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Image: “This micrograph stained with chlorazol black revealed an Entamoeba histolytica cyst.” by CDC/ Dr. George Healy – This media comes from the Centers for Disease Control and Prevention’s Public Health Image Library (PHIL), with identification number #1474. License: Public Domain
Overview of Amebiasis
Amebiasis is an infection of the intestine caused by E. histolytica, a protozoan parasite. Humans and other non-human primates are the main hosts. Infection occurs through ingestion of water and food contaminated with feces. The contaminated food/water contains cysts of E. histolytica. It can cause extraintestinal invasive infections.
- dispar and E. histolytica are the only pathogenic amoebas known to humans. The protozoan enters the digestive tract and multiplies in the terminal ileum or colon to the trophozoite stage. Trophozoite of E. Histolytica is a motile form.
Inside the intestine, the parasite causes necrosis and perforation of the intestinal wall. It can also penetrate the intestinal wall and reach the liver via the portal vein, where it may cause a fatal liver abscess.
Cysts are excreted into the feces. The life cycle of E. histolytica is completed at this stage. It can survive in the environment for weeks to months. Factors that increase the chances of amebiasis include travel to an endemic area, consumption of contaminated food/water, and anal intercourse. The infection can cause an asymptomatic disease, to mild to moderate intestinal cases (ulcers, dysentery), to severe or chronic cases.
Amebiasis is prevalent highly in developing countries where general sanitation standards may not be high.
The classic symptoms of amebiasis are bloody or watery diarrhea, fever (10–30 % patients), anorexia, and weight loss. Rectal bleeding can occur without diarrhea in children. Amebiasis remains asymptomatic in most individuals and resolves on its own. Invasive infections result when E. histolytica reaches organs such as the liver via the systemic circulation. If the infection is limited to the intestine it is called a non-invasive infection.
Treatment is directed at disinfecting luminal trophozoites (non-invasive infection), as well as those in the tissue (invasive infection) if the infection has spread that far. Not all drugs can disinfect the cysts of E. histolytica. Drugs are usually active either in the tissues or the intestinal lumen, thereby necessitating a combination therapy for symptomatic amebiasis. Patients should be educated to avoid contaminated food/water or undercooked food.
Functional and Chemical Classification of Amebicides
Tissue amebicides
- Nitro-imidazoles: metronidazole, tinidazole, etc
- Alkaloid derivatives: emetine and dehydroemetine
Luminal amebicides
- Amide: diloxanide furoate, nitazoxanide
- 8-hydroxyquinolines: iodoquinol
- Antibiotics: tetracycline and erythromycin
Amebicidal Drugs
Tissue amebicides
Metronidazole
- A nitro-imidazole
- Selectively toxic to anaerobic microorganisms
- Half-life is 7.5 hours
| Mechanism of action | The nitro-group of nitro-imidazoles undergoes bioactivation by ferredoxin (which is present only in anaerobic microorganisms) to form reactive toxic metabolites (nitro radicals) that inhibit nucleic acid synthesis. This occurs by nitro-radicals competing with the energy metabolism pathway of anaerobic microorganisms. |
| Clinical uses |
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| Adverse effect/toxicity |
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| Drug interactions |
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Tinidazole
Tinidazole is a second-generation nitro-imidazole. Its efficacy equals that of metronidazole, but it has a shorter course of treatment (3 days vs. 10 days for metronidazole), a longer half-life (12–14 hours), and is more expensive.
| Mechanism of action | Similar to metronidazole |
| Clinical uses |
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| Adverse effect/toxicity | Similar to metronidazole |
| Drug interactions | Similar to metronidazole |
| Contraindications | Similar to metronidazole |
Other nitro-imidazoles
- Secnidazole, ornidazole, and satranidazole
- Similar to metronidazole/tinidazole but with a longer half-life
- Satranidazole has much better tolerability (fewer side effects).
Nitazoxanide
Nitazoxanide is a new nitrothiazole antiparasitic drug and prodrug, which is converted to tizoxanide. It is effective against numerous intestinal protozoa and helminths.
| Mechanism of action | Inhibits pyruvate: ferredoxin oxidoreductase enzyme (PFOR), which is essential for electron transport energy metabolism in anaerobic organisms. |
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| Adverse effect/toxicity |
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| Drug interactions | None documented. However, the active metabolite tizoxanide is highly protein-bound. Therefore, its use with other highly protein-bound drugs must be avoided or done with caution. |
Emetine & Dehydroemetine
These drugs are rarely used because of toxicity concerns; for example, severe amebiasis when metronidazole cannot be used. They act only against trophozoites, not against the cysts. In cases of severe amebiasis, these drugs should be administered subcutaneously or in intramuscularly (and not intravenously) for at least 3–5 days.
| Mechanism of action | Block ribosomal movement along mRNA |
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Luminal amebicides
Diloxanide furoate
- Amide
- Directly kills luminal trophozoites
- Does not kill the tissue trophozoites
- No antibacterial action
- Prodrug (releases free diloxanide as the active metabolite)
| Mechanism of action | Unknown |
| Clinical uses |
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| Adverse effect/toxicity |
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| Drug interactions | No known interactions |
Iodoquinol
Iodoquinol directly kills amebic trophozoites, thereby inhibiting cyst production. It is ineffective as a tissue amebicide.
| Mechanism of action | Unknown |
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Paromomycin
- Aminoglycoside
- May be superior to diloxanide in asymptomatic infection
- It is only active as luminal amebicide
| Mechanism of action | Protein synthesis inhibitor by interacting with the 30S ribosomal subunit. |
| Clinical uses |
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| Adverse effect/toxicity | Headaches, dizziness, rashes, arthralgia (due to considerable systemic absorption, e.g., in a patient with renal insufficiency) |
| Contraindications |
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Other antibiotics
Some antibiotics can be used in the treatment of intestinal amebiasis. Examples include erythromycin and tetracycline.
Summary of Antiamebicidal Drug Treatment
| Form of amebiasis | Treatment |
| Asymptomatic intestinal infection |
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| Amebic colitis (mild to moderate) |
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| Amebic colitis (severe) |
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| Extraintestinal amebiasis |
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*Diloxanide furoate is not available in the United States.
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