Amoebiasis: A parasitic infection

Amoebiasis: A parasitic infection


Amoebiasis is an infection caused by Entamoeba histolytica with or without symptoms (WHO 1969). Synonyms include entamoebiasis, amoebiosis, amoebic dysentery or bloody flux. Entamoeba dispar is a harmless commensal, which is indistinguishable from E. histolytica. The other members of the group infecting humans are E. moshkovskii, E. hartmannii, E. gingivalis, Endolimax nana and Iodamoeba butschlii.

Theoretically, ingestion of even one viable cyst can cause infection. Trophozoites are digested and destroyed by the gastric acid, hence, cannot cause infection even though they are ingested. 

The cyst divides into four initially, which divide again into eight daughter amoebae after an incubation period of 1–4 weeks, which may, however, be from few days to a year. These grow and mature into adult amoebae in about 7–10 days and stay as boarders in the large intestine, mainly the cecum and the sigmoid, feeding on intraluminal cellular debris and the bacteria.

The infection is usually asymptomatic. Under unfavorable conditions and as the liquid stool becomes solid during its passage down the colon, the vegetative forms become cysts and are passed in the feces. Most individuals are asymptomatic cyst shredders.

Symptoms of the disease

Most often, clinical manifestations are insidious and intermittent, commencing as abdominal discomfort, bloating, irregular bowel habits, intermittent dysentery with or without blood/mucous, tenesmus with bloody mucoid diarrhea, constitutional symptoms, abdominal tenderness, toxic megacolon, and finally symptoms and signs of peritonitis secondary to perforation.


Extraintestinal manifestations are primarily those of hepatic involvement. These include fever, pain in right lower chest, which may be related to respiration, appetite disturbances, breathlessness, cough with or without expectoration and breathlessness, occasionally mild jaundice, rarely convulsions.


These are secondary to severe toxemia, perforation of the bowel, toxic megacolon, rupture of the hepatic abscess into pleura, lung, peritoneum, pericardium, skin and subcutaneous tissue. Extraintes-tinal spread metastasizing in the brain and bones is uncommon. Formation of a granuloma in the bowel wall mimicking a malignant growth, the amoeboma, is also not common.

Rarely, a large hepatic abscess producing obstructive jaundice can occur. Fever, leukocy-tosis with elevated polymorphs, rise in hepatic enzymes and serum bilirubin are the accompaniments of the complications.


High degree of suspicion in endemic areas is a prerequisite. Fresh liquid stool examination showing hematophagus trophozoites with Charcot-Leyden crystals is characteristic. Stool examination, preferably for three consecutive days is advocated.

Presence of only cysts in asymptomatic individuals is not diagnostic, since the cysts of E. dispar, which is noninvasive and harmless are indistinguishable from those of invasive E. histolytica. Sigmoidoscopic scrapings of ulcers showing hematophagus trophozoites are diagnostic. So also is the finding of amoebae from the walls of hepatic abscess.

Ultrasound (USG) scan of the abdomen helps in the delineation of hepatic abscesses. X-ray of the chest helps in the detection of spread to the pleura, lung or pericardium. X-ray of the abdomen is useful for the diagnosis of peritonitis and toxic megacolon. Computed tomography/magnetic resonance imaging help in the diagnosis of intracranial spread of amoebiasis.

Conditions to be kept in the mind are different types of E. coli and the Shigella enteric infections in acute presentation and tuberculosis in subacute or chronic presentation. Antibody detection at the end of 1 week of invasive amoebiasis, indirect hemagglutination assay (IHA) and enzyme-linked immuno-sorbent assay (ELISA) are diagnostic. Polymerase chain reaction (PCR) in advanced centers is confirmatory.


Asymptomatic cyst shredders, need/should not be treated–WHO guidelines.

Combination therapy with luminal and tissue amoebicides is highly recommended. Introduction of nitroimidazole derivatives has revolutionized the treatment of amoebiasis. Usage of cardiotoxic emetine and the relatively less toxic dehydroemetine are now of historical interest.
Though metronidazole and other derivatives are highly toxic to the vegetative forms and to a lesser extent the cysts, a course of luminal amoebicides is recommended for complete cure.

Tissue Amoebicides

  • Metronidazole: 500 mg IV 8th hourly. For 7–10 days for extraintestinal amoebiasis.400 mg thrice daily orally for 7–10 days (40–60 mg/kg body weight in children)
  • Tinidazole: 2 g as single dose for 2–3 days. 300 mg twice daily orally for 7 days (50–60 mg/kg body weight in children)
  • Ornidazole: 1.5 g once daily for 3 days. 500 mg twice daily orally for 7–10 days (40 mg/kg body weight in children)
  • Secnidazole: 2 g as single dose
  • Nitazoxanide: 500 mg twice daily for 3 days (age above 12 years), 200 mg twice daily for 3 days (4–11 years) or 100 mg. Twice daily (1–3 years)
  • Chloroquine: 300 mg twice daily followed by 300 mg daily for 21 days as an adjunct to metronidazole.

Luminal Amoebicides

These are recommended to prevent relapses following the course of tissue amoebicides:

  • Diloxanide furoate: 500 mg thrice daily for 10 days (20 mg/kg body weight in children)
  • Quinodocholor: 500 mg twice daily for 10 days
  • Iodochlorhydroxyquin: 500 mg twice daily for 10 days
  • Paromomycin: 30 mg/kg body weight thrice daily for 7 days (25 mg/kg body weight in children).


Surgical drainage of hepatic abscess is not mandatory, though some authorities advocate to speed up the recovery process and to bring down the systemic manifestations. This is restricted to large abscess with imminent danger of rupture and to those in the left lobe of liver and on the undersurface of liver. Closure of the cavity takes weeks to months. Rupture in to pleura needs to be drained. Rupture in to the pericardial cavity needs emergency drainage in view of the danger of cardiac tamponade.

Contraindications of amoebic drugs

Metronidazole and tinidazole are contraindicated to people who have taken alcohol. To reduce the development of drug resistant bacteria and maintain the effectiveness of flagyl (metronidazole) and other antibacterial drugs. Metronidazole should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria. Metronidazole has shown to be carnigenic

in mice and rats. unnecessary use of drug should be avoided.

Side effects and adverse effects of amoebic drugs 

  • Stomach pain, diarrhea
  • dizziness, loss of balance
  • vaginal itching and discharge
  • dry mouth or unpleasant metallic taste
  • cough, sneezing, runny or stuffy nose
  • swollen or sore tongue
  • anorexia,constipation
  • headache, 

uncomfortable side effects

  • fevers
  • pain with urination mouth sores
  • tingling or pickling sensations that may become permanent
  • brain disease
  • seizures

serious side effects but unlikely to occur

  • unsteadness
  • seizures
  • mood changes
  • numbness



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