Strongyloidiasis

Strongyloidiasis is a parasitic infection caused by the nematode Strongyloides stercoralis, which is endemic to warm, moist climates. Infection occurs when larvae penetrate the skin, most commonly the feet (primarily occurs through walking barefoot). Symptoms may be absent, mild, or severe and include a pruritic serpiginous rash, cough, abdominal pain, and diarrhea. Immunosuppressed patients are at risk for hyperinfection syndrome and disseminated strongyloidiasis, which can lead to severe disease and septic shock. Patients with planned immunosuppression (e.g., due to transplant) and those at risk for strongyloidiasis (e.g., due to time spent in an endemic region) should be screened for strongyloidiasis to prevent complications. Diagnosis is made based on serology and/or visualization of the larvae in stool specimens. Strongyloidiasis is treated with ivermectin or a benzimidazole.

• Pathogen
  • Strongyloides stercoralis (threadworm) ^[1]
  • Threadworms are nematodes.
• Mode of transmission: percutaneous penetration of larvae (primarily via the feet)
• Life cycle ^[2]
  • The Strongyloides eggs hatch in the human intestine and release larvae→ larvae are excreted in feces and contaminate the soil→ larvae penetrate the intact skin of the definitive host upon contact with contaminated soil → larvae migrate to the lungs via the bloodstream → larvae migrate to the pharynx via the alveoli and bronchial system → larvae are swallowed, causing autoinfection → larvae mature into adult, egg-producing worms in the intestine → eggs develop into infectious larvae and are excreted in feces
  • Hyperinfection: Some larvae may penetrate the intestinal wall and enter the bloodstream (rare).

Strongyloides infection most commonly occurs in warm, moist climates (e.g., the tropics) in individuals who often walk barefoot.

Patients are usually asymptomatic or have mild symptoms during acute and chronic infections. ^[3]

Acute infection ^[4]

• Cutaneous phase
  • Swelling, erythema, maculopapular rash at the site of larval penetration
  • Larva currens: raised serpiginous lesions or urticarial tracts; due to migration of larvae under the skin in autoinfection; pathognomonic of S. stercoralis infection
  • Pruritus
• Pulmonary phase: dry cough and wheezing, hemoptysis, rarely pneumonia (Loeffler syndrome)
• Intestinal phase: inflammation (e.g., duodenitis)
  • Abdominal pain
  • Diarrhea
  • Anorexia, nausea, vomiting

Incubation periods range from 1–4 weeks. Acute local cutaneous reaction to larval penetration is immediate. Pulmonary symptoms develop ∼ 1 week and intestinal symptoms ∼ 3–4 weeks after initial infection. ^[4]

Chronic infection ^[4]

• Diarrhea, occasional vomiting, constipation
• Larva currens

General principles ^[5]

Studies are performed based on immune status and presentation.

• Immunocompetent: either serology or stool microscopy
• Immunosuppressed: both serology and stool microscopy
• Hyperinfection and/or disseminated strongyloidiasis: additional microscopy of other body fluids based on organ involvement

Confirmatory studies ^[3][6]

Indications for testing

• Clinical features of strongyloidiasis
• Screening for strongyloidiasis in patients with either:
  • Immunocompetence and a high risk of exposure
  • Current or planned immunosuppression and an intermediate or high risk of exposure

Modalities

• Stool microscopy: serial stool examination with mobile rhabditiform larvae ^[4]
• Microscopy of other body fluids : evidence of filariform larvae in patients with hyperinfection and/or disseminated strongyloidiasis
• Serology: detection of IgG against Strongyloides ^[5][6]

Additional studies

• CBC: : may show eosinophilia ^[3]
• Diagnostic studies for sepsis: e.g., blood cultures to rule out bacterial infections

Approach ^[4]

• Initiate sepsis management in severe infections.
• Start antiparasitic therapy in all patients with confirmed strongyloidiasis.
• Consider empiric therapy in immunosuppressed patients with a risk of exposure if diagnostic testing cannot be obtained. ^[5]
• Repeat serology at 6 months and 12 months after treatment to assess for decreasing titers. ^[5]

Hyperinfection syndrome is a medical emergency and should be treated immediately without waiting for a definitive diagnosis. ^[4]

Pharmacotherapy ^[6]

• Uncomplicated infection (i.e., acute or chronic strongyloidiasis)
  • Preferred: ivermectin for 1–2 days ^[6]
  • Alternative: benzimidazoles, e.g., albendazole (off-label) for 7 days ^[4][6]
• Severe infection (i.e., hyperinfection syndrome or disseminated strongyloidiasis)
  • Preferred: ivermectin ^[6]
  • Duration: until stool and/or sputum specimens are negative for 2 weeks ^[3][6]
  • Reduce immunosuppressive drugs if feasible.

Ivermectin should not be given to patients with concurrent Loa loa infection due to the risk of fatal encephalopathy. ^[7]

• Primary prevention ^[3]
  • Wear shoes if walking on soil.
  • Avoid contact with sewage and fecal matter.
• Secondary prevention: See “Screening for strongyloidiasis.” ^[3]

Hyperinfection syndrome

Increased rate of autoinfection due to immunosuppression (e.g., steroid therapy, HTLV-1 coinfection, transplant) that can cause:

• GI symptoms, e.g., abdominal pain, ileus, mucosal ulcers
• Pulmonary symptoms, e.g., pneumonitis, respiratory failure
• Dermatologic signs, e.g., recurrent rash, larva currens
• Complications of dissemination, e.g., organ dysfunction, septic shock, meningitis

Strongyloides completes its entire life cycle within its host. In the intestinal tract, eggs hatch into larvae that reinfect the host and may result in a high parasitic burden within a short period of time. ^[6]

We list the most important complications. The selection is not exhaustive.