A 28-year-old woman from West Bengal presents with recurrent fever 6 months after an initial malaria episode treated with chloroquine. She reports fever with chills every 48 hours, and her thick blood smear shows ring forms and trophozoites. Thin smear reveals hypochromic RBCs with stippling. She denies recent travel outside the endemic area. What is the most likely diagnosis, and what is the underlying mechanism?

Explanation

## Relapse vs. Recrudescence in Plasmodium Infection **Key Point:** Reappearance of fever 6 months after initial treatment in an endemic area, without recent travel, is characteristic of P. vivax relapse caused by hypnozoite activation from the liver. ### Distinction Between Relapse and Recrudescence | Feature | Relapse | Recrudescence | |---------|---------|---------------| | Definition | Reactivation of dormant parasites (hypnozoites) from liver | Persistence of blood parasites due to inadequate drug levels | | Causative organisms | P. vivax, P. ovale | P. falciparum, P. malariae | | Time to recurrence | Weeks to months (even years) | Days to weeks after treatment | | Mechanism | Hypnozoite dormancy in hepatocytes | Incomplete erythrocytic clearance | | Chloroquine response | Poor (requires primaquine) | Good (chloroquine-sensitive strains) | | Thick smear findings | Ring forms, trophozoites | Ring forms, trophozoites | | Geographic pattern | Tropical/subtropical P. vivax | Chloroquine-resistant P. falciparum | **High-Yield:** P. vivax and P. ovale have a pre-erythrocytic dormant stage (hypnozoite) in hepatocytes that can reactivate months or years after the initial infection, even if blood parasites are cleared. Chloroquine kills only blood schizonts and gametocytes, NOT hypnozoites. Primaquine is required to eliminate hypnozoites. ### Why This Patient Has Relapse 1. **Timing:** 6 months post-treatment = classic relapse window for P. vivax 2. **Clinical features:** 48-hourly fever = tertian pattern = P. vivax 3. **Morphology:** Ring forms + stippling in hypochromic RBCs = P. vivax 4. **No recent travel:** Rules out new reinfection; indicates reactivation of dormant parasites 5. **Prior chloroquine treatment:** Chloroquine does not eliminate hypnozoites; primaquine is needed **Mnemonic:** **VIVAX RELAPSE = Hypnozoite Reactivation; FALCI RECRUDESCENCE = Incomplete Blood Clearance** ### Pathophysiology of Hypnozoite Activation ```mermaid flowchart TD A[Mosquito injects sporozoites]:::action --> B[Sporozoites reach liver]:::action B --> C[Pre-erythrocytic schizogony]:::action C --> D{P. vivax/ovale?}:::decision D -->|Yes| E[Some schizonts → hypnozoites dormant in hepatocytes]:::outcome D -->|No| F[All schizonts → merozoites to blood]:::outcome E --> G[Blood parasites cleared by chloroquine]:::action G --> H[Hypnozoites remain dormant]:::outcome H --> I[Months/years later: spontaneous or stress-triggered activation]:::action I --> J[New erythrocytic schizogony]:::action J --> K[Recurrent fever = RELAPSE]:::urgent F --> L[Chloroquine clears blood parasites]:::action L --> M{Adequate drug levels?}:::decision M -->|No| N[Parasites persist in blood]:::outcome M -->|Yes| O[Cure]:::outcome N --> P[Recurrent fever = RECRUDESCENCE]:::urgent ``` **Clinical Pearl:** Patients with P. vivax infection in endemic areas should receive primaquine (0.5 mg/kg/day for 14 days) to prevent relapse. G6PD deficiency screening is mandatory before primaquine use, as it causes hemolysis in G6PD-deficient patients. [cite:Park 26e Ch 3; Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases Ch 275]