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To simplify the discussion, the antimalarial drugs are covered in a separate section at the end of this chapter. Some of the more common protozoal diseases are listed in the following table.

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Protozoa Disease
Entamoeba histolytica Amebiasis (diarrhea)
Balantidium coli Balantidial dysentery
Trichomonas vaginalis Trichomoniasis (genital infection)
Giardia lamblia Giardiasis (diarrhea)
Leishmania Leishmaniasis (three types)
Trypanosoma brucei African sleeping sickness
Trypanosoma cruzi Chagas disease (South American)

Of the drugs used in these diseases, metronidazole is most important for you to know. Of the diseases listed, trichomoniasis and giardiasis are most common in the United States and both are treated with metronidazole. This along with a few more details will go a long way. You need to be aware of the other drugs and where to find the information about treatment for these diseases.

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Antiprotozoal Drugs


benznidazole (Chagas)

eflornithine (African sleeping sickness)

miltefosine (leishmaniasis)

nitazoxanide (cryptosporidiosis)

nifurtimox (Chagas)

pentamidine (African sleeping sickness)

sodium stibogluconate (leishmaniasis)

suramin (African sleeping sickness)



METRONIDAZOLE is effective in the treatment of vaginal trichomoniasis, giardiasis, and all forms of amebiasis.

First note that metronidazole, fenbendazole, and tinidazole end in “azole” but are not antifungal agents. Sorry!

Metronidazole is one of the most effective drugs against anaerobic bacteria and several protozoal species. It is highly effective in the treatment of trichomoniasis. It penetrates protozoal and bacterial cell walls but cannot enter mammalian cells. The drug must be activated once it has entered the cell. The activating enzyme, nitroreductase, is found only in anaerobic organisms. The reduced metronidazole inhibits DNA replication by causing breaks and inhibiting repair of the DNA.

The most common side effects are nausea, vomiting, and diarrhea. The drug can turn the urine dark or red-brown and cause a metallic taste in the mouth. Metronidazole can cause a disulfiram-like reaction when taken with alcohol. The disulfiram-like effect consists of abdominal cramping, vomiting, flushing, or headache after drinking alcohol.


Malaria is caused by a single-cell protozoa, the plasmodium. There are over 50 species of plasmodia, but only 4 are infectious to humans: Plasmodium malariae, Plasmodium ovale, Plasmodium vivax, and Plasmodium falciparum. Plasmodium vivax is the most prevalent, but P. falciparum is the most serious and lethal form of malaria.

To understand the drugs and the rationale behind treatment of patients with malaria, it is important to understand the life cycle of the malaria organism (Figure 36–1).


As a reminder, the life cycle of the malaria-causing protozoa is presented. Plasmodium vivax and Plasmodium ovale are the two species that can take up residence in the liver.

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