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Infectious Diseases > MEDICAL TOPICS
Amebiasis
Article Last Updated: Feb 21, 2007
AUTHOR AND EDITOR INFORMATION
Section 1 of 11  
Author: Kepler Davis, MD, Assistant Chief of Infectious Disease Service, Chairman of Infection Control Committee, Dwight D Eisenhower Army Medical Center
Coauthor(s):
J Robert Cantey, MD, Chief of Infectious Disease, Veterans Affairs Medical Center, Director, Professor, Department of Medicine, Division of Infectious Diseases, Medical University of South Carolina;
Robert Swords, MD, Fellow, Department of Medicine, Division of Infectious Diseases, Medical University of South Carolina
Editors: Klaus-Dieter Lessnau, MD, FCCP, Clinical Assistant Professor of Medicine, New York University School of Medicine; Medical Director, Pulmonary Physiology Laboratory, Director of Research in Pulmonary Medicine, Department of Medicine, Section of Pulmonary Medicine, Lenox Hill Hospital; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Michael Stuart Bronze, MD, Professor, Stewart G Wolf Chair in Internal Medicine, Department of Medicine, University of Oklahoma Health Science Center; Eleftherios Mylonakis, MD, Clinical and Research Fellow, Department of Internal Medicine, Division of Infectious Diseases, Massachusetts General Hospital; Burke A Cunha, MD, Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital
Author and Editor Disclosure
Synonyms and related keywords:
enteric amebiasis, amebic colitis, amebic liver abscess, hepatic amebiasis, pleuropulmonary amebiasis, cerebral amebiasis, Entamoeba histolytica, E histolytica
Background
Amebiasis is an infection caused by the protozoal organism Entamoeba histolytica and includes amebic colitis and liver abscess. In developed countries, infection occurs primarily among travelers to endemic regions, recent immigrants from endemic regions, homosexual males, immunosuppressed persons, and institutionalized individuals. Transmission usually occurs by food-borne exposure, particularly when food handlers are shedding cysts or food is cultivated in feces-contaminated soil, fertilizer, or water. Less common means of transmission include contaminated water, oral and anal sexual practices, and direct rectal inoculation through colonic irrigation devices.
In 1875, in St. Petersburg, Russia, Fedor Losch was credited with the initial documentation of amebae in stool. Losch described the amebae in the stool as having a "round, pear shaped or irregular form and which are in a state of almost continuous motion."
In 1890, Sir William Osler reported the first North American case of amebiasis, when he observed amebae in stool and abscess fluid from a physician who had previously resided in Panama.
In 1913, in the Philippines, Walker and Sellards documented the cyst form of E histolytica as the infective form of the parasite; in 1925, Dobell further described the organism's life cycle.
Pathophysiology
E histolytica is a pseudopod-forming, nonflagellated protozoal parasite that exerts a lytic effect on tissue, a characteristic for which the organism is named. Host cells are killed via the induction of apoptosis; therefore, the parasite does not kill the host cell but rather induces its self-destruction. Ingestion of the cyst is followed by excystation in the small bowel and trophozoite colonization of the colon. Upon colonization of the colonic mucosa, the trophozoite may encyst and be excreted in the feces or it may invade the intestinal mucosal barrier, thereby gaining access to the circulation, resulting in involvement of the liver, lung, and other sites. Whether infection results in colonization or invasion, the E histolytica strain and its interaction with bacterial flora; host genetic susceptibility; and factors such as malnutrition, sex, age, and immunocompetence may influence the infection.
Frequency
United States
The overall prevalence of Entamoeba infection is approximately 4%; however, certain high-risk groups have a much higher incidence of infection and disease. In 1993, a total of 2970 cases of amebiasis were reported to the Centers for Disease Control and Prevention (CDC); 33% occurred in Hispanic immigrants and 17% in immigrants from Asia or the Pacific Islands.
International
Entamoeba species infect approximately 10% of the world's population. The prevalence of infection is as high as 50% in areas of Central and South America, Africa, and Asia.
