Strongyloidiasis

Pathogenesis and How the Parasite Invades

Cause and Life Cycle

• Main cause: Strongyloides stercoralis (the intestinal threadworm).
• Infective form: Filariform larvae (L3) living freely in moist soil and water contaminated with feces.
• Main route of entry: L3 larvae actively burrow through intact skin (usually the feet, between the toes or other areas touching soil), enter the veins, travel through the heart and lungs, climb the airways to the throat, and are swallowed. In the gut they mature into adult females that live in the lining of the small intestine.

Autoinfection and Larva Currens

• The autoinfection cycle:
  • Eggs laid by the adult female hatch into rhabditiform larvae (L1) inside the gut.
  • L1 larvae can transform directly into infective L3 larvae within the small intestinal lining or on the skin around the anus.
  • These L3 larvae pass through the gut wall (internal autoinfection) or burrow through the perianal skin (external autoinfection) to re-enter the bloodstream, keeping the infection going for 20–40 years.
• How larva currens ("racing larva") develops:
  • When L3 larvae burrow through the skin of the perianal folds, buttocks or groin, they move freely through the upper dermis at very high speed (5–15 cm per hour).
  • As they travel, they release parasite antigens that set off a local type I hypersensitivity reaction (histamine release from mast cells), producing a raised, intensely itchy red band that rapidly extends along the larva's path.

Clinical Features at Each Stage

Chronic Skin Sign: Larva Currens (Racehorse Larva)

• Appearance: A raised red wheal forming a wavy or straight band clearly visible on the skin.
• Speed: Moves very quickly (5–15 cm per hour), visibly changing position within a few hours.
• Typical sites: Concentrated around the anus, buttocks, thighs, groin and lower abdomen, because autoinfective larvae emerge from the anal area.
• Associated symptoms: Intense waves of itching and burning; the rash may vanish within hours and then reappear elsewhere as the allergic reaction recurs.

Acute and Chronic Gut and Lung Symptoms

• Lungs (Loeffler syndrome): Larvae passing through the lungs cause a dry cough, throat irritation, wheezing, low-grade fever and patchy infiltrates on chest X-ray.
• Digestive tract: Upper abdominal pain (easily mistaken for peptic ulcer disease), chronic diarrhea alternating with constipation, bloating, weight loss and malnutrition.

Hyperinfection Syndrome and Disseminated Strongyloidiasis

• Triggers: Occurs in people with impaired cell-mediated immunity, especially those taking corticosteroids (even at low doses or for short periods), people infected with HTLV-1, and organ transplant recipients.
• Mechanism: Corticosteroids cause a sudden surge in egg laying by female worms and speed up the transformation of L1 into L3 larvae inside the gut.
• Consequences: Millions of L3 larvae tear through the bowel lining and spread to organs throughout the body (brain, liver, kidneys, heart). They carry gram-negative gut bacteria into the blood, causing septic shock and recurrent bacterial meningitis, with mortality of 80–90% if not treated promptly.

How Strongyloidiasis Is Diagnosed at Haute-Derma

History and Risk Factor Screening:

• We record any contact with moist soil, farm work, and travel to or residence in endemic areas.
• Careful medication history: Current or planned use of corticosteroids, anti-rejection drugs, biologic therapy or chemotherapy.

Clinical Assessment and Dermoscopy: We gauge how fast the red band is spreading (marking its edge with a pen and checking for movement after 1–2 hours) and use dermoscopy to look at vascular congestion in the skin.

Parasitology Tests:

• Direct and concentrated stool examination (Baermann method / agar plate culture): Looks for live L1/L3 larvae in stool. Because larvae are shed intermittently, at least 3 stool samples on different days are needed to improve sensitivity.
• Perianal swab / tape test: Detects larvae on the perianal skin in patients with larva currens.

Serology and Blood Count:

• Complete blood count: The eosinophil count is markedly raised (often above 10–15%) in uncomplicated chronic infection. (Note: eosinophils may fall sharply or disappear altogether in hyperinfection syndrome.)
• ELISA for IgG against Strongyloides: Highly sensitive (over 90%) and very useful for screening for silent chronic infection.

Treatment and Recovery Care at Haute-Derma

Principles of Drug Treatment

• Essential goal: To kill all adult worms and larvae in the body and fully break the autoinfection cycle. Unlike with other worms, simply reducing the parasite load is not acceptable.
• Screening before immunosuppressive treatment: Always clear Strongyloides infection before starting any prolonged course of systemic corticosteroids.

Specific Treatment Regimens

• Ivermectin (first-line):
  • Dose: 200 µg/kg body weight per day.
  • Chronic infection / uncomplicated larva currens: A single dose. To make sure the autoinfection cycle is fully eradicated, our standard protocol at Haute-Derma repeats the same dose after 2 weeks.
  • Hyperinfection / disseminated infection: Ivermectin 200 µg/kg given every day until stool and body fluid tests have been negative for larvae for at least 2 consecutive weeks.
• Albendazole (second-line):
  • Used only when ivermectin is unavailable or contraindicated.
  • Dose: 400 mg twice daily for 3–7 consecutive days (considerably less effective than ivermectin at eradicating the infection).

Skin Symptom Management and Supportive Care

• Second-generation H1 antihistamines: Ease the intense itching caused by the allergic reaction as the larvae die.
• Local skin care: Barrier-repair moisturizers, plus a topical antibacterial if scratched skin shows signs of secondary bacterial infection.
• Absolute contraindication: Do not apply potent topical steroids to larva currens before specific anti-Strongyloides treatment has been given.