NAME: Entamoeba histolytica
SYNONYM OR CROSS REFERENCE: Amebic dysentery, invasive amebiasis Footnote 1.
CHARACTERISTICS: Entamoeba histolytica is a pseudopod-forming nonflagellate protozoan parasite. E. histolytica is a member of the phylum Sarcomastigophora and subphylum Sarcodina, class Lobosa, order Amoebida, and family Entamoebidae Footnote 2Footnote 3. The life cycle consists of two stages: an ameboid trophozoite and infectious cyst forms Footnote 1-3. Following ingestion of infective cysts, and passage through the stomach, the organism excysts and emerges in the large intestine as an active trophozoite. Trophozoites multiply by simple division and encyst as they move by means of cytoplasmic protrusions called pseudopodia further down the large bowel Footnote 3-6. Cysts are then expelled with the feces and may remain viable in a moist environment for weeks to months Footnote 6. The invasive trophozoites are 10 to 60 μm in diameter, have a single nucleus with a central karyosome, and are facultative anaerobes and require complex axenic media for growth Footnote 3. Food vacuoles are common in the cytoplasm of active trophozoites and may contain host erythrocytes in samples from diarrheic feces. Cysts are 10 to 15 μm, round, contain four or fewer nuclei, glycogen, and ribosomal assemblies called chromatoid bodies and are surrounded by a chitin wall Footnote 5.
PATHOGENICITY/TOXICITY: Infections with E. histolytica can be asymptomatic, symptomatic without tissue invasion or symptomatic with tissue invasion Footnote 2. Most infections (90%) with E. histolytica are asymptomatic, with just passage of cysts in stools of infected individuals Footnote 2Footnote 7Footnote 8. Symptomatic patients present mainly with intestinal diseases Footnote 2Footnote 3. Amebic dysentery is characterized by diarrhea with severe cramping, lower abdominal pain, low-grade fever, and presence of blood or mucous in the stool for long periods of time Footnote 2Footnote 3Footnote 8. Ulcers may be produced due to intestinal invasion by trophozoites Footnote 2. Less common, fulminant amebic colitis/dysentery with profuse bloody diarrhea, fever, pronounced leucocytosis, abdominal pain with peritoneal signs, and extensive involvement of the colon may occur in pregnant women, debilitated or immunocompromised individuals Footnote 3Footnote 5. Rare, but serious complications of the disease include abdominal perforation, acute necrotizing colitis, toxic megacolon, peritonitis or ameboma Footnote 2Footnote 8. Patients with acute necrotizing colitis present with fever, bloody mucoid diarrhea, and abdominal pain with rebound tenderness, and peritoneal irritation Footnote 8. Ameboma is a benign tumor-like lesion, consisting of granulamatous tissue, which occurs due to chronic ulceration Footnote 2. Extraintestinal manifestations of E. histolytica infection include liver abscess, lung abscess resulting in pneumonia or empyema, and rarely cerebral abscess Footnote 2Footnote 3Footnote 5. Extension of abscess into the pericardium is the most serious manifestation, resulting in cardiac compression and death or chronic pericardial disease Footnote 2Footnote 3. Cutaneous amebiasis has also been reported in children with E. histolytica infection Footnote 9.
EPIDEMIOLOGY: E. histolytica is a worldwide distributed pathogen and is the second leading cause of parasitic death Footnote 2Footnote 5Footnote 7. It infects an estimated 500 million people per year, causes liver abscess and colitis in 50 million, and results in death of an estimated 40,000-100,000 individuals annually Footnote 2Footnote 5Footnote 7. Infections due to E. histolytica are more common in tropical and subtropical areas such as Mexico, South and West Africa, western South America and South Asia Footnote 2Footnote 3. Infection rates are also high in temperate areas with poor sanitation Footnote 2. Most cases of amebiases in the Unites States occur in immigrants from endemic areas, in HIV infected patients and people living in states that border Mexico Footnote 2Footnote 5. Individuals travelling to endemic areas are also at risk of developing amebiases due to E. histolytica Footnote 5. Amebic liver abscess is rare (occurring in less than 5% of cases), but is the most common extraintestinal manifestation of E. histolytica infection Footnote 10. It is endemic in Mexico, the Indian subcontinent, Indonesia, sub-Saharan and tropical regions of Africa, and parts of Central and South America Footnote 10.
HOST RANGE: Humans and non-human primates Footnote 5.
INFECTIOUS DOSE: Average infectious dose is >1000 organisms. Ingestion of one cyst, however, has also been reported to cause disease Footnote 3.
MODE OF TRANSMISSION: Transmission can occur through fecal-oral route (ingestion of food and water, contaminated with feces containing E. histolytica cysts)1-3Footnote 8. Sexual transmission can also occur Footnote 3Footnote 5.
INCUBATION PERIOD: Intestinal diseases due to E. histolytica may occur within a few days or may take months Footnote 2. Amebic liver abscess associated with E. histolytica usually appears 8 to 20 weeks after the patient has left an endemic area Footnote 10.