Mortality/Morbidity
- E histolytica probably is second only to malaria as a protozoal cause of death. The prevalence of amebic colitis and liver abscess is estimated at 40-50 million cases annually worldwide, resulting in 40,000-110,000 deaths.
- Asymptomatic intestinal infection occurs in 90-99% of infected individuals. Most infected individuals eliminate the parasite from the gut within 12 months; however, colonization with E histolytica carries a low but definite risk of developing into invasive amebiasis.
- Case fatality rates of amebic colitis range from 1.9-9.1%. Amebic colitis is complicated by fulminant or necrotizing colitis in approximately 0.5% of cases, with a resultant mortality rate of greater than 40%. Amebic liver abscess is complicated by intraperitoneal rupture in 2-7% of patients, with sudden perforation causing a high mortality rate.
Sex
- Amebic liver abscess is 7-12 times more common in men than in women, although the sex distribution is equal in children.
- Amebic colitis affects both sexes equally.
Age
Young children appear to be at higher risk for fulminant invasive disease, resulting in a higher mortality rate.
History
- Amebic colitis
- Patients with amebic colitis typically present with a history of several weeks of abdominal pain, diarrhea, and bloody stools.
- Fever is uncommon and occurs in approximately 10-30% of patients.
- Because of the gradual onset of disease, weight loss is a common complaint and may be accompanied by symptoms of volume depletion (eg, orthostasis).
- Uncommon manifestations include ameboma, fulminant colitis, and rectovaginal fistulas.
- Fulminant or necrotizing colitis is the most serious manifestation, which occurs in approximately 0.5% of patients and has a mortality rate of greater than 40%.
- Predisposing factors for fulminant colitis include poor nutrition, pregnancy, corticosteroid use, and very young age.
- Amebic liver abscess
- Patients with amebic liver abscess typically present with a 1- to 2-week history of fever and right upper quadrant abdominal pain.
- Unlike amebic colitis, amebic liver abscess is associated with fever in 85-90% of patients.
- Patients with a single abscess may be more likely to present subacutely, with prominent weight loss, and fewer than half the patients have fever and abdominal pain.
- Most patients with liver abscess (ie, 60-70%) do not have concomitant colitis, although a history of dysentery within the previous year may be obtained.
- History of alcohol abuse is common, but how this condition may contribute to the development of a liver abscess still is unclear.
- Pleuropulmonary amebiasis
- This condition is a rare but serious complication of amebic liver abscess, usually caused by rupture of a superior right upper lobe abscess with erosion through the diaphragm.
- Patients with this complication present with cough, pleuritic chest pain, dyspnea, and, occasionally, necrotic sputum.
- Intraperitoneal rupture of amebic liver abscess occurs in 2-7% of patients. These patients can present with a rigid abdomen that may be diagnosed erroneously as a perforated viscus.
- Cerebral amebiasis is a rare cause of brain abscess and is characterized by an abrupt onset of mental status change and/or focal neurologic deficits. Progression to death occurs over 12-72 hours without adequate therapy.
Physical
- Amebic colitis
- Fever (10-30%)
- Weight loss (40%)
- Diffuse abdominal tenderness (12-85%)
- Heme-positive stools (70-100%)
- Patients with fulminant colitis are more likely to present with abdominal pain, distension, and rebound tenderness.
- Amebic liver abscess
- Fever (85-90%)
- Right upper quadrant abdominal tenderness (84-90%)
- Weight loss (33-50%)
- Hepatomegaly (30-50%)
- Jaundice (6-10%)
Causes
- Amebiasis is an infection caused by the protozoal organism E histolytica, which causes colitis and liver abscess.
- Transmission usually occurs by food-borne exposure, particularly when food handlers are shedding cysts or food is cultivated in feces-contaminated soil, fertilizer, or water. Less common means of transmission include contaminated water, oral and anal sexual practices, and direct rectal inoculation through colonic irrigation devices.
