Strongyloidiasis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Pathophysiology

Pathogenesis

Strongyloides is classified as a soil-transmitted helminth. This means that the primary mode of infection is through contact with soil that is contaminated with free-living larvae. When the larvae come in contact with skin, they are able to penetrate it and migrate through the body, eventually finding their way to the small intestine where they burrow and lay their eggs. Unlike other soil-transmitted helminths such as hookworm and whipworm whose eggs do not hatch until they are in the environment, the eggs of Strongyloides hatch into larvae in the intestine. Most of these larvae will be excreted in the stool, but some of the larvae may molt and immediately re-infect the host either by burrowing into the intestinal wall, or by penetrating the perianal skin. This characteristic of Strongyloides is termed auto-infection. The significance of auto-infection is that unless treated for Strongyloides, persons may remain infected throughout their lifetime.

Transmission

• Contact with soil and auto-infection, there have been rare cases of person-to-person transmission in:
• Organ transplantation
• Institutions for the development nfeclly disabled
• Long-term care facilities
• Daycare centers

Incubation period

Most people do not know when their exposure occurred. For those who do, a local rash can occur immediately. The cough usually occurs several days later. Abdominal symptoms typically occur approximately 2 weeks later, and larvae can be found in the stool about 3 to 4 weeks later.

Life cycle

• The Strongyloides life cycle is more complex than that of most nematodes with its alternation between free-living and parasitic cycles, and its potential for autoinfection and multiplication within the host.
• Two types of cycles exist:

Free-living cycle:

• The rhabditiform larvae passed in the stool can either become infective filariform larvae (direct development) or free living adult males and females
• These adult forms mate and produce eggs from which rhabditiform larvae hatch, eventually become infective filariform larvae.
• The filariform larvae penetrate the human host skin to initiate the parasitic cycle.

Parasitic cycle:

• Filariform larvae in contaminated soil penetrate the human skin and by various, often random routes, migrate into the small intestine.
• Historically it was believed that the larvae migrate via the bloodstream to the lungs, where they are eventually coughed up and swallowed.
• However, there is also evidence that larvae can migrate directly to the intestine via connective tissues.
• In the small intestine they molt twice and become adult female worms.
• The females live threaded in the epithelium of the small intestine and by parthenogenesis produce eggs which yield rhabditiform larvae.
• The rhabditiform larvae can either be passed in the stool or can cause autoinfection.
• In autoinfection, the rhabditiform larvae become infective filariform larvae, which can penetrate either the intestinal mucosa (internal autoinfection) or the skin of the perianal area (external autoinfection); in either case, the filariform larvae may disseminate throughout the body.
• To date, occurrence of autoinfection in humans with helminthic infections is recognized only in Strongyloides stercoralis and Capillaria philippinensis infections.
• In the case of Strongyloides, autoinfection may explain the possibility of persistent infections for many years in persons who have not been in an endemic area and of hyperinfections in immunosuppressed individuals.

Life cycle of Strongyloides stercoralis

Microscopic findings

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