Strongyloides stercoralis is a nematode with a unique life cycle, as it can alternate between free-living and parasitic life cycles. A person can become infected with this nematode if they come in contact with the infective filariform larvae that can penetrate human skin and enter the circulatory system, and ultimately end up the in the lungs. The host then coughs up and swallows the larvae which allow the larvae to enter the small intestine where they develop into female adults. The female worms are small and they live in tunnels between enterocytes in the small intestine. In the small intestine, the female adults produce eggs via parthenogenesis which develop into rhabditiform larvae and are excreted into the environment through feces. In a process known as auto-infection, the rhabditiform larvae develop into filariform larvae which penetrate the intestinal mucosa and can increase the number of infections in the host. At this point in S. stercoarlis’ life cycle, the rhabditiform larvae can either develop into the directly transmitted infective filariform larvae by molting twice, or molt four times and produce male and female adults that are free living. In the external environment, these free living adults can mate and produce eggs that at any point can develop into the infective filariform larvae and enter the parasitic life cycle by penetrating a mammalian host.
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