Strongyloides Stercoralis: Roles of the Male and Female
Explore the reproductive biology of *Strongyloides stercoralis*, where females sustain infection alone, while males contribute to a separate, free-living generation.
Strongyloides stercoralis is an intestinal nematode, or roundworm, with a lifecycle that allows it to alternate between a parasitic form inside a host and a free-living form in the environment. This adaptability is managed through different roles for its male and female worms. Its survival strategy involves three distinct routes: a parasitic cycle, a free-living cycle, and an autoinfection cycle.
The Parasitic Cycle in the Host
The parasitic phase of Strongyloides stercoralis occurs entirely within a host and is notable for the exclusive presence of female worms. These parasitic females are parthenogenetic, meaning they reproduce without males. After maturing, a female worm embeds herself deep within the mucosal lining of the small intestine, primarily the duodenum and jejunum, where she can lay several dozen eggs per day.
These thin-shelled eggs hatch rapidly within the intestinal crypts, releasing first-stage rhabditiform larvae. These larvae are motile and feed on intestinal content as they move through the gut. The rhabditiform larvae are then excreted from the host with feces, which begins the free-living portion of the parasite’s lifecycle.
The Free-Living Cycle in the Environment
Once rhabditiform larvae are passed into warm, moist soil, the free-living cycle can begin. This part of the lifecycle involves both male and female worms and sexual reproduction. The rhabditiform larvae in the soil molt to develop into free-living adult males and females. These non-parasitic adults survive by feeding on bacteria in the soil.
The free-living males and females mate in the soil, and the fertilized females then lay eggs. These eggs hatch, releasing a new generation of rhabditiform larvae. This new generation of larvae faces a developmental choice, influenced by environmental cues that are not fully understood. Some larvae will develop into another generation of free-living adults, continuing the cycle in the soil.
Other larvae will develop into infective third-stage larvae, known as filariform larvae. These filariform larvae are non-feeding and are specifically adapted for infection. They can survive in the soil for several weeks while waiting for a host, which they infect by penetrating the skin. This pathway allows the parasite to amplify its numbers in the environment before finding a new host to begin the parasitic cycle again.
Distinguishing Male and Female Forms
The physical characteristics of Strongyloides stercoralis adults reflect their different roles. The parasitic female is the largest form, a slender, transparent worm measuring 2.0 to 3.0 mm in length. Her long, filariform esophagus takes up about a third of her body length, an adaptation for living in host tissue.
In contrast, the free-living adults found in the soil are smaller and stouter. The free-living female measures up to 1.0 mm long and has a rhabditiform pharynx, which is a more muscular esophagus suited for feeding on bacteria. Her reproductive system is also robust, with a vulva located near the middle of her body.
The free-living male is even smaller, reaching about 0.75 to 0.9 mm in length. His definitive features are his reproductive structures: a ventrally curved tail used during copulation and two spicules inserted into the female’s vulva to transfer sperm. These morphological differences are related to their separate functions.
The Autoinfection Pathway
A unique aspect of the Strongyloides stercoralis lifecycle is autoinfection, a process that allows the infection to persist for decades. This pathway begins when some rhabditiform larvae develop into infective filariform larvae within the gut instead of being passed with feces. This transformation allows the parasite to complete its lifecycle without ever leaving the host.
These newly formed infective larvae can penetrate the wall of the lower intestine or the perianal skin to re-enter the body. They migrate through the bloodstream to the lungs, are coughed up and swallowed, and return to the small intestine to mature into a new generation of parthenogenetic females.
In individuals with compromised immune systems, this cycle can accelerate uncontrollably, leading to hyperinfection. During hyperinfection, larvae spread widely throughout the body to organs not normally involved in the lifecycle, such as the brain and liver. This can lead to severe and often fatal disseminated disease.