A 32-year-old woman from Mumbai presents with fever, severe headache, and myalgia for 4 days. She reports that her 8-year-old son had similar symptoms 2 weeks ago, which resolved after 5 days. On examination, she has a petechial rash on her trunk and mild hepatomegaly. Platelet count is 85,000/μL, and NS1 antigen is positive. What is the most likely epidemiological classification of her dengue infection?

Explanation

## Epidemiological Classification of Dengue Infection **Key Point:** The temporal relationship between the mother's and child's illness, combined with the presence of thrombocytopenia and petechial rash, indicates secondary dengue infection with enhanced disease severity. ### Why This Is Secondary Dengue 1. **Temporal clustering**: The child's dengue 2 weeks prior followed by the mother's illness suggests household transmission and prior exposure to a different dengue serotype. 2. **Antibody-dependent enhancement (ADE)**: In secondary dengue, pre-existing antibodies from the first infection enhance viral uptake into immune cells, increasing viral load and disease severity. 3. **Clinical severity markers**: Thrombocytopenia (85,000/μL) and petechial rash are hallmarks of dengue hemorrhagic fever (DHF), which occurs predominantly in secondary infections. ### Epidemiological Risk Factors for Secondary Dengue | Feature | Primary Dengue | Secondary Dengue | |---------|---|---| | Prior dengue exposure | No | Yes (different serotype) | | Antibody status | Negative | Positive (heterologous) | | Viral load | Moderate | High (ADE-mediated) | | Risk of DHF/DSS | <1% | 5–30% | | Platelet nadir | Usually >100,000 | Often <100,000 | | Petechiae/bleeding | Rare | Common | **High-Yield:** Secondary dengue infections account for ~80% of dengue hemorrhagic fever cases in endemic areas. The presence of a household contact with recent dengue is a strong epidemiological clue. **Clinical Pearl:** In India, dengue is endemic in urban and periurban areas (Mumbai, Delhi, Bangalore). Household clustering is common because *Aedes aegypti* breeds in stored water containers and bites multiple family members in close proximity. **Mnemonic — ADE (Antibody-Dependent Enhancement):** - **A**ntibodies from first infection - **D**o not neutralize second serotype - **E**nhance viral entry and replication ### Why Thrombocytopenia Matters Platelet count <100,000/μL in the presence of fever and rash is a WHO criterion for dengue hemorrhagic fever, which is almost exclusively a secondary infection phenomenon [cite:Park 26e Ch 3].