- Humans are the only reservoir, and infection occurs by ingestion of mature cysts in food or water, or on hands contaminated by faeces.
- The cysts of E. histolytica enter the small intestine and release active amoebic parasites (trophozoites), which invade the epithelial cells of the large intestines, causing flask-shaped ulcers. Infection can then spread from the intestines to other organs, eg liver, lungs and brain, via the venous system.
- Asymptomatic carriers pass cysts in the faeces and the asymptomatic carriage state can persist indefinitely. Cysts remain viable for up to 2 months.
- Invasive amoebiasis most often causes an amoebic liver abscess but may affect the lung, heart, brain, urinary tract and skin.
E. histolytica (the cause of invasive amoebiasis) must be differentiated from Entamoeba dispar, which is a normal commensal of the gastrointestinal tract.
- About 10 percent of the world's population is infected with E.histolytica.
- It is the third most common cause of death (after schistosomiasis and malaria) from parasitic infections.
- Very common in South and Central America, West Africa and South-East Asia. Rare in temperate climates.
- About 90% of infections are asymptomatic and the remaining 10% produce a spectrum of disease varying from dysentery to amoebic liver abscess.
Incubation period may be as short as 7 days and tissue invasion mostly occurs during first 4 months of infection.
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- The most common type of amoebic infection is the asymptomatic passage of cysts.
- Symptomatic patients initially have lower abdominal pain and diarrhoea and later develop dysentery (with blood and mucus in stool).
- Amoebic colitis with dysentery: loose stools with fresh blood. Patient is usually generally well with mild or moderate abdominal pain. Symptoms often fluctuate over weeks or even months with patient becoming debilitated.
- Abdominal tenderness in one or both iliac fossae but may be generalised. Palpably thickened gut. Low fever. Abdominal distension in more severely ill patients passing relatively small amounts of stool sometimes.
- Amoebic colitis without dysentery: change in bowel habit, blood stained stools, flatulence and colicky pain, tenderness in right iliac fossa or other places over colon. May disappear or progress to dysentery.
- Rectal bleeding: may occasionally be only sign, with or without tenesmus (common in children).
- Fulminant colitis: more likely in children and patients taking steroids; high grade fever, severe abdominal pain, increasing distension of abdomen with vomiting plus watery diarrhoea. Absent bowel sounds. X-ray may show free peritoneal gas with acute gaseous dilatation of the colon.
- Localised perforation and appendicitis: deep ulcer may cause sudden perforation with peritonitis or may leak causing pericolic abscess or retroperitoneal infection. May also resemble simple appendicitis, often with signs of dysentery.
- Usually no current and often no history of dysentery.
- Usually occurs within 8 weeks to 1 year of infection.
- Presents with sweating and pyrexia, painful liver or diaphragm together with weight loss often appearing insidiously, but pain may appear abruptly.
- Fever is typically remitting with prominent evening rise with brief rigors and profuse sweating
- Often anaemia and dry painful cough.
- Liver enlargement with localised tenderness in right hypochondrium, epigastrium and intercostal spaces overlying liver.
- May find epigastric mass from left-lobe lesion.
- Upward enlargement may cause bulging of right chest wall with raised upper level of liver dullness on percussion. May hear reduced breath sounds or crepitations at right lung base.
- Abscess may extend into adjacent structures, usually the right chest, peritoneum and pericardium. If extends into lung produces hepatobronchial fistula with expectoration of brownish, necrotic liver tissue. May also cause peritonitis, pericarditis, brain abscess or genitourinary disease.
- Other causes of infective colitis, ulcerative colitis, colorectal carcinoma.
- In chronic infection, other possible diagnoses include Crohn's disease, ileocaecal tuberculosis, diverticulitis, anorectal lymphogranuloma venereum.
- Amoebic liver abscess has to be differentiated from pyogenic abscess which may occur particularly in older patients with underlying bowel disease or after surgery.
- Full blood count (leucocytosis), raised ESR, abnormal liver function tests (raised alkaline phosphatase and transaminases)
- Stool examination:
- Microscopic stool examination for trophozoites from a single stool sample in amoebic colitis is only up to 50% sensitive but examination of 3 stool samples (taken over a period of up to 10 days) improves the sensitivity to 85-95%.
