Giardiasis and Amoebiasis

Prepared and reviewed by the PrepElephant Academic Review Team

First published 14 Oct 2026 · Spot an error?

Reference basis: Nelson Textbook of Pediatrics; Ghai Essential Pediatrics; Harriet Lane Handbook

On this page

Direct answer
What you must remember
Separating two protozoal infections from the same clinic
Where the exam tries to catch you
Frequently asked questions
Related topics

Direct answer

Giardiasis and amoebiasis are protozoal infections of the intestine, caused by Giardia lamblia and Entamoeba histolytica respectively. Giardia causes greasy, foul-smelling, non-bloody diarrhoea with malabsorption, while Entamoeba histolytica causes dysentery with mucus and blood and can invade the liver to form an abscess. Metronidazole is the standard drug for both, and amoebiasis also needs a luminal agent to clear the cysts from the gut.

What you must remember

Giardia lamblia: a flagellate acquired from cysts in contaminated water, causing watery, greasy diarrhoea with bloating and malabsorption, usually without blood.
Giardia diagnosis: stool antigen detection or microscopy for cysts, with repeat samples because shedding is intermittent.
Entamoeba histolytica: causes colitis with mucoid, bloody stools and few leucocytes, and the trophozoites may contain ingested red cells.
Amoebic liver abscess: the right lobe is most often affected, presenting with fever, right upper quadrant pain and a tender liver.
Confirmation: ultrasound of the liver shows the abscess, and serology supports invasive disease because stool is often negative in liver abscess.
Drugs: metronidazole treats giardiasis and invasive amoebiasis, and diloxanide furoate is the classic luminal amoebicide.
Aspiration: reserved for large or left-lobe abscesses, those at risk of rupture, or those not responding to drugs.

Separating two protozoal infections from the same clinic

A 6-year-old from a rural area has three weeks of greasy diarrhoea and bloating, with no blood in the stool. Another child has bloody, mucoid stools with cramps and a tender liver. The approach separates them by stool and organ findings, in a fixed order.

Ask about the stool first. Non-bloody, greasy, floating stools with bloating point to Giardia, whereas mucoid bloody stools with cramps and tenesmus point to amoebic colitis.
Check for systemic disease. Fever, right upper quadrant pain or an enlarged, tender liver suggest an amoebic liver abscess, and ultrasound of the liver confirms the lesion.
Send stool tests. Use a stool antigen test or microscopy for cysts and trophozoites, and remember that stool may be negative when the liver is the site of infection.
Treat the tissue infection. Metronidazole is given for giardiasis and for invasive amoebiasis, with the dose and duration set by the child's weight and the guideline in use.
Clear the lumen. Follow invasive treatment with a luminal amoebicide such as diloxanide furoate, because metronidazole alone does not reliably clear cysts from the gut.
Drain selectively. Aspirate the abscess only when it is large, in the left lobe, at risk of rupture or not responding to drugs.

Where the exam tries to catch you

Both infections are easy marks when the drug and the stool clue are linked.

Trap: giving metronidazole alone for intestinal amoebiasis Correct: a luminal amoebicide is needed after tissue treatment, since cysts persist in the gut and can reinfect the child.
Trap: reading many pus cells in the stool as amoebiasis Correct: amoebic dysentery typically shows few leucocytes, while bacterial dysentery shows many.
Trap: calling every liver abscess pyogenic Correct: in endemic areas, amoebic liver abscess is a classic cause, and metronidazole treats it while serology supports the diagnosis.

Frequently asked questions

Which stool finding distinguishes giardiasis from amoebic dysentery?

Giardiasis produces non-bloody, greasy, foul-smelling stools, whereas amoebic dysentery produces mucoid stools containing blood and few leucocytes. These stool findings are only a guide, so antigen testing or microscopy is needed for confirmation.

What is the first-line drug for giardiasis?

Metronidazole is the first-line drug for giardiasis, with tinidazole and nitazoxanide used as alternatives. A repeat stool test is advised if symptoms persist or relapse after treatment.

Why is a luminal amoebicide needed after treating amoebic liver abscess?

A luminal amoebicide such as diloxanide furoate is needed because intestinal cysts survive the tissue-active drug given for invasive disease. Without luminal treatment, the child can continue passing cysts and may relapse.

When is aspiration indicated for an amoebic liver abscess?

Aspiration of an amoebic liver abscess is reserved for large lesions, left-lobe abscesses, those at risk of rupture, or those not responding to drugs. Most amoebic liver abscesses respond to metronidazole alone, so aspiration is not a routine step.