Chapter 32: Helminths

The discussion of intestinal nematodes begins with Ascaris lumbricoides, which demonstrates a complex developmental pathway initiated by ingestion of embryonated eggs, followed by larval migration through the lungs before establishing mature populations within the small intestine. Hookworms, particularly Necator americanus, employ a distinct transmission strategy by penetrating intact skin and traveling through the circulatory system to reach pulmonary tissues before final intestinal colonization. Strongyloides stercoralis presents unique clinical challenges through its capacity for internal reinfection, where larvae can penetrate either the intestinal epithelium or perianal tissues to perpetuate infection within a single host. Trichuris trichiura and Enterobius vermicularis represent additional intestinal infections, with the latter causing characteristic intense perianal itching and requiring specialized diagnostic approaches such as the scotch tape examination method. The chapter then addresses nematodes that colonize blood and tissue compartments, including Onchocerca volvulus responsible for river blindness, Wuchereria bancrofti and Brugia malayi which cause lymphatic filariasis and its severe manifestation elephantiasis, and Dracunculus medinensis known as the Guinea worm. The coverage of trematodes focuses on Schistosoma species, blood flukes that establish themselves in specific venous beds and depend on freshwater snails as obligate intermediate hosts. The final section examines cestodes, contrasting Taenia solium and Taenia saginata in terms of transmission routes and pathogenic consequences, with emphasis on the serious neurological complications of cysticercosis resulting from T. solium egg ingestion. Additional tapeworm infections including Diphyllobothrium latum and Echinococcus species are reviewed, noting their potential to cause substantial organ damage through formation of cystic larval structures.