A 28-year-old woman from Kerala presents with a 5-day history of high fever (39.5°C), severe joint pain (especially wrists, ankles, and knees), and a maculopapular rash on the trunk and extremities. She also reports myalgia and headache. On examination, she has conjunctival injection without exudate, and joint swelling is noted. Laboratory investigations show thrombocytopenia (platelets 95,000/μL) and elevated transaminases (AST 120 U/L, ALT 95 U/L). A rapid NS1 antigen test is negative, but IgM antibodies against a mosquito-borne virus are positive. Which of the following is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Chikungunya ### Key Distinguishing Features **Key Point:** Chikungunya is characterized by the triad of **fever, severe polyarthralgia, and rash**, with joint pain being the hallmark and often persisting for weeks to months (chronic arthralgia). ### Comparative Clinical Presentation | Feature | Dengue | Chikungunya | Zika | |---------|--------|-------------|------| | **Fever pattern** | Biphasic ("saddle-back") | High, sustained | Low-grade, brief | | **Joint/Muscle pain** | Moderate myalgia | **Severe polyarthralgia** (wrists, ankles, knees) | Mild arthralgia | | **Rash** | Maculopapular, petechial | Maculopapular, pruritic | Fine maculopapular, pruritic | | **Conjunctivitis** | Without exudate | Rare | **Common (conjunctival injection)** | | **Thrombocytopenia** | **Marked** (often <100,000) | Mild | Absent | | **Transaminitis** | **Marked elevation** | Mild | | **Chronic sequelae** | Dengue hemorrhagic fever | **Chronic arthralgia (months)** | Congenital abnormalities (microcephaly) | ### Why This Case Points to Chikungunya 1. **Severe polyarthralgia** — The patient's prominent joint pain affecting wrists, ankles, and knees is pathognomonic for chikungunya. Dengue causes myalgia, not arthralgia. 2. **Negative NS1 antigen** — Rules out acute dengue (NS1 is positive in the first 5 days of dengue). 3. **Mild thrombocytopenia** — Platelets of 95,000/μL are only mildly reduced; dengue typically causes more severe thrombocytopenia (often <50,000). 4. **Conjunctival injection** — Present in this case; more common in chikungunya and Zika than dengue. 5. **IgM positivity** — Confirms acute arboviral infection; chikungunya IgM appears by day 3–5 of illness. **High-Yield:** Chikungunya = **Chik-JOINT-gunya** — the disease defined by debilitating joint pain. ### Epidemiology **Clinical Pearl:** Chikungunya is transmitted by *Aedes aegypti* and *Aedes albopictus* mosquitoes. Kerala and other southern Indian states have endemic transmission, making chikungunya a top differential in febrile arthralgia presentations from these regions. ### Pathophysiology Chikungunya virus replicates in joint synovium and causes an inflammatory arthritis. The joint pain is often severe enough to immobilize patients (the name "chikungunya" means "that which bends up" in Bantu, referring to the stooped posture from joint pain). ### Management Implications - **Supportive care** — NSAIDs for joint pain (paracetamol is preferred in dengue due to bleeding risk, but NSAIDs are acceptable in chikungunya). - **No specific antiviral** — Ribavirin is not effective. - **Monitoring** — Watch for rare neurological complications (Guillain-Barré syndrome).