What is Amoebiasis (Amoebic Dysentery)

Amoebiasis is an infectious disease caused by a one-celled parasite called Entamoeba histolytica, which causes both intestinal and extraintestinal infections. Two species of Entamoeba are morphologically indistinguishable: Entamoeba histolytica is pathogenic and Entamoeba dispar harmlessly colonizes the colon.
Amoebas adhere to and kill the cells of the colon and cause dysentery with blood and mucus in the stool. Amoebas also secrete substances called proteases that degrade lining of the colon and permit invasion into the bowel wall and beyond. Amoebas can spread via the circulation to the liver and cause liver abscesses. The infection may spread further by direct extension from the liver or through the bloodstream to the lungs, brain, and other organs.

Statistics on Amoebiasis (Amoebic Dysentery)

Amoebiases occurs worldwide, although much higher rates of incidence are found in the tropics and subtropics. About 5,000 to 10,000 cases are diagnosed each year in the US, leading to about 20 deaths annually.

Risk Factors for Amoebiasis (Amoebic Dysentery)

Although anyone can have this disease, it is most common in people who live in developing countries that have poor sanitary conditions. In the United States, amoebiasis is most often found in immigrants from developing countries. It also is found in people who have traveled to developing countries and in people who live in institutions that have poor sanitary conditions. It also commonly affects active homosexual men.

Progression of Amoebiasis (Amoebic Dysentery)

The incubation period is highly variable and may be even as short as a few days or as long as several months or even a year.
The presenting features may be gradual, severe or fulminating, and include:

  • Gradual onset colitis: Mild intermittent diarrhoea and abdominal discomfort, usually progressing to bloody diarrhoea with mucous. Systemic manifestations such as nausea, headache, low grade fever and anorexia are often present.
  • Severe acute amoebic dysentery: Episodes of frequent semi liquid stools containing blood, mucous and trophozoites.

Abdominal findings range from mild tenderness to frank abdominal pain with high fevers and systemic symptoms. The liver is often tender. Between relapses, symptoms diminish to recurrent cramps and loose or soft stools, but emaciation and anaemia continue.

How is Amoebiasis (Amoebic Dysentery) Diagnosed?

Stool examination is the commonest examination done for diagnosis. The finding of trophozoites are diagnostic. White blood cells and pus are also often present. Since trophozoites are killed rapidly by water or drying, at least three fresh stool specimens have to be examined for a positive diagnosis. Fresh stool or concentrated stool examination is positive in 75 to 95 percent of patients.
A blood test can also be performed, and is positive in more than 90 percent of patients with invasive amoebiasis.
Barium studies are contraindicated in acute amoebic colitis for fear of perforation.
An ultrasound, CT and MRI scans of the abdomen can be useful in diagnosing hepatic amoebiasis. Since abscesses resolve slowly or may even increase in size during treatment, clinical response is more important in the follow-up rather than repeated scans.
Acute intestinal amoebiasis should be differentiated from organisms causing traveller’s diarrhoea (due to Escherischia Coli) and also inflammatory bowel disease.
Amoebic liver abscess has to be differentiated from pyogenic abscess which are seen in older patients with underlying bowel disease or after surgery.

Prognosis of Amoebiasis (Amoebic Dysentery)

Prognosis is generally good with treatment unless complications of abscess rupture occurs, this is when surgical intervention may be required.

How is Amoebiasis (Amoebic Dysentery) Treated?

General therapy relieves symptoms, replaces blood, and corrects fluid and electrolyte losses. Antibiotics, such as Metronidazole are necessary, and are given for 5 days for amoebic dysentery and for 10-14 days if there is a liver abcess or extraintestinal spread. Large abcesses in the liver may require drainage, using an ultrasound scan to localise the abcess accurately and position the drainage needle.

Amoebiasis (Amoebic Dysentery) References

  1. Kumar P, Clark M (eds). Clinical Medicine (4th edition). Edinburgh: WB Saunders Company; 1999. [Book]
  2. Longmore M, Wilkinson I, Torok E. Oxford Handbook of Clinical Medicine (5th edition). Oxford: Oxford University Press; 2001. [Book]
  3. Amebiasis [online]. Whitehouse Station, NJ: Merck Manual of Diagnosis and Therapy; 2004 [cited 3 January 2005]. Available from: URL link