How to determine if schistosomiasis has been eliminated from the body?

Determining Schistosomiasis Elimination from the Body

Confirming elimination of schistosomiasis requires demonstrating both absence of viable worms (through negative antigen tests like CCA/CAA) and absence of egg excretion, with follow-up testing at 3-6 months post-treatment to account for the parasite's life cycle.

Diagnostic Algorithm for Assessing Cure

Immediate Post-Treatment Assessment (Not Recommended)

• Testing immediately after praziquantel treatment is unreliable because adult worms may persist for weeks before dying, and antigen levels decline gradually 1
• Eggs may continue to be excreted from tissue deposits even after worms are killed 2

Optimal Timing: 3-6 Months Post-Treatment

• Wait at least 3-6 months after praziquantel treatment before assessing cure 3
• This allows time for adult worms to die, antigen levels to decline, and residual egg excretion to cease 3

Multi-Modal Testing Approach

Primary Tests (Use Both):

• Antigen Detection (CCA/CAA in urine or serum): These detect active worm infections and are 6-10 times more sensitive than egg detection 1, 4

  • Circulating Cathodic Antigen (CCA) in urine is commercially available and practical 2
  • Negative CCA/CAA indicates no viable worms producing antigen 1

• Egg Detection (stool or urine microscopy): Traditional Kato-Katz for intestinal species or urine filtration for S. haematobium 1

  • Requires multiple samples (at least 3 specimens on different days) due to low sensitivity, especially in low-intensity infections 5, 4
  • Reading multiple slides per sample (3 slides minimum) increases detection by 20% 5

Supplementary Tests:

• Serology (antibody tests): Useful for detecting exposure but cannot distinguish active from past infection 6, 3
  • Antibodies persist for years after cure, making them unreliable for confirming elimination 3
  • Only use serology if both antigen and egg tests are negative but clinical suspicion remains high 6

Critical Pitfalls to Avoid

The "Egg-Negative but Worm-Positive" Problem

• A person can have viable worms producing antigens but no detectable eggs in stool/urine 2, 1
• This occurs because:
  • Egg excretion may be sporadic and below detection thresholds 2
  • Low worm burdens produce few eggs 2
  • Single stool/urine samples miss intermittent egg shedding 5
• Relying solely on egg detection will miss 80-90% of infections in low-prevalence areas 2, 1

Trace Readings on CCA Tests

• Faint bands ("trace" results) on POC-CCA should be considered positive, not negative 2
• These trace-positive individuals may still excrete eggs sporadically and maintain transmission 2

Insufficient Sampling

• Single stool or urine samples have only 74% sensitivity compared to 96% for multiple samples 4
• Always collect at least 3 specimens on different days if using microscopy alone 4, 5

Recommended Testing Protocol

For Individual Patients:

1. At 3-6 months post-treatment, perform:

   • POC-CCA urine test (or CAA if available) 1, 4
   • Three stool samples on different days with Kato-Katz (3 slides per sample) for S. mansoni 5, 4
   • OR three urine samples on different days with filtration for S. haematobium 4

2. Interpretation:

   • Cure confirmed: Negative CCA/CAA AND negative eggs on all samples 1, 4
   • Treatment failure: Positive CCA/CAA OR eggs detected 1
   • Uncertain: Negative eggs but trace/positive CCA → repeat testing in 3 months or consider PCR if available 2, 7

3. If PCR available: Use as confirmatory test when antigen and microscopy results are discordant 7, 3

   • PCR is up to 4 times more sensitive than microscopy in early or low-intensity infections 3

Special Considerations

• Eosinophilia may persist for months after successful treatment and is not a reliable marker of active infection 3
• Imaging (ultrasound, CT, MRI) assesses organ damage but cannot confirm parasite elimination 8
• In low-transmission areas, even single worm pairs can maintain transmission cycles, making sensitive antigen detection essential 2