Strongyloides stercoralis (Threadworm) — Notes OmpathStudy

Introduction Strongyloides stercoralis is the smallest nematode known to cause human infection. It primarily infects the small intestine (duodenum and jejunum) of humans. Unlike most nematodes, larvae, not eggs, are excreted in feces and detected in stool examination. Life Cycle Natural host : Humans (also found in dogs and cats). Infective form : Filariform larva (L3 stage) . Mode of infection : Skin penetration by L3 larvae , usually through barefoot contact with contaminated soil . Autoinfection (Internal & External). Ingestion of contaminated food/water (less common). Types of Development Direct Development (Parasitic Life Cycle) The rhabditiform larvae (L1) , which hatch from eggs in the intestine, are excreted in feces . In soil, the larvae molt twice to become the infective filariform larvae (L3) . These larvae penetrate human skin , enter the venous circulation , travel to the heart and lungs , escape into the alveoli , and migrate to the pharynx , where they are swallowed . They mature into adult female worms in the small intestine within 15–20 days . Indirect Development (Free-living Cycle) Rhabditiform larvae (L1) in feces develop into free-living adult males and females in soil . These free-living worms mate , producing eggs that hatch into new rhabditiform larvae . These larvae may either:Continue the free-living cycle. Develop into infective filariform larvae (L3) , which penetrate human skin and initiate the parasitic cycle . Autoinfection External autoinfection :Some rhabditiform larvae transform into infective L3 larvae while passing through the gut. These L3 larvae penetrate the perianal skin during defecation , causing perianal creeping eruption (cutaneous larva migrans) . Internal autoinfection :Occurs inside immunocompromised individuals . The rhabditiform larvae (L1) mature into filariform larvae (L3) within the intestine . These L3 penetrate the intestinal mucosa , enter the circulation , and complete their cycle without exiting the body. This can lead to hyperinfection syndrome , causing systemic complications. Pathogenesis & Clinical Features Most infections are asymptomatic . Severe disease occurs in immunocompromised patients . Cutaneous Manifestations Dermatitis, erythema, and itching at the site of larval penetration. Allergic responses in previously infected individuals. Perianal pruritus and urticaria in chronic infections. Larva currens ("racing larvae"):Rapidly progressing serpiginous urticarial tracks due to migrating larvae. Starts perianally and spreads rapidly . Pulmonary Manifestations Occurs during larval migration through the lungs . Small hemorrhages in alveoli and bronchioles. Bronchopneumonia , which may progress to chronic bronchitis and asthma-like symptoms . Strongyloides larvae may be found in sputum . Intestinal Manifestations Symptoms resemble peptic ulcer disease or malabsorption syndrome . Mucus diarrhea and abdominal pain. Severe infections cause honeycombing of intestinal mucosa , leading to sloughing and dysenteric stools . Complications : Protein-losing enteropathy . Paralytic ileus . Hyperinfection Syndrome & Disseminated Strongyloidiasis Occurs in immunocompromised individuals (HIV, chemotherapy, steroids, malnutrition) . Massive autoinfection leads to filariform larvae entering the arterial circulation . Larvae may lodge in organs such as the heart, lungs, brain, kidneys, pancreas, liver, and lymph nodes . Complications : Brain abscess, meningitis, peritonitis . Septicemia , as larvae carry intestinal bacteria into circulation . Diagnosis 1. Microscopy Direct stool examination (wet mount) to detect larvae (not eggs) . Concentration methods (Formol-ether concentration, Baermann's funnel gauze method). Baermann's test : Larvae actively migrate out of feces onto gauze. Sputum, duodenal aspirates, or jejunal biopsy may also show larvae. 2. Stool Culture Used when larvae are scanty in stool . Techniques : Agar plate culture . Charcoal culture method . 3. Serology ELISA (95% sensitivity) detects Strongyloides antibodies . Limitations :Cross-reactivity with other helminths. Antigens may not always be available. 4. Imaging X-ray, CT scan, or MRI may help detect intestinal and pulmonary involvement . Treatment & Prophylaxis Treatment Ivermectin (preferred): 200 µg/kg daily for 2 days . For disseminated strongyloidiasis , treatment should be extended to at least 5–7 days . Albendazole ( less effective ): 400 mg daily for 3 days . Hyperinfection treatment : Ivermectin for 7+ days until larvae disappear from stool/sputum. Prevention & Control Proper disposal of feces to prevent soil contamination. Avoid walking barefoot in endemic areas. Avoid contact with contaminated soil and surface water . Treat all infected individuals to prevent transmission. Screen immunocompromised patients before starting corticosteroids or chemotherapy. Critical Notes & Additional Points Strongyloides stercoralis has a unique free-living cycle , allowing it to persist in the environmen