Causal Agents:
The human hookworms include two nematode (roundworm) species, Ancylostoma duodenale and Necator americanus.  (Adult females: 10 to 13 mm [A. duodenale], 9 to 11 mm [N. americanus]; adult males: 8 to 11 mm [A. duodenale], 7 to 9 mm [N. americanus]).  A smaller group of hookworms infecting animals can invade and parasitize humans (A. ceylanicum) or can penetrate the human skin (causing cutaneous larva migrans), but do not develop any further (A. braziliense, A. caninum, Uncinaria stenocephala).  Occasionally A. caninum larva may migrate to the human intestine causing eosinophilic enteritis; this may happen when larva is ingested rather than through skin invasion.

Life Cycle:

Eggs are passed in the stool , and under favorable conditions (moisture, warmth, shade), larvae hatch in 1 to 2 days.  The released rhabditiform larvae grow in the feces and/or the soil , and after 5 to 10 days (and two molts) they become become filariform (third-stage) larvae that are infective .  These infective larvae can survive 3 to 4 weeks in favorable environmental conditions.  On contact with the human host, the larvae penetrate the skin and are carried through the veins to the heart and then to the lungs.  They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed .  The larvae reach the small intestine, where they reside and mature into adults.  Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall with resultant blood loss by the host .  Most adult worms are eliminated in 1 to 2 years, but longevity records can reach several years.
Some A. duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle).  In addition, infection by A. duodenale may probably also occur by the oral and transmammary route.  N. americanus, however, requires a transpulmonary migration phase.

Geographic Distribution:
The second most common human helminthic infection (after ascariasis).  Worldwide distribution, mostly in areas with moist, warm climate.  Both N. americanus and A. duodenale are found in Africa, Asia and the Americas.  Necator americanus predominates in the Americas and Australia, while only A. duodenale is found in the Middle East, North Africa, and southern Europe.