Abdominal Abscess
Arteriovenous Malformations
Campylobacter Infections
Diverticulitis
Escherichia Coli Infections
Hepatocellular Adenoma
Inflammatory Bowel Disease
Pyogenic Hepatic Abscesses
Salmonellosis
Shigellosis
Other Problems to be Considered
Echinococcal cyst
Ischemic colitis
Inflammatory bowel disease
Lab Studies
- Microscopy
- Microscopic examination of a single stool specimen from a patient with amebic colitis is only 33-50% sensitive.
- Results on a repeated stool examination in patients with proven amebic liver abscess are positive in 8-40% of cases.
- Identification of the parasite in a liver abscess aspirate is only 20% sensitive.
- The World Health Organization (WHO) recommends that intestinal infection be diagnosed with an E histolytica-specific test, thus rendering the classic stool ova and parasite examination obsolete in this setting; however, finding quadrinucleated cysts or trophozoites containing ingested erythrocytes in stool is considered by many to be diagnostic for amebic colitis.
- Antigen detection
- A stool antigen test specific for E histolytica is available from TechLab (Blacksburg, Va).
- This test uses monoclonal antibodies specific for the galactose (Gal)/N-acetyl-D-galactosamine (GalNAc) lectin of E histolytica and has a sensitivity of 87% and specificity of greater than 90% compared with culture.
- Early data on antigen detection tests for serum and liver abscess aspirates are promising.
- Serology
- Serum antiamebic antibody tests are an important adjunct to antigen detection, particularly in cases of amebic liver abscess, in which the parasite is found less commonly in the stool.
- Tests for antibodies to amebae are 90% sensitive for amebic liver abscess and 70% sensitive for amebic colitis.
- A problem with serological tests is that the antibody persists for years after the initial infection; therefore, differentiating between current and previous infection may be difficult, especially in individuals from endemic areas.
- Polymerase chain reaction
- Preliminary results of polymerase chain reaction (PCR) and DNA probes for strain-specific detection of E histolytica in stool and liver abscess aspirates appear encouraging.
- Sensitivity when performed on stool samples is estimated at 87%.
Imaging Studies
- Ultrasound: An amebic liver abscess will appear as a homogenous hypoechoic round lesion.
- CT scan: With contrast, an amebic liver abscess appears as a rounded, low-attenuation lesion with an enhancing rim. The abscess may be homogenous or septated, with or without observable fluid levels.
Procedures
- Liver aspiration
- If an amebic or pyogenic cause of liver abscess cannot be differentiated on clinical grounds and the patient is not stable enough to await serology or antigen detection, liver aspiration under CT scan or ultrasound guidance should be performed.
- Aspiration of an amebic liver abscess yields an odorless yellow-brown liquid that should be sent for microscopic examination, culture, and antigen detection.
- Colonoscopy
- Colonoscopy is recommended if stool and serologic study results are negative in a patient thought to have amebic colitis.
- A relative contraindication to endoscopy is if the patient is thought to have fulminant colitis, in which an increased risk of intestinal perforation exists.
- The appearance of amebic colitis may resemble that of inflammatory bowel disease, with a friable and diffusely ulcerated mucosa. In addition, an ameboma may be present in the form of an annular lesion, which usually occurs in the cecum and ascending colon and often is visually indistinguishable from colonic carcinoma.
Histologic Findings
Endoscopically obtained biopsy specimens should be taken from the edge of ulcers and evaluated for motile trophozoites. Pathologic evaluation of colonic lesions may vary from mucosal thickening to flask-shaped ulcerations to necrosis. Periodic acid-Schiff stains E histolytica magenta and may increase detection of the parasite in biopsied material.
Medical Care
Most individuals with E histolytica infection may be treated on an outpatient basis. Exceptions include the following:
- Patients with severe colitis requiring intravenous volume replacement
- Patients with fulminant colitis that may require surgical intervention
- Patients with liver abscess of uncertain etiology or not responding to therapy
- Patients with suspected liver abscess rupture
Surgical Care
- Indications for surgery in fulminant amebic colitis include perforation and persistence of abdominal distension and tenderness despite antiamebic therapy.