- E. histolytica should be differentiated from other Entamoeba spp. The World Health Organisation now recommends that intestinal amoebiasis should be diagnosed with specific stool E. histolytica testing (eg cultures, antigen testing or polymerase chain reaction) rather than examining stool for ova and parasites.
- Serology: antibody testing is positive in 95% of cases of liver abscess, 60% of invasive bowel disease and nearly 100% of patients with amoeboma.
- Polymerase chain reaction (PCR) tests (faeces, abscess aspirate or other tissues).
- Barium studies are contraindicated in acute amoebic colitis because of the risk of perforation.
- Ultrasound, CT and MRI scans of the abdomen can be useful in diagnosing hepatic amoebiasis.
- Ultrasound or CT-guided liver abscess aspiration.
- Proctoscopy, sigmoidoscopy or colonoscopy: mucosal scrapings for biopsy and E. histolytica testing.
- Abscesses resolve slowly and may increase in size during treatment and so clinical response is more important in monitoring progress rather than repeated scans.
- Fluid and electrolyte replacement, gastric suction and blood transfusion may be required.
- Diloxanide furoate is the drug of choice for asymptomatic patients with E. histolytica cysts in the faeces (metronidazole and tinidazole are relatively ineffective).
- Metronidazole is the first choice for treatment of acute invasive amoebic dysentery. Tinidazole is also effective.
- Treatment with metronidazole or tinidazole is followed by a 10-day course of diloxanide furoate to destroy any amoebae in the gut.
- Diloxanide furoate is also given as a 10-day course for chronic infections.
- Amoebic abscesses of the liver:
- Metronidazole and tinidazole are effective for amoebic abscesses of the liver.
- Diloxanide furoate is ineffective against hepatic amoebiasis but a 10-day course should be given at the completion of metronidazole or tinidazole treatment to destroy any amoebae in the gut.
- Surgical drainage of an uncomplicated amoebic liver abscess is unnecessary and should be avoided.
- However the abscess should be aspirated if there is a risk that it may rupture or if metronidazole leads to no improvement after 72 hours of treatment. Aspiration may need to be repeated.
- Percutaneous catheter drainage improves the outcome for patients with amoebic empyema or amoebic pericarditis.
- Laparotomy is required for rupture of a liver abscess.
- Amoebic colitis may lead to fulminant or necrotising colitis, toxic megacolon, amoeboma or a rectovaginal fistula.
- Amoebic liver abscess: may extend and/or rupture into the abdomen or chest, or disseminate and cause a brain abscess.
- In uncomplicated disease, mortality rate is less than 1% but is much higher in complicated severe disease, eg fulminant amoebic colitis, chest involvement or cerebral amoebiasis.
- More severe illness occurs in children (especially neonates), the immunosuppressed, malnourished, pregnancy and post-partum.
- Recurrence is common if amoebae are not completely eradicated.
- The bowel heals rapidly and completely; hepatic abscesses usually disappear within 8 months to 2 years.
- Successful control of amoebiasis depends on prevention of infection through adequate sanitation, safe food and water and good personal hygiene of the population.
- No vaccine is yet available.
Further reading & references
- Stanley SL Jr; Amoebiasis. Lancet. 2003 Mar 22;361(9362):1025-34.
- Lacasse A, Cleveland KO; Amebiasis. eMedicine. Updated: Feb 9, 2009.
- Amebiasis, DPDx, Centers for Disease Control & Prevention
- Jackson TF; Entamoeba histolytica and Entamoeba dispar are distinct species; clinical, Int J Parasitol. 1998 Jan;28(1):181-6.
- Heckendorn F, N'Goran EK, Felger I, et al; Heckendorn F, N'Goran EK, Felger I, et al; Species-specific field testing of Entamoeba spp. in an area of high endemicity. Trans R Soc Trop Med Hyg. 2002 Sep-Oct;96(5):521-8.
- British National Formulary; 59th Edition (March 2010) British Medical Association and Royal Pharmaceutical Society of Great Britain, London.
|Original Author: Dr Colin Tidy||Current Version: Dr Colin Tidy|
|Last Checked: 25/08/2010||Document ID: 1798 Version: 22||© EMIS|
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