During a dengue outbreak, a 22-year-old man presents on day 5 of illness with fever, thrombocytopenia (platelets 28,000/μL), and positive tourniquet test. NS1 antigen test is negative. Anti-dengue IgM and IgG are BOTH positive with high IgG titers. This serological pattern suggests:
- A Primary dengue infection — early IgM and background IgG from maternal antibodies
- B Secondary dengue infection — IgG from prior sensitization amplifies the current antibody response ✓
- C Cross-reactive antibodies from recent Zika virus infection
- D False-positive IgM due to concurrent malaria infection (rheumatoid factor interference)
Explanation
In secondary dengue infection, the pattern shows: NS1 antigen may become negative by day 4–5 as antigen is cleared by antibodies, IgG is present early and at high titers (boosted secondary response from prior sensitization to a different dengue serotype), and IgM is also present but may be lower than in primary infection. Primary dengue shows high IgM with low/absent IgG. Secondary dengue is more likely to progress to severe dengue (dengue hemorrhagic fever/dengue shock syndrome) due to antibody-dependent enhancement (ADE) — pre-existing non-neutralizing cross-reactive antibodies from the first serotype infection enhance uptake of the second serotype into Fc receptor-bearing cells, amplifying viral replication.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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Written and medically reviewed by the StethoPrep medical team.