Trophozoite of Entamoeba histolytica in saline preparation of stool as shown above image. Trophozoite has the following features-
Entamoeba histolytica is an invasive enteric protozoan parasite that is the cause of amebiasis. It derives its name from its ability to lyse virtually every tissue in the human body and in the bodies of experimental animals.
Trophozoites of Entamoeba histolytica resides in the mucosa and submucosa of the large intestine of man.
Entamoeba histolytica exists in 3 morphological forms-
Trophozoite
Cyst
Entamoeba histolytica Passes life cycle in only one host i.e. Man
2 phases of development-trophozoite and cyst with the transitory stage of pre cystic phase
Infective form- mature cyst (Quadrinucleate cyst)
Mode of infection – feco-oral route
Site of localization– large intestine (caecum)
Infective stage- mature quadrinucleate cyst
The reservoir of infection- human
Mode of infection- fecal-oral route
1. fecal contamination of drinking water, vegetables, and food
2. eating of uncooked vegetables and fruit which have been fertilized with infected human feces
3. handling of food by infected individuals (cyst passers or cyst carriers)
Incubation period– 4-5 days
Pathogenic lesion-
a. primary or intestinal – infection is entirely limited to LI
b. secondary or extraintestinal or metastatic lesion
Intestinal ulcer
Site – lesions may be
a. Generalized-whole length of LI up to internal sphincter is involved
b. Localized
Ileocecal region- commonest site
Sigmoido-rectal region
Shape – flask-shaped ulcer
Ulcers maybe
a. superficial-up to muscularis mucosa
b. deep- into the submucous layer-
Extraintestinal lesion
Fulminant amoebic colitis
Chronic amoebic colitis
It is clinically similar to inflammatory bowel disease (IBD). There is recurrent episodes of bloody diarrhea and vague abdominal discomfort.
Amebomas
It is a less common form of intestinal disease, arises from the formation of annular colonic granulation in response to the infecting organisms, which results in a large local lesion of the bowel.
Amoebic peritonitis
It is generally secondary to a ruptured liver abscess. Patients present with fever and a rigid distended abdomen. Amoebic appendicitis in countries of high prevalence, amoebiasis occasionally presents as acute appendicitis.
It is divided into two parts. One is a diagnosis of intestinal amoebiasis whereas another diagnosis of extra intestinal amoebiasis.
Lab Diagnosis of Intestinal amoebiasis
Specimen: fresh stool, colonoscopic biopsy, or scraping from the margin of the lesion
Microscopic Examination
Method: Wet mount preparation with normal saline and iodine
permanent stain- trichrome stain, iron, and hematoxylin stain
Stool leukocytes may be found, but in fewer numbers than in shigellosis
trophozoites that contain ingested red blood cells (RBCs) – is diagnostic of E. histolytica infection
Examination of a single stool sample has a sensitivity of only 33-50%; however, examination of 3 stool samples over no more than 10 days can improve the detection rate to 85-95%.
Culture
It is common in following media like
Antigen detection
Enzyme-linked immunosorbent assay (ELISA) is used to detect antigens from E. histolytica in stool samples.
Antibody detection
Methods-ELISA, latex agglutination test, Indirect hemagglutination, indirect fluorescent assay, RIA, and CCIE.
Diagnosis of extraintestinal amoebiasis
Blood tests
Tissue amebicides
intestinal wall, liver, and other metastatic lesions: emetine, dehydroemetine-parenteral
Liver and lungs only: 4-aminoquinolone ( chloroquine)
Luminal amebicides
Direct-acting:
i) halogenated hydroxyquinolones-diodoquin, clioquinol
ii) Dichloroacetamide group- diloxanide
iii) Antibiotics-paromomycin ( brand name humatin -oral)
b. Indirect acting-tetracycline
Both luminal and tissue amebicides: oral Metronidazole, tinidazole, nitroimidazole
Personal prophylaxis:
Community prophylaxis:
About amoebic versus bacillary dysentery, click on this title.