From PCR to Duplex RAA for Schistosoma Detection: A Practical Guide for Field-Ready Species-Level Diagnostics

Part II of our Isothermal Masterclass Series reveals how a duplex RAA assay can overcome the limits of PCR to enable rapid, field-ready detection of Schistosoma species, using isothermal chemistry engineered for real-world conditions.

  
Schistosomiasis remains one of the most persistent parasitic diseases in Africa and parts of South America. Accurate detection, and particularly species identification, is essential for patient management, environmental surveillance, and tracking transmission dynamics. Yet PCR-based diagnostics often fail to translate into endemic settings where electricity, trained staff, and stable reagents are limited. 

In the second session of our Isothermal Masterclass Series, Ejona Gjika from Xpedite Diagostics presented a clear and technically rigorous blueprint for building a duplex RAA assay capable of detecting both Schistosoma mansoni and Schistosoma haematobium within a single reaction. 

Her work illustrates how isothermal amplification, when paired with precise oligo design and optimization, can produce diagnostics suitable for environmental and clinical use far beyond conventional laboratories. 
 

RAA as a POC-optimal solution 

Feature 

PCR 

RAA 

Amplification type 

Thermal cycling 

Isothermal (37–42 °C) 

Time to result 

1–2 hours 

15–30 min 

Equipment required 

Thermocycler 

Portable heater+ readout platform 

Consumables/ waste generated 

High 

Low 

Field deployable 

No 

Yes 

Sensitivity 

High 

High 

Energy/power need 

High 

Low 


When literature-based assays fail the validation test 

Ejona began with two published assays (Zhao S., et al, 2023). 

S. haematobium cox1 RAA 

  • Published LOD: ~10 copies/µL 
  • In-house: ~177 copies, only detected at 15 min 

S. mansoni mitochondrial minisatellite RAA 

  • Published LOD: ~1 copy 
  • In-house: ~1960 copies, only weakly detected 

The discrepancies underscore a core principle of isothermal testing: performance claims rarely transfer across reagents and laboratories without redesign and test. 

This failure triggered a deeper reassessment of target choice. 
 

Redesigning the S. mansoni assay using the SM1–7 repeat 

Ejona turned to SM1–7, a tandem repeat representing ~12% of the S. mansoni genome. Its high copy number makes it inherently suitable for sensitive detection.  
 
For complete design guidelines, reference here. 

Adapting SM1–7 to RAA required: 

  • 30–35 nt primers 
  • 46–52 nt probe with dspacer modification and 3′ block 
  • FAM labeling and ROX-labeled 
  • ~100–150 bp amplicon 

This redesign produced a dramatic improvement: LOD from 1960 copies to 122 copies, with no cross-reactivity against S. haematobium. 

 

Engineering a functional duplex: temperature, stoichiometry, kinetics 

With the redesigned SM1–7 assay, Ejona optimized the duplex by addressing three parameters: 

1. Temperature 

The assays originally preferred different temperatures (39 °C vs 42 °C). The compromise 41 °C supported robust amplification of both species. 

2. Primer/probe concentrations 

To avoid competition: 

  • S. mansoni SM1–7: full concentration 
  • S. haematobium cox1: 50% concentration 

This asymmetry stabilized duplex performance. 

3. Reaction time 

Amplification converged on a 10–15 minute window across matrices. 

 

Performance across environmental and clinical samples 

Ejona evaluated the duplex with matrices relevant to real-world surveillance.

Snail extracts (benchmarked to qPCR) 

When using the Sm1-7 qPCR assay, the samples were all positive with Ct values ranging from 12.01 to 27.6. The duplex RAA reproduced these values reliably maintaining a visibly high correlation between a low qPCR-derived Ct and early time-to-positivity in RAA. 

Plasma cfDNA from human infections 

  • Five cfDNA extracts from the plasma of 5 patients with confirmed S. manosni infection via microscopy of their stool samples:  29-43 eggs/ g of stool per patient 
  • All detected in <5 minutes 
  • High parasitemia was confirmed throughout the 5 patients, but there was no clear correlation between egg count in stool and RAA time-to-positivity 

Together, these results demonstrate that the assay functions across both high-inhibitor environmental samples and clinically relevant fluids. 
 

Mixing strategy: a critical variable in isothermal chemistry 

RAA initiates when magnesium is added. Without effective mixing, early reaction zones may amplify unevenly. Ejona’s work highlighted that: 

  • S. mansoni sensitivity improved from 2400 copies to 120 copies with automated steel bead mixing 
  • S. haematobium detection improved near the LOD 
  • Reaction variability decreased substantially 

Mixing is often ignored in isothermal work, yet here it proved essential. 

 

Looking for setting up your isothermal diagnostic assay? 

We support organizations and researchers developing rapid parasitology diagnostics, environmental surveillance tools, and field-deployable isothermal assays. 

 

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