A 28-year-old woman from Mumbai presents with sudden onset fever, severe headache, and myalgia for 3 days. On examination, she has a maculopapular rash sparing palms and soles, and a positive tourniquet test. Her platelet count is 95,000/μL and haematocrit is 38%. She reports that her 5-year-old son had similar symptoms 2 weeks ago, which resolved completely. What is the most likely diagnosis and what is the epidemiological significance of her presentation?

Explanation

## Clinical Presentation Analysis This patient presents with: - Fever, headache, myalgia (classic dengue triad) - Maculopapular rash sparing palms and soles (dengue-specific distribution) - Positive tourniquet test (indicates thrombocytopenia and capillary fragility) - Mild thrombocytopenia (95,000/μL) and normal haematocrit ## Epidemiological Context The **prior infection in her son 2 weeks ago** is the critical epidemiological clue. This suggests: 1. **Household dengue transmission** — likely the same serotype circulating in the community 2. **Sequential infection** — she is now experiencing infection with a *different* dengue serotype than her son (or re-exposure to the same serotype) 3. **Secondary dengue infection** — she has pre-existing dengue immunity from a prior exposure (possibly asymptomatic or unrecognized) ## Antibody-Dependent Enhancement (ADE) **Key Point:** Secondary dengue infection carries the highest risk for dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS) due to **antibody-dependent enhancement**. **High-Yield:** ADE mechanism: - Non-neutralizing antibodies from the first infection bind to the new serotype - These antibody-virus complexes enhance viral entry into macrophages via Fc receptors - Results in higher viraemia and greater cytokine release - Peak risk occurs **during the critical phase (days 3–7 of illness)**, not immediately ## Risk Stratification | Feature | Primary Dengue | Secondary Dengue | |---------|---|---| | **Immune status** | Naive | Pre-existing antibodies | | **Viraemia level** | Moderate | High (due to ADE) | | **DHF risk** | ~1% | ~15–20% | | **Timing of DHF** | Rare in first infection | Days 3–7 (critical phase) | | **Serotype specificity** | Lifelong immunity to that serotype only | Cross-reactive antibodies to all serotypes | **Clinical Pearl:** The presence of a prior dengue infection in a household member within 2 weeks strongly suggests secondary dengue in this patient, even without serological confirmation. This is the epidemiologically most significant scenario because it identifies her as **high-risk for severe dengue**. ## Why This Is the Answer Option 1 (Primary dengue) is incorrect because the household exposure history and the timing suggest prior sensitization. Option 3 is incorrect because dengue immunity is serotype-specific, not pan-serotype. Option 4 is incorrect because DHF does not develop within 48 hours; it develops during the critical phase (typically days 3–7).