Chapter 31: Protozoans

Protozoans are single-celled eukaryotic organisms that represent a major class of human parasites, transmitted through various routes and capable of infecting multiple body systems. This chapter systematically categorizes protozoal diseases by anatomical location and transmission mechanism, beginning with intestinal infections caused by organisms such as Entamoeba histolytica and Giardia lamblia, which exist in two distinct life stages: the hardy, transmissible cyst form and the metabolically active trophozoite form responsible for pathology. The intestinal section emphasizes how certain protozoa exploit immunocompromised hosts, including Cryptosporidium, Isospora, and Cyclospora, which produce severe diarrhea in patients with compromised immune systems. The urogenital tract diseases section focuses on Trichomonas vaginalis, a sexually transmitted parasite requiring coordinated treatment of both partners to interrupt the cycle of reinfection. Free-living amoebas that cause central nervous system infections are examined separately, highlighting the acute meningitis risk from Naegleria fowleri acquired through recreational water exposure and the chronic keratitis associated with Acanthamoeba in contact lens wearers. The chapter devotes substantial coverage to blood and tissue protozoa, particularly the Plasmodium species responsible for malaria, detailing the complex vector transmission by Anopheles mosquitoes and the biphasic life cycle involving multiplication within hepatic cells followed by colonization and rupture of erythrocytes. Related tick-borne pathogens like Babesia microti are contrasted with malaria parasites to highlight epidemiologic distinctions. The final sections address blood-dwelling flagellates including Leishmania species transmitted by sandflies and two medically significant trypanosomes: Trypanosoma brucei, causing African sleeping sickness through tsetse fly transmission and affecting the nervous system, and Trypanosoma cruzi, transmitted by reduviid insects and responsible for Chagas disease with its characteristic chronic cardiac and gastrointestinal complications.