Protozoal infections: amebiasis, giardiasis, trichomoniasis, toxoplasmosis and cryptosporidiosis

Single-celled parasites that cause diarrhoea, liver abscess, a sexually transmitted infection and dangerous disease in pregnancy or weak immunity.

Updated Nov 12, 2026

Protozoa are single-celled eukaryotic parasites. Five of them cause most non-malarial protozoal disease seen in clinics: Entamoeba histolytica, Giardia, Trichomonas vaginalis, Toxoplasma gondii and Cryptosporidium. They differ in where they live (gut lumen, gut wall, genital tract, tissues), and that location decides which drug reaches them. Malaria, Chagas disease and leishmaniasis have their own pages.

Amebiasis (Entamoeba histolytica)

Infection follows swallowing cysts in food or water contaminated with faeces. In the gut the cysts release trophozoites. Many people carry them in the lumen without symptoms; others develop invasive colitis with bloody diarrhoea, and the parasite can travel to the liver, where amoebic liver abscess is the commonest extra-intestinal form. Most cases occur in low- and middle-income countries. Microscopy cannot tell E. histolytica from harmless look-alikes such as E. dispar, so antigen or molecular tests are used.

Invasive disease: a nitroimidazole (metronidazole or tinidazole) acts in tissues.
Then a luminal agent (paromomycin or iodoquinol) eradicates parasites left in the bowel lumen and prevents relapse and spread.

Giardiasis (Giardia duodenalis)

Giardia attaches to the small-intestinal lining and causes watery, greasy diarrhoea, bloating and weight loss; protozoa are a typical cause when travellers' diarrhoea lasts more than two weeks. It spreads through contaminated water and food and from person to person. Effective drugs are tinidazole, nitazoxanide and metronidazole, all with activity against this anaerobic parasite.

Trichomoniasis (Trichomonas vaginalis)

This flagellate infects the vagina and urethra and is the most prevalent non-viral sexually transmitted infection worldwide. Many people have no symptoms. Nitroimidazoles (metronidazole, tinidazole; secnidazole is another member) are the only class with proven efficacy. Current sexual partners are treated at the same time to prevent reinfection, and women are retested some months later. Metronidazole is considered low risk in pregnancy, whereas tinidazole is avoided. Some infections resist metronidazole, which leads to longer or higher-intensity nitroimidazole courses and specialist advice.

Toxoplasmosis (Toxoplasma gondii)

In healthy people it is usually mild and needs no treatment. It is dangerous in three settings: a first infection during pregnancy (congenital toxoplasmosis), reactivation in immunosuppression such as advanced HIV (brain abscesses), and eye disease (retinochoroiditis).

Standard regimen: pyrimethamine plus sulfadiazine, with folinic acid (leucovorin) to protect the bone marrow from pyrimethamine.
Alternatives: clindamycin in place of sulfadiazine for sulfa allergy; trimethoprim-sulfamethoxazole or atovaquone.
Pregnancy: spiramycin is generally used early when fetal infection is not documented; the pyrimethamine combination later or when the fetus is infected.
In AIDS, maintenance therapy continues until antiretroviral therapy restores immunity.
Drugs act on actively dividing tachyzoites and do not eradicate tissue cysts, so relapse is possible.

Cryptosporidiosis (Cryptosporidium)

Cryptosporidium causes watery diarrhoea and resists chlorine, so it spreads in swimming pools and water supplies. In immunocompetent people it is usually self-limited, and nitazoxanide is the approved treatment. In people with weakened immunity, including advanced HIV, nitazoxanide has not proven better than placebo; restoring immunity with antiretroviral therapy is the key step there.

How the drugs fit the parasites

Nitroimidazoles: activated by reduction inside anaerobic protozoa, forming toxic radicals that damage DNA; covers Entamoeba (tissue), Giardia and Trichomonas.
Luminal agents: poorly absorbed, so they stay in the bowel lumen where cysts live.
Nitazoxanide: interferes with energy metabolism of anaerobic parasites; used for Giardia and Cryptosporidium.
Antifolates (pyrimethamine with a sulfonamide): block folate synthesis in Toxoplasma; folinic acid protects the host.
In Colombia and the rest of Latin America, Chagas disease and leishmaniasis are also important protozoal diseases, treated with different drugs (see their pages).

Test yourself

Think of the answer, then tap the question to see it.

Why is invasive amebiasis treated with a luminal agent after metronidazole?

Metronidazole acts in tissues but leaves parasites in the bowel lumen; a poorly absorbed luminal agent clears them and prevents relapse and spread.

Which drug class is the only one with proven efficacy against Trichomonas vaginalis?

The nitroimidazoles, such as metronidazole and tinidazole.

Why is folinic acid given with pyrimethamine for toxoplasmosis?

Pyrimethamine blocks folate metabolism and can suppress the bone marrow; folinic acid bypasses the block in human cells and protects them.

In a person with advanced HIV and cryptosporidiosis, what is the most important treatment step?

Restoring immunity with antiretroviral therapy; nitazoxanide has not proven effective in severely immunocompromised people.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Amebiasis (U.S. Centers for Disease Control and Prevention (DPDx), 2026); Travelers' Diarrhea (U.S. Centers for Disease Control and Prevention (Yellow Book), 2026); Trichomoniasis - STI Treatment Guidelines (U.S. Centers for Disease Control and Prevention, 2026); Clinical Care of Toxoplasmosis (U.S. Centers for Disease Control and Prevention, 2026); Clinical Care of Crypto (U.S. Centers for Disease Control and Prevention, 2026).

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