- Surgical intervention for intrathoracic or intraperitoneal rupture of an amebic liver abscess rarely is indicated, except in cases complicated by secondary bacterial infection.
Asymptomatic colonization with E histolytica is a risk factor for future development of invasive disease; therefore, affected patients should be treated, and a luminal agent (eg, iodoquinol, paromomycin, diloxanide furoate) should suffice. Patients with invasive amebiasis (eg, colitis, liver abscess) should be treated with metronidazole or tinidazole plus a luminal agent to eradicate colonization.
Drug Category: Amebicides
Parasite biochemical pathways are sufficiently different from the human host to allow selective interference by chemotherapeutic agents in relatively small doses.
| Drug Name | Iodoquinol (Yodoxin) |
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| Description | Amebicidal against E histolytica. Considered effective against trophozoite and cyst forms. |
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| Adult Dose | 650 mg PO tid for 20 d |
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| Pediatric Dose | 10-13 mg/kg PO tid for 20 d |
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| Contraindications | Documented hypersensitivity; hepatic dysfunction |
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| Interactions | None reported |
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| Pregnancy | C - Safety for use during pregnancy has not been established.
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| Precautions | Caution in thyroid disease; protein-bound serum iodine levels may increase during treatment with iodoquinol, interfering with certain thyroid function tests |
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| Drug Name | Paromomycin (Humatin) |
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| Description | Amebicidal and antibacterial aminoglycoside obtained from a strain of Streptomyces rimosus, active in intestinal amebiasis against trophozoite and cyst forms of E histolytica. Recommended for the treatment of Diphyllobothrium latum, Taenia saginata, Taenia solium, Dipylidium caninum, and Hymenolepis nana. |
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| Adult Dose | 25-35 mg/kg/d PO divided tid for 7 d |
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| Pediatric Dose | Administer as in adults |
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| Contraindications | Documented hypersensitivity; intestinal obstruction |
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| Interactions | Nephrotoxic potential may increase with concurrent administration of other aminoglycosides, penicillins, cephalosporins, amphotericin B, and loop diuretics; may decrease serum concentrations of digoxin |
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| Pregnancy | C - Safety for use during pregnancy has not been established.
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| Precautions | Because of narrow therapeutic index and toxic hazards associated with extended administration, not for long-term therapy; caution in renal failure, hypocalcemia, myasthenia gravis, and conditions that depress neuromuscular transmission; adjust dose in renal impairment; may cause abdominal cramping, nausea, emesis, and diarrhea; malabsorption, ototoxicity, and nephrotoxicity can occur if administered in high doses or in ulcerative colitis |
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| Drug Name | Diloxanide (Entamide, Furamide) |
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| Description | Dichloroacetamide derivative. Amebicidal against trophozoite and cyst forms of E histolytica. Not available in United States. |
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| Adult Dose | 500 mg PO tid for 10 d |
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| Pediatric Dose | <2 years: Not recommended
>2 years: 20 mg/kg PO divided tid for 10 d |
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| Contraindications | Documented hypersensitivity |
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| Interactions | None reported |
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| Pregnancy | C - Safety for use during pregnancy has not been established.
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| Precautions | GI adverse effects (eg, flatulence, abdominal cramps, nausea, emesis, diarrhea) may occur |
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| Drug Name | Metronidazole (Flagyl) |
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| Description | Active against various anaerobic bacteria and protozoa. Appears to be absorbed into cells. Intermediate metabolized compounds are formed and bind DNA and inhibit protein synthesis, causing cell death. Antimicrobial effect may be due to production of free radicals.
Indicated for invasive E histolytic infections. |
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| Adult Dose | 500-750 mg PO tid for 10 d |
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| Pediatric Dose | 35-50 mg/kg PO divided tid for 10 d |
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| Contraindications | Documented hypersensitivity |
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| Interactions | Cimetidine may increase toxicity of metronidazole; may increase effects of anticoagulants; may increase toxicity of lithium and phenytoin; ingestion of ethanol during therapy may induce a disulfiramlike reaction with abdominal cramps, nausea, and emesis |
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| Pregnancy | B - Usually safe but benefits must outweigh the risks.
