Entamoeba histolytica (True Pathogen – Amebiasis)

Organism:
This organism belongs to the amebae, is a true pathogen, and causes amebiasis.  Both the trophozoite (usual size, 12-60 µm) and cyst forms (usual size, 10-20 µm) can be found in clinical specimens.

 

Entamoeba histolytica                                
Note:  ingested RBCs in troph

     

Entamoeba histolytica/E. dispar

 Amebiasis, flask-shaped ulcer                           Amebic ulcer, courtesy of CDC

Life Cycle:
Large bowel, organisms passed in feces, AND/OR
Large bowel, dissemination via blood (liver, lung, brain, pericardium, other tissues).

Acquired:
Fecal-oral transmission via cyst form; contaminated food and water

Epidemiology:
Worldwide, primarily human-to-human transmission

Clinical Features:
Intestinal: Diarrhea, dysentery or asymptomatic; it is estimated that only a small proportion (2 to 8%) of infected individuals will have invasive disease beyond the lumen of the bowel. Also, organisms may be spontaneously eliminated with no disease symptoms.
Extraintestinal:  Right upper quadrant pain, fever.  Blood flow draining the intestine tends to return to the liver, most commonly the upper right lobe. The organisms present in the submucosa can therefore be carried via the bloodstream to the liver. Onset of symptoms may be gradual or sudden; upper right abdominal pain with fever from 38 to 39°C is the most consistent finding. Weakness, weight loss, cough, and sweating are less commonly seen. There tends to be hepatomegaly with tenderness; however, liver function tests may be normal or slightly abnormal, with jaundice being very rare. There may be changes at the base of the right lung owing to the elevated diaphragm. The abscess can be visualized radiologically, sonically, or by radionuclear scan, and the majority of patients have a single abscess in the right lobe of the liver. The most common complication is rupture of the abscess into the pleural space. An abscess can also extend into peritoneum and through the skin. Hematogenous spread to the brain, as well as to the lung, pericardium, and other sites, is possible. 

Clinical Specimen:
Intestinal:  Stool, sigmoidoscopy specimens
Extraintestinal:  Liver aspirate, biopsy specimen, serum for antibody

Laboratory Diagnosis:
Intestinal:  Ova and Parasite examination (concentration, permanent stained smear); fecal immunoassay to detect antigen of Entamoeba histolytica/E. dispar group or the true pathogen, E. histolytica.
Extraintestinal:  Trichrome, PAS stains, routine histology (H&E stain); test for antibody.  E. histolytica cysts and trophozoites are found in the stools of only a few patients with liver abscess. About 60% of these patients have no intestinal symptoms or any history of dysentery.

Organism Description:
Trophozoite:  Evenly arranged nuclear chromatin, central karyosome, fine cytoplasm (may contain ingested RBCs)
Cyst:  May contain chromatoidal bars with smooth, rounded edges; mature cyst contains 4 nuclei (rarely more seen).

Laboratory Report:
Without confirmation using the specific immunoassay to detect the true pathogen, E. histolytica, the report must indicate:  Entamoeba histolytica/E. dispar group (indicate cysts and/or trophozoites); this report indicates that the organism morphology cannot be used to differentiate the true pathogen, E. histolytica, vs. the nonpathogen, E. dispar.
Report Comment:  Submit a fresh stool if you want confirmation of the true pathogen (Entamoeba histolytica).  The laboratory will then test the fresh stool (fresh, frozen, some acceptable in Cary-Blair) for the presence of the true pathogen, Entamoeba histolytica, antigen.  If confirmation of E. histolytica is not performed, then the physician will usually treat if the patient is symptomatic.

Treatment: 
Garcia, L.S. 2007.  Diagnostic Medical Parasitology, 5th ed., ASM Press, Washington, D.C.

Control:
Improved hygiene, adequate disposal of fecal waste, adequate washing of contaminated fruits and vegetables

Comments:
E. histolytica (true pathogen, cause of amebiasis) cannot be differentiated from the nonpathogen, E. dispar (although when trophozoites are found to contain ingested RBCs, it is more likely to be E. histolytica and will be reported as such).  E. dispar does not normally ingest RBCs.

Research has centered on explaining the molecular differences between pathogenic E. histolytica and nonpathogenic E. dispar.  However, the molecules considered the most important for host tissue destruction (amebapore, galactose/N-acetyl galactosamine inhibitable lectin, and cysteine proteases) can be found in both organisms.  Pathogenicity differences may be related to the composition and properties of the surface coat components (or pathogen-associated molecular patterns, PAMPS), and the ability in innate immune response to recognize these components, thus eliminating the organisms. Targets of the host immune system modulation appear to be both neutrophils and macrophages, which are unable to abort the infection even when present at the site of the lesion.  The total body of evidence supports the differentiation of the pathogenic E. histolytica from the nonpathogenic E. dispar as two distinct species.