Ancylostoma duodenale, Necator americanus

Necator americanus and Ancylostoma duodenale are the most common hookworms of humans. Both are small roundworms (nematodes) with a length of about 1 cm. They live attached to the mucosa with their mouth parts which are characteristic: Ancylostoma has four teeth, Necator has two cutting plates. Hookworms continually suck blood, causing a considerable daily blood loss in heavy infections.



Infective third-stage larvae (filariform larvae) actively invade the skin of the host, usually through the feet. They migrate via the bloodstream to the pulmonary capillaries. In the lung, they grow bigger and moult, then penetrate the capillary wall and enter the alveoli. From the alveoli, larvae reach the trachea and pharynx. They are then swallowed and pass into the small intestine, where they mature. The adults attach to the wall of the jejunum and can survive for about a year. Females lay thousands of eggs daily, which are passed out with the faeces.
The first larvae develop in the egg within 1 to 2 days under optimal conditions (high humidity, warm temperature). They hatch, and free-living larvae feed on bacteria. Then they moult twice and develop into filariform larvae.