Causal Agents:
The human hookworms include the nematode species, Ancylostoma duodenale and Necator americanus.  A larger 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 larvae may migrate to the human intestine, causing eosinophilic enteritis.  Ancylostoma caninum larvae have also been implicated as a cause of diffuse unilateral subacute neuroretinitis.

Life Cycle (intestinal hookworm infection):

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 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 blood vessels 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 the longevity may 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.

Life Cycle (cutaneous larval migrans):

Cutaneous larval migrans (also known as creeping eruption) is a zoonotic infection with hookworm species that do not use humans as a definitive host, the most common being A. braziliense and A. caninum.  The normal definitive hosts for these species are dogs and cats.  The cycle in the definitive host is very similar to the cycle for the human species.  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 filariform (third-stage) larvae that are infective .  These infective larvae can survive 3 to 4 weeks in favorable environmental conditions.  On contact with the animal host , the larvae penetrate the skin and are carried through the blood vessels 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.  Some larvae become arrested in the tissues, and serve as source of infection for pups via transmammary (and possibly transplacental) routes .  Humans may also become infected when filariform larvae penetrate the skin .  With most species, the larvae cannot mature further in the human host, and migrate aimlessly within the epidermis, sometimes as much as several centimeters a day.  Some larvae may persist in deeper tissue after finishing their skin migration.

Geographic Distribution:
Hookworm is the second most common human helminthic infection (after ascariasis).  Hookworm species are worldwide in 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.