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Chapter 141 : Nematodes
Soil-transmitted helminths (intestinal nematodes) are the most common infections globally with more than one billion people infected, especially in resource-poor settings where sanitation is inadequate. Studies strongly suggest that nematodes are actually related to the arthropods and priapulids in a newly recognized group, the Ecdysozoa. The Ascaris lumbricoides adult worm is the largest of the human pathogenic nematodes, 15 to 35 cm in length. Ancylostoma and Necatorare the two genera of Ancylostomatidae that infect humans. The major clinical manifestation of hookworm infection is iron deficiency anemia due to intestinal blood loss and depletion of iron stores. Some rhabditiform larvae develop into infective filariform larvae in the bowel lumen, penetrate the intestinal mucosa or perianal skin, and repeat the cycle of maturation within the same host. This process of autoinfection, albeit uncommon among intestinal nematodes, results in chronic infections that may persist for 40 years or more. The clinical features are related to the intensity of infection, as is the case with the other intestinal nematodes. In developed countries, any helminth eggs or larvae found in feces are significant and treatment is recommended, even if the patient is asymptomatic. However, treatment of asymptomatic cases is not necessary in developing countries as many of these parasites are endemic and reinfection is common.