COMMUNICABILITY: Amebiases can spread within familiesFootnote 5. Person-person transmission occurs through fecal-oral-route under conditions of poor hygiene (45 million cysts are passed in the stool daily) Footnote 3. Sexual transmission occurs mainly in homosexual males Footnote 3.
DRUG SUSCEPTIBILITY: Trophozoites can be killed by metronidazole, nitroimidazole derived compounds such as tinidazole, secnidazole and ornidazole; emetine, and chloroquine Footnote 11. Diloxanide furoate, diiodohydroxyquin, and paromomycin have activity against cysts Footnote 2Footnote 11.
DRUG RESISTANCE: Laboratory induced resistance against metronidazole has been reported Footnote 11.
SUSCEPTIBILITY/RESISTANCE TO DISINFECTANTS: E. histolytica cysts are the most highly resistant to disinfectants, after bacterial spores Footnote 12, and are most susceptible to ozone, followed by chlorine dioxide, 8 ppm iodine, and free chlorine Footnote 12Footnote 13.
PHYSICAL INACTIVATION: Amebic cysts can be killed by heating at 50-56°C, by solar irradiation 1 Footnote 14 or by freezing Footnote 15. Pasteurization or brief boiling is sufficient to eliminate the risk of transmission of E. histolytica Footnote 15.
SURVEILLANCE: Monitor for symptoms. Diagnosis consists of: 1) monitoring for clinical symptoms; 2) endoscopic examination and demonstration of abscesses in liver, lung or other organs by sonography, or CT scan; 3) microscopic detection of trophozoites and cysts along with ingested RBC’s in clinical samples such as liquid feces and sigmoidoscopic aspirates; 4) detection of E. histolytica antigen (such as lectin); 5) detection of specific antibodies (for e.g. IgA antibody against lectin) by serological methods such as ELISA or EIA; 5) PCR or real-time PCR to detect trophozoite or cyst DNA; 6) in-vitro cultivation of the amebae is useful for chronic amebiasis with few cysts Footnote 1-3.
Note: All diagnostic methods may not be available in all countries.
FIRST AID/TREATMENT: Asymptomatic patients carrying the pathogen can be treated with luminal amebicides (kills cysts) such as oral paromomycin, oral diloxanide furoate, and oral iodoquinol Footnote 1Footnote 2Footnote 8. Amebic dysentery/colitis or amebic abscess can be treated with tissue amebicides (kills trophozoites) such as oral metronidazole, or oral tinidazole Footnote 1Footnote 8. Oral paromomycin, diloxanidfuroate or iodoquinol are prescribed for elimination of cysts following treatment with tissue amebicides Footnote 1Footnote 2Footnote 8. Therapeutic aspiration/drainage of an amebic liver may be required, alone or along with antiparasitic therapy, in patients who do not respond to therapy Footnote 8.
LABORATORY-ACQUIRED INFECTIONS: Laboratory acquired infections have been reported with Entamoeba spp. Footnote 16.
PRIMARY HAZARDS: Ingestion Footnote 16.
SPECIAL HAZARDS: None.
RISK GROUP CLASSIFICATION: Risk Group 2.
CONTAINMENT REQUIREMENTS: Containment Level 2 facilities, equipment, and operational practices for work involving infectious or potentially infectious materials, animals, or cultures Footnote 18.
PROTECTIVE CLOTHING: Lab coat. Gloves when direct skin contact with infected materials or animals is unavoidable. Eye protection must be used where there is a known or potential risk of exposure to splashes Footnote 18.
OTHER PRECAUTIONS: All procedures that may produce aerosols, or involve high concentrations or large volumes should be conducted in a biological safety cabinet (BSC). The use of needles, syringes, and other sharp objects should be strictly limited. Additional precautions should be considered with work involving animals or large scale activities Footnote 18.
SPILLS: Allow aerosols to settle, and wearing protective clothing, gently cover the spill with paper towels and apply an appropriate disinfectant, starting at the perimeter and working towards the centre. Allow sufficient contact time before clean up Footnote 19Footnote 20.
DISPOSAL: Decontaminate all wastes that contain or have come in contact with the infectious organism before disposing by autoclave, chemical disinfection, gamma irradiation, or incineration Footnote 18.
STORAGE: The infectious agent should be stored in leak-proof containers that are appropriately labelled Footnote 18.
REGULATORY INFORMATION: The import, transport, and use of pathogens in Canada is regulated under many regulatory bodies, including the Public Health Agency of Canada, Health Canada, Canadian Food Inspection Agency, Environment Canada, and Transport Canada. Users are responsible for ensuring they are compliant with all relevant acts, regulations, guidelines, and standards.
UPDATED: December 2011
PREPARED BY: Pathogen Regulation Directorate, Public Health Agency of Canada
Although the information, opinions and recommendations contained in this Pathogen Safety Data sheet are compiled from sources believed to be reliable, we accept no responsibility for the accuracy, sufficiency, or reliability or for any loss or injury resulting from the use of the information. Newly discovered hazards are frequent and this information may not be completely up to date.
Copyright © Public Health Agency of Canada, 2011 Canada