What is strongyloidiasis and how is it diagnosed?

Strongyloidiasis: What It Is and How to Test for It

Strongyloidiasis is a neglected tropical disease caused by the roundworm Strongyloides stercoralis, which requires diagnosis through specialized stool examination techniques, serology, or molecular methods due to its often low parasite burden and irregular larval output. 1

What is Strongyloidiasis?

Strongyloidiasis is a helminth infection caused primarily by the roundworm Strongyloides stercoralis. This parasitic infection:

• Is most prevalent in Southeast Asia and the Western Pacific, though cases have been documented worldwide 1
• Affects an estimated 613.9 million people annually, mainly in tropical and subtropical regions 2
• Has a reported seroprevalence of approximately 4% in the United States, primarily among immigrants 2

Life Cycle and Infection Process

The infection occurs through a unique life cycle:

• Infection begins when infective filariform larvae penetrate human skin directly 1
• Parasitic females reside in the small intestine and reproduce via parthenogenesis
• Eggs hatch inside the host before rhabditiform larvae are excreted in feces
• A distinctive feature is the ability for autoinfection, where rhabditiform larvae can develop into infectious filariform larvae within the gut and reinfect the same host 1

Clinical Presentation

Strongyloidiasis can present in several ways:

• Many infected individuals remain asymptomatic
• Symptomatic cases may exhibit non-specific gastrointestinal, respiratory, or skin symptoms 1
• In immunocompromised individuals, autoinfection can lead to hyperinfection and disseminated strongyloidiasis, which carries a mortality rate of up to 87% 3, 2

How to Test for Strongyloidiasis

According to the Infectious Diseases Society of America (IDSA) guidelines, diagnosis of strongyloidiasis can be challenging due to low parasite burden and irregular larval output 4. A single stool examination using conventional techniques fails to detect larvae in up to 70% of cases 3.

Recommended Diagnostic Methods

1. Stool Examination:

   • Ova and parasite examination including permanent stained smears 4
   • Specialized techniques to improve sensitivity:
     • Baermann technique or agar plate culture 4
     • Modified Baermann method using a test tube with a rubber stopper instead of a funnel 5

2. Serological Testing:

   • Strongyloides-specific immunoassay 4
   • Note: Serological tests may show cross-reactivity with other parasites like hookworms, filariae, and schistosomes 3
   • May be negative in hyperinfection cases 3

3. Molecular Methods:

   • Nucleic acid amplification tests (NAAT) 4
   • Droplet digital PCR (ddPCR) has shown 98% sensitivity and 90% specificity compared to microscopic methods 6
   • Real-time PCR has demonstrated 82% sensitivity and 76.7% specificity 6

Testing Algorithm

For optimal diagnosis, follow this approach:

1. Initial Testing: Order both stool examination and serology

   • Stool examination should include specialized techniques (Baermann or agar plate culture)
   • Serology is particularly important for screening at-risk populations

2. For Negative Initial Results with High Clinical Suspicion:

   • Repeat stool examinations (at least 3 samples on different days)
   • Consider molecular testing methods if available

3. For Immunocompromised Patients:

   • More aggressive diagnostic approach with multiple stool samples
   • Consider empiric treatment if clinical suspicion is high, even with negative test results 4

Treatment Considerations

The FDA-approved treatment for strongyloidiasis is ivermectin:

• Standard dosing: 200 μg/kg as a single oral dose 7
• For immunocompromised patients: Consider repeated courses of therapy at 2-week intervals 7
• For hyperinfection syndrome: Ivermectin 200 μg/kg on days 1,2,15, and 16 4

Special Considerations

• Immunocompromised Patients: Proactive screening is essential before initiating immunosuppressive therapy, particularly corticosteroids 2
• Follow-up: Repeated stool examinations are necessary to document clearance of infection 7
• Risk Factors: HTLV-1 infections, hypogammaglobulinemia, immunosuppressant use (particularly steroids), alcoholism, and malnutrition increase the risk of severe disease 2

Common Pitfalls in Diagnosis

• Relying on a single stool sample examination (misses up to 70% of cases)
• Using only direct microscopy without concentration techniques
• Failure to consider strongyloidiasis in immunocompromised patients with unexplained symptoms
• Not screening high-risk individuals before immunosuppressive therapy

By understanding the nature of strongyloidiasis and employing appropriate diagnostic techniques, clinicians can better identify and manage this potentially serious parasitic infection.