Medically Important Protozoa

Key points

• • Protozoa are single-celled eukaryotes, larger and more complex than bacteria; many have a hardy cyst stage for spread and an active trophozoite stage that causes disease.
• • Entamoeba histolytica is swallowed as a cyst in contaminated food or water and causes amoebic dysentery and liver abscesses.
• • Giardia lamblia infects the upper small intestine and causes fatty, foul-smelling diarrhoea, often caught from contaminated water.
• • Leishmania is spread by the bite of sandflies and causes either skin sores or a serious disease of the internal organs (visceral leishmaniasis, or kala-azar).
• • Trypanosomes cause African sleeping sickness (spread by the tsetse fly) and, in South America, Chagas disease (spread by the triatomine bug).
• • Trichomonas vaginalis is a protozoan spread by sexual contact that causes a common genital infection, recognised by its jerky movement under the microscope.