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| Precautions | Adjust dose in hepatic disease; monitor for seizures and development of peripheral neuropathy; alcoholic beverages should be avoided during administration and for 3 d after |
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| Drug Name | Tinidazole (Fasigyn) |
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| Description | 5-Nitroimidazole derivative used for susceptible protozoal infections. Indicated to treat intestinal amebiasis and amebic liver abscess caused by E histolytica in adults and children aged 3 years and older. |
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| Adult Dose | Intestinal amebiasis: 600 mg PO bid for 5 d; alternatively, 2 g PO qd for 3 d with food
Hepatic amebic abscess: 2 g PO qd for 3-5 d with food |
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| Pediatric Dose | <3 years: Not established
>3 years:
Intestinal amebiasis: 50 mg/kg/d PO for 3 d with food; not to exceed 2 g/dose
Amebic liver abscess: 50 mg/kg/d PO for 3-5 d with food; not to exceed 2 g/dose, limited data exist for pediatric patients treated > 3 d (monitor closely) |
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| Contraindications | Documented hypersensitivity; first trimester of pregnancy |
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| Interactions | Limited data exist; interaction information based on experience with other nitroimidazole derivatives (ie, metronidazole); may prolong PT when coadministered with warfarin; avoid alcoholic beverages and preparations that contain ethanol or propylene glycol during and 3 d following administration (may cause disulfiramlike reaction); may increase serum levels of lithium, phenytoin, cyclosporine, tacrolimus, and fluorouracil; CYP450 inducers (eg, phenobarbital, rifampin, phenytoin) may increase elimination; CYP450 inhibitors (eg, cimetidine, ketoconazole) may decrease elimination; concurrent administration with cholestyramine may decrease oral bioavailability; oxytetracycline may antagonize effect |
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| Pregnancy | C - Safety for use during pregnancy has not been established.
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| Precautions | Carcinogenicity has been observed in mice and rats treated long-term with metronidazole (another nitroimidazole), although not observed with tinidazole, use cautiously; seizures and peripheral neuropathy have been reported; caution with history of blood dyscrasia; may cause metallic/bitter taste, nausea, anorexia, vomiting, weakness, fatigue, dizziness, or headache; if administered on day of hemodialysis, administer additional dose equivalent to one-half of recommended dose following dialysis |
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Further Inpatient Care
- Patients with suspected fulminant colitis or liver abscess rupture should be admitted to the hospital for further evaluation.
Further Outpatient Care
- Follow-up stool examination after therapy completion is recommended to ensure eradication of colonization.
Deterrence/Prevention
- Prevention of amebiasis is accomplished by eradicating fecal contamination of food and water through improved sanitation, hygiene, and water treatment. Amebic cysts are not killed by soap or low concentrations of chlorine or iodine; therefore, when in an area of endemicity, water should be boiled and vegetables should be washed with a detergent soap and soaked in acetic acid or vinegar for 10-15 minutes before consumption.
- Avoiding sexual practices that allow fecal-oral contact may reduce the risk of sexual transmission of infectious cysts.
- Screen family members or close contacts of an index case.
- Efforts to develop a vaccine are underway but have been complicated by an incomplete understanding of the mechanisms of immunity in humans and the lack of well-studied intestinal models of infection.
Complications
- Amebic colitis
- Fulminant or necrotizing colitis
- Toxic megacolon
- Ameboma
- Rectovaginal fistulas
- Amebic liver abscess
- Intrathoracic or intraperitoneal rupture with or without secondary bacterial infection
- Direct extension to pleura or pericardium
- Brain abscess
Prognosis
- The prognosis of cure following treatment for invasive amebiasis is good; however, prior infection and treatment do not protect against future colonization or recurrent invasive disease.
Patient Education
- Individuals who travel to areas where E histolytica is prevalent should be advised on the proper handling of food and water.
- Uncooked vegetables should be washed and soaked in acetic acid or vinegar for 10-15 minutes.
