardium.
Preventive measures include reducing fecal contamination of food and water and emphasizing the use of safe sexual practices, particularly in men who have sex with men. An effective vaccine would be instrumental in improving health in developing countries, particularly in children. Once considered a fatal infection, an amebic liver abscess is now considered a very treatable condition.
Diligence in maintaining local water sources and emphasis on counseling men who have sex with men regarding safe sexual practices are of paramount importance in preventing amebiasis. Development of an effective vaccine would be instrumental in protecting children in developing nations.
The care of amebic liver abscess depends on the complexity of the liver abscess. Even though drugs are the first choice to manage a liver abscess, there are no randomized trials to demonstrate that combination therapy is better than monotherapy. Another consideration is that drugs like chloroquine also have adverse effects that may affect vision. Hence, where possible, multiple drug combinations should be avoided. The treatment of amebic liver abscess requires an interprofessional team to lower the morbidity and mortality of the disorder. 
When an interprofessional team approach is undertaken, the prognosis for most patients with an amebic liver abscess is excellent. To avoid complications like rupture into the lung, pericardium, or abdomen, patients with amebic liver abscess need a prompt referral to an infectious disease expert for treatment. Current evidence reveals that drainage of complex abscess can improve outcomes as opposed to medical management. (Level III)