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The hookworm, which includes Ancylostoma duodenale and Necator americaus, is another common type of nematode, or roundworm. They have similar life cycles and cause an infection known as ancylostomiasis. Necator americanus is the prevailing species found in the southeastern section of the United States. Hookworms derived their name from the fact that their heads bend towards their underside, in a hook-like fashion.

People are most likely to become infected with hookworm because of unsanitary conditions, especially with regards to human waste. Conditions are particularly favourable in areas with tropical or subtropical climates, with good soil moisture and where humans defecate on the ground. Hookworm larvae will not survive, however, if the soil is too moist or too dry.

Hookworm eggs are passed onto the ground in contaminated human faeces where they develop into infective larvae. When the soil is cool, the worms crawl to a moist area of the ground and extend their bodies into the air. They hold this position until it becomes too hot or they come into contact with skin, usually on the sole of the feet. The larvae can remain infective in the soil for several weeks. An infection occurs when these larvae then penetrates the human skin and enters the bloodstream. This may cause a rash where the larvae enters the body which is sometimes referred to as "ground itch".

Having broken through the skin, the hookworm larvae then enter the bloodstream and are carried to the lungs through the blood and the lymphatic system. The larvae matures in the lungs and then migrates from the lungs up to the windpipe (trachea) where they are swallowed once again and are carried back down to the intestine. This life cycle is referred to as "tracheal migration". They then attach to the linings of the intestine by mouthpieces containing cutting plates and proceed to suck blood and tissue juices from their human host. This enables the worm to lie flat along the intestine without being dislodged by passing faecal material.

Adult hookworms average about 10mm in length. They live in the small intestine of the human host and, after mating, the female's eggs are passed into the faeces. A female hookworm can produce between 10,000 and 25,000 thin-shelled eggs per day, depending on the species. Roughly 48 hours after being laid the hookworm egg hatches and the resulting larvae reaches an infective stage about five days later.

There are few symptoms of a hookworm infection. Diarrhoea may occur as the worm starts to mature in the intestines and prior to eggs appearing in the faeces. As well as the rash previously mentioned, other noticeable symptoms might include abdominal pain, intestinal cramps, colic, and nausea.

Studies have shown that humans can tolerate being host to small to moderate numbers of hookworms without problems, provided that they stay in good health and get enough iron in their diet. If the number of worms becomes very large, however, a person can develop anaemia. This is because of the loss of blood resulting from all the hookworms attaching themselves to the intestine and sucking blood and tissue juices from the human host. It has been estimated that one adult hookworm can drain 0.1mL of blood per day so a relatively mild infestation of 1500 hookworms would cause the loss of 150mL of blood each day. If the host is a child then the loss of excessive amounts of blood caused by a severe hookworm infestation could cause growth retardation.

It requires a doctor, with reference to a pathology laboratory, to make a diagnosis of hookworm infestation. A laboratory expert will examine the stool specimen, counting the number of hookworm eggs that are present. If there are more than about 2,000 eggs then the doctor will assume that the infestation is large enough to possibly cause anaemia. Because they all live in similar environments it is possible to experience polyparasitic infections that also include roundworms (Ascaris lumbricoides) or whipworms.

Hookworm can be treated with Vermox or Combantrin-1, which contain mebendazole, or Combantrin, which has pyrantel embonate as its active ingredient.