- Local water should be boiled.
Medical/Legal Pitfalls
- Failure to perform follow-up stool examination may facilitate persistent colonization with continued risk for invasive disease.
| Media file 1:
Trichrome stain of Entamoeba histolytica trophozoites in amebiasis. Two diagnostic characteristics are observed. Two trophozoites have ingested erythrocytes, and all 3 have nuclei with small, centrally located karyosomes. |
 |  View Full Size Image | |
Media type: Photo
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| Media file 2:
Trichrome stain of an Entamoeba histolytica cyst in amebiasis. Each cyst has 4 nuclei with characteristically centrally located karyosomes. Cysts measure 12-15 mm. |
 | View Full Size Image | |
Media type: Photo
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- Bassily S, Farid Z, el-Masry NA. Treatment of intestinal E. histolytica and G. lamblia with metronidazole, tinidazole and ornidazole: a comparative study. J Trop Med Hyg. Feb 1987;90(1):9-12. [Medline].
- Gonzalez-Ruiz A, Haque R, Rehman T. Diagnosis of amebic dysentery by detection of Entamoeba histolytica fecal antigen by an invasive strain-specific, monoclonal antibody-based enzyme-linked immunosorbent assay. J Clin Microbiol. Apr 1994;32(4):964-70. [Medline].
- Guerrant RL, Brush J, Ravdin JI. Interaction between Entamoeba histolytica and human polymorphonuclear neutrophils. J Infect Dis. Jan 1981;143(1):83-93. [Medline].
- Hai AA, Singh A, Mittal VK. Amoebic liver abscess. Review of 220 cases. Int Surg. Apr-Jun 1991;76(2):81-3. [Medline].
- Haque R, Ali IK, Akther S. Comparison of PCR, isoenzyme analysis, and antigen detection for diagnosis of Entamoeba histolytica infection. J Clin Microbiol. Feb 1998;36(2):449-52. [Medline].
- Haque R, Neville LM, Hahn P. Rapid diagnosis of Entamoeba infection by using Entamoeba and Entamoeba histolytica stool antigen detection kits. J Clin Microbiol. Oct 1995;33(10):2558-61. [Medline].
- Kagan IG. Serologic diagnosis of parasitic diseases. N Engl J Med. Mar 19 1970;282(12):685-6. [Medline].
- McAuley JB, Juranek DD. Paromomycin in the treatment of mild-to-moderate intestinal amebiasis. Clin Infect Dis. Sep 1992;15(3):551-2. [Medline].
- McAuley JB, Herwaldt BL, Stokes SL. Diloxanide furoate for treating asymptomatic Entamoeba histolytica cyst passers: 14 years'' experience in the United States. Clin Infect Dis. Sep 1992;15(3):464-8. [Medline].
- Petri WA, Singh U, Ravdin JI. Enteric amebiasis. In: Guerrant RL, Walker DH, Weller PF, eds. Tropical Infectious Diseases: Principles, Pathogens, and Practice. Vol 1. Philadelphia, Pa:. Churchill Livingstone;1999:685-97.
- Petri WA Jr, Singh U. Diagnosis and management of amebiasis. Clin Infect Dis. Nov 1999;29(5):1117-25. [Medline].
- Petri WA Jr. Recent advances in amebiasis. Crit Rev Clin Lab Sci. Jan 1996;33(1):1-37. [Medline].
- Ravdin JI. Entamoeba histolytica (Amebiasis). In: Mandell: Principles and Practice of Infectious Diseases. 5th ed. Philadelphia, Pa:. Churchill Livingstone;2000:2798-807.
- Ravdin JI. Amebiasis. Clin Infect Dis. Jun 1995;20(6):1453-64; quiz 1465-6. [Medline].
- Tachibana H, Kobayashi S, Okuzawa E. Detection of pathogenic Entamoeba histolytica DNA in liver abscess fluid by polymerase chain reaction. Int J Parasitol. Dec 1992;22(8):1193-6. [Medline].
Amebiasis excerpt Article Last Updated: Feb 21, 2007
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