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amebiasis

Etiology: -infection with Entamoeba histolytica Epidemiology: 1) infection generally acquired by ingestion of contaminated food 2) more common in developing countries where sanitation may be suboptimal 3) one outbreak caused by a contaminated colonic irrigation machine 4) cysts are ingested & transform into trophozoites in the intestine 5) trophozoites proliferate by binary fission in the colon 6) both cysts & trophozoites may be passed in the feces 7) only mature cysts are infective Pathology: - host defenses, previous exposure, diet & strain of E. histolytica influence severity of infection Clinical manifestations: 1) acute manifestations (amebic dysentery) a) fulminant onset b) cramping abdominal pain c) diffuse abdominal tenderness d) bloody diarrhea e) tenesmus f) fever g) dehydration h) invasion of the intestinal mucosa (colon & terminal ileum) may lead to perforation & peritonitis 2) chronic manifestations (amebic colitis) a) intermittent symptoms b) mild, crampy abdominal pain c) diarrhea containing mucus or blood d) fever e) tenderness of cecum & ascending colon during cramping f) liver tenderness (amebic hepatitis) g) friction rub with liver abscess (5% of symptomatic patients) Laboratory: 1) stool antigen testing more sensitive than ova & parasites - Entamoeba histolytica antigen in stool 2) stool ova & parasites a) motile (trophozoites) or encysted organisms b) trophozoites 1] measure 10-60 um 2] commensal forms are generally 15-20 um 3] invasive forms are generally > 20 um 4] progressively motile on wet mounts with hyaline finger-like pseudopods 5] unstained nucleus is not visible 6] with invasive disease, some trophozoites may contain ingested erythrocytes (pathognomonic) 7] both pathogenic & commensal forms may contain ingested bacteria in the cytoplasm 8] stained specimens, peripheral nuclear chromatin is evenly distributed along nuclear membrane c) cysts 1] spherical in shape 2] measure 10-20 um in diameter (generally 12-15 um) 3] precyst has single nucleus without refractile cyst wall 4] mature cyst has 4 nuclei, each 1/6 the diameter of the cyst 5] glycogen is generally diffuse 6] chromotoid bodies: elongated bars 3) serology for Entamoeba histolytica a) antibodies are present in both invasive & intestinal disease b) ELISA or EIA 1] 90% of patients with amebic abscess are positive 2] 70% of patients with intestinal disease are positive 3] 10% of asymptomatic carriers are positive c) indirect hemagglutination (IHA) titers remain increased for years after invasive infection d) counterimmunoelectrophoresis (CIE) becomes negative after cure of invasive disease 4) cultures are not widely used 5) polymerase chain reaction (PCR) - Entamoeba histolytica DNA 6) complete blood count (CBC) a) leukocytosis without eosinophilia b) mild anemia may be present 7) serum alkaline phosphatase may be elevated with hepatic involvement Special laboratory: 1) flexible sigmoidoscopy a) ulcers of various depth with raised edges b) small hemorrhages c) hyperemia d) trophozoites on aspirate of mucosal lesions 1] indicate invasive disease 2] highest yield at ulcer edge 2) fine needle aspiration of liver abscess Radiology: 1) barium enema may show irregular distribution of barium in cecum & ascending colon 2) computed tomography (CT), ultrasound or MRI of abdomen may show liver abscess Differential diagnosis: 1) infectious agents causing dysentery a) parasitic - Giardia lamblia - Cryptosporidia - Isospora - schistosomiasis b) bacterial - Escherichia coli - Campylobacter - Salmonella - Shigella - Vibrio sp - Yersinia - tuberculosis c) viral gastroenteritis 2) other causes or dysentery a) diverticulitis b) irritable bowel syndrome c) regional enteritis d) malabsorption syndrome e) exacerbation of inflammatory bowel disease (see management) f) colon cancer g) lymphoma 3) other causes of hepatic abscess a) pyogenic abscess b) hepatocellular carcinoma c) echinococcal cyst Complications: - intestinal perforation - peritonitis Management: 1) asymptomatic individual passing cysts in feces a) treatment recommended because subsequent tissue invasion may occur b) iodoquinol* (Yodoxin) 650 mg PO TID for 21 days c) paromomycin* (Humatin) 25-30 mg/kg divided TID for 7 days d) diloxanide furoate (Furamide) 500 mg TID for 10 days (available from CDC, if b & c not options) 2) symptomatic intestinal disease a) metronidazole (Flagyl) 750 mg PO TID for 10 days b) tinidazole is alternative for parasitic clearance c) dehydroemetine (Mebadin) for severe disease (available from CDC) d) follow with intraluminal agent (iodoquinol, paromomycin) to eradicate cysts 3) hepatic abscess a) metronidazole (Flagyl) 750 mg PO TID for 10 days b) dehydroemetine (Mebadin) followed by chloroquine is an alternative c) aspiration of abscess 1] no improvement after 72 hours of therapy 2] severe liver tenderness or swelling d) follow with intraluminal agent (iodoquinol, paromomycin) to eradicate cysts 4) glucocorticoid therapy (for inflamatory bowel disease) will exacerbate amebiasis & may result in toxic megacolon 5) diet a) avoid fecally contaminated food and water b) vegetables grown in endemic areas are often contaminated c) water must be boiled to eradicate organism d) vegetables should be cleaned with a detergent & soaked in vinegar or acetic acid for 10 minutes to eradicate cysts e) peel all fruit f) avoid sexual practices that promote fecal-oral spread * intraluminal agent

Specific

amoebic meningoencephalitis chronic intestinal amebiasis (amebic colitis)

General

protozoan infection parasitic gastroenteritis

References

  1. DeGowin & DeGowin's Diagnostic Examination, 6th edition, RL DeGowin (ed), McGraw Hill, NY 1994, pg 863
  2. Clinical Diagnosis & Management by Laboratory Methods, 19th edition, J.B. Henry (ed), W.B. Saunders Co., Philadelphia, PA. 1996, pg 1271-74
  3. Saunders Manual of Medical Practice, Rakel (ed), WB Saunders, Philadelphia, 1996, pg 899-901
  4. Medical Knowledge Self Assessment Program (MKSAP) 14, 16, 17, 18. American College of Physicians, Philadelphia 2006, 2012, 2015, 2018.