Necator Americanus - Lifecycle

Lifecycle

This worm starts out as an unembryonated egg in the soil. After 24–48 hours under favorable conditions, the eggs become embryonated and hatch. This first juvenile stage 1 is known as 'rhabditiform'. The rhabditiform larvae grow and molt in the soil, transforming into a juvenile stage 2. The juvenile stage 2 molts once more until reaching the juvenile 3 stage, which is also called 'filariform'; this is also the infective form. The transformation from rhabditiform to the filariform usually takes five to 10 days. This larval form is able to penetrate human skin, travel through the blood vessels and heart, and reach the lungs. Once there, they burrow through the pulmonary alveoli and travel up the trachea, where they are swallowed and are carried to the small intestine, where they mature into adults and reproduce by attaching themselves to the intestinal wall, causing an increase of blood loss by the host. The eggs end up on the soil after leaving the body through the feces. On average, most adult worms are eliminated in one to two years. The N. americanus life cycle only differs slightly from that of A. duodenale. N. americanus has no development arrest in immune hosts and it must migrate through the lungs.

Cutaneous larva migrans, a zoonotic infection, occurs when humans become accidental hosts. The definitive hosts for these species are cats and dogs. The cycle in the definitive host is similar to that in humans, the difference is when the filariform larvae burrow through human skin. Because the filariform can reproduce and mature only in its definitive host, it wanders throughout the epidermal layer, where it cannot penetrate deeply into the human skin. The trail of the filariform penetration is visible across the skin, since it can only burrow through its outer layers.