A 28-year-old woman from Kerala presents to the emergency department on day 4 of acute fever with severe joint pain affecting her wrists, ankles, and knees bilaterally. She reports the fever subsided 2 days ago but the arthralgia has worsened. On examination, she has mild conjunctival injection and a maculopapular rash over her trunk and extremities. Laboratory investigations show platelet count 180,000/μL, normal haemoglobin, and negative dengue NS1 antigen. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Chikungunya Fever ### Key Clinical Features **Key Point:** Chikungunya is characterized by the triad of fever, arthralgia, and rash, with distinctive **biphasic fever pattern** and **incapacitating joint pain** that persists for weeks to months. ### Why This Patient Has Chikungunya 1. **Fever pattern**: Biphasic fever (subsided 2 days ago, now in the arthralgia phase) 2. **Arthralgia characteristics**: Severe, symmetric, polyarticular (wrists, ankles, knees) — classic for Chikungunya 3. **Rash**: Maculopapular rash typical of Chikungunya 4. **Negative dengue NS1**: Rules out dengue fever 5. **Platelet count normal**: Dengue typically causes thrombocytopenia; Chikungunya does not 6. **Geographic context**: Kerala is endemic for Chikungunya ### Comparative Virology Features | Feature | Dengue | Chikungunya | Zika | |---------|--------|-------------|------| | **Fever pattern** | Continuous/biphasic | Biphasic (characteristic) | Continuous | | **Arthralgia severity** | Mild to moderate | Severe, incapacitating, persistent | Mild | | **Joint involvement** | Large joints | Small joints (wrists, ankles) | Mild arthralgia | | **Rash** | Maculopapular, petechial | Maculopapular | Fine maculopapular | | **Thrombocytopenia** | Common (< 100,000) | Rare | Absent | | **Conjunctivitis** | Mild | Mild | Prominent (without exudate) | | **Duration of arthralgia** | Days to weeks | Weeks to months | Days | ### High-Yield Mnemonic: CHIK **Key Point:** **C**yclone of **H**ot joints, **I**ncapacitating pain, **K**eeps you down for weeks ### Clinical Pearl **Clinical Pearl:** The persistent, incapacitating polyarthralgia affecting small joints (wrists, ankles) that outlasts the fever is pathognomonic for Chikungunya. Patients often report being unable to walk or perform daily activities. This post-viral arthralgia can persist for months, unlike dengue arthralgia which resolves within weeks. ### Pathophysiology Chikungunya virus (Alphavirus, Togaviridae family) replicates in joint synovial tissue, causing direct synovial inflammation and immune-mediated joint damage. This explains the prolonged arthralgia even after viral clearance. ### Diagnostic Confirmation - **Acute phase (days 1–5)**: RT-PCR of serum or plasma - **Convalescent phase (day 5+)**: IgM ELISA (positive by day 5–7) - **IgG ELISA**: Appears after 2 weeks, indicates past infection [cite:Park 26e Ch Communicable Diseases]