Hookworm Disease (Necatoriasis and Ancylostomiasis)

Hookworm disease is caused by either of two nematodes, Necator americanus (New World hookworm) or Ancylostoma duodenale (Old World hookworm). N. americanus is found in the Western Hemisphere in tropical and subtropical areas and also in Africa and Asia. A. duodenale is found in the Mediterranean region and in parts of Europe and Asia; it is rarely, if ever, seen in the Western Hemisphere. The worms measure 7 to 9 mm, but Ancylostoma may be much larger than Necator and is more prolific, producing 10,000 to 30,000 eggs.

The life cycles of A. duodenale and N. americanus are essentially the same ( Fig. 121.1 ), and the infection is acquired by skin penetration when walking barefoot in soil contaminated with hookworms. The worms attach to the small intestine, where they feed on the blood and lymph of the host, and the fertilized female lays eggs. Large numbers of eggs are passed in the feces. Rhabditiform larvae develop in warm moist soil, and the larvae penetrate human skin, pass through the circulation to the lungs, and are then swallowed to complete the life cycle in the duodenum (see Fig. 123.1 ). The worms may live for years in the host.

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Hookworm Disease: Life Cycle of Necator americanus (Necatoriasis) and Ancylostoma duodenale (Ancylostomiasis).

An estimated 1 billion persons harbor hookworms, which surprisingly were not identified as human parasites until the mid-19th century. They heavily infect children but can be unusually virulent and cause chronic infections and anemia in elderly persons. They probably are a major cause of iron deficiency anemia worldwide. Each worm can cause 0.03 to 0.26 mL of blood loss per day. Ancylostoma is larger and more aggressive in its drainage. Hookworms develop in tropical and subtropical areas, where larvae can grow in the soil.