Strongyloides stercoralis (Threadworm)

## **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 day