Life Cycle of a Stercoralis

Strongyloides stercoralis is a small, thin, white nematode, or roundworm, that infects the intestines of human beings. Most commonly this type of parasite is referred to as a pinworm or threadworm.

Living in the soil, the threadworm is common in wet tropical areas, including the southern United States, and infection often takes place by walking barefoot in contaminated areas.

The lifecycle of Strongyloides stercoralis is complex compared to that of other types of nematodes. It can alternate between free-living and parasitic life and can reproduce either sexually or without mating.

Larva

• The filariform larvae in contaminated soil penetrate through human skin to infect the host. These larvae then travel to the host's lungs and then to the pharynx and are swallowed into the small intestines. In the small intestines, the larvae molt twice until they reach the adult threadworm stage.

Adult

• The female threadworm anchors herself with her mouth to the small intestines of her human host. The adult female may grow to two to three mm in length. The female threadworm can reproduce without mating, and the adult may produce several dozen eggs a day. The eggs hatch in the intestines of the infected human and the larvae pass into the feces and become free-living or stay in the body to hatch with the larvae returning to the host's lungs. Once infected, the threadworm may continue in the human host for more than 30 years.

Treatment

• Many times infection of threadworms produces no symptoms. In severe infections, symptoms may include abdominal pain, diarrhea, lung problems, rashes and more serious complications. To diagnose an infection, the physician will examine a stool sample under a microscope. Treatment may include the medications Ivermectin or albendazole to end the nematode's life cycle within the human host.

Free-Living

• The free-living larvae will molt twice and infect another human host or will molt four times to become free-living adult male and female threadworms.

Mating

• Free-living threadworms will mate and produce eggs. From these eggs, the larvae that hatch can develop into a new generation of free-living threadworms or infect a human host and begin the parasitic life cycle again.