Hookworms

Hookworms are soil-transmitted parasitic roundworms (nematodes). The two species that infect the human intestine are Ancylostoma duodenale and Necator americanus, which together infect hundreds of millions of people in warm, humid regions with poor sanitation. Infection classically begins when bare skin — often the feet — contacts contaminated soil.

The life cycle explains the clinical picture. Filariform larvae penetrate intact skin, enter the bloodstream, travel to the lungs, ascend the airway, and are swallowed. In the small intestine, adult worms attach to the mucosa with cutting mouthparts and feed on blood. A local itchy rash ("ground itch") can mark the entry site, transient pulmonary symptoms may accompany lung migration, and eosinophilia is a typical laboratory clue during tissue invasion. Diagnosis rests on finding characteristic eggs in the stool.

The hallmark consequence is chronic blood loss: heavy infection produces iron deficiency microcytic anemia and can impair growth and cognition in children. A related presentation, cutaneous larva migrans, occurs when animal hookworm species such as Ancylostoma braziliense penetrate human skin but cannot complete their life cycle, leaving intensely itchy serpiginous tracks. Treatment of intestinal hookworm infection uses bendazole-class antiparasitics.

Hookworms are a standard parasitology topic on the USMLE Step 1 exam, which favors vignettes linking barefoot soil exposure, eosinophilia, and microcytic anemia to the skin-lung-gut life cycle — and contrasts hookworms with Strongyloides, the other nematode whose larvae penetrate the skin.