Hookworm is a roundworm infestation affecting the small intestine and lungs. The worms are about 1/2 inch long.

The disorder is caused by infestation with the roundworms Necator americanus, Ancylostoma duodenale, Ancylostoma ceylanicum, or Ancylostoma braziliense. The first two occur in humans only. The last types also occur in animals.

Hookworm disease is widespread in the moist tropics and subtropics, and it affects over 1 billion people worldwide. In developing nations, the disease indirectly causes the death of many children by increasing their susceptibility to other infections that could normally be tolerated.

There is very little risk of contracting the disease in the U.S. because of advances in sanitation and control of wastes.

The larvae (immature form of the worm) penetrate the skin, where an itchy rash called ground itch may develop. The larvae migrate to the lungs via the bloodstream, enter the airways and cause coughing.

After traveling up the windpipe, the larva are swallowed. When the larvae are swallowed, they infect the small intestine and develop into adult worms. Adult worms and larvae are excreted in the feces.

Most people have no symptoms once the worms enter the intestines. However, iron deficiency anemia caused by loss of blood may result from heavy infestation.