A 32-year-old woman from rural Odisha presents with fever, chills, and headache for 5 days. She had returned from a malaria-endemic area 10 days ago. Blood smear shows P. vivax parasitemia (2% RBCs infected). She is started on chloroquine 600 mg base stat, then 300 mg at 6 hours, 24 hours, and 48 hours. On day 7 of treatment, she develops recurrent fever with parasitemia on repeat blood smear. Which additional drug is most appropriate to prevent relapse?

Explanation

## Clinical Context This patient has P. vivax malaria with recrudescence (or relapse) after chloroquine therapy. P. vivax and P. ovale form hypnozoites in the liver that can cause relapses weeks to months after the initial attack. ## Why Primaquine Is Correct **Key Point:** Primaquine is the only antimalarial that acts on hypnozoites and prevents relapse in P. vivax and P. ovale infections. **High-Yield:** Primaquine dosing is 15 mg base (or 26.5 mg salt) daily for 14 days for P. vivax. For P. ovale, the same regimen is used. This is standard post-chloroquine therapy for these species. **Clinical Pearl:** Before prescribing primaquine, G6PD deficiency must be ruled out because primaquine causes hemolysis in G6PD-deficient patients. A G6PD assay should be done; if normal, primaquine can be given safely. ## Mechanism Primaquine is an 8-aminoquinoline that is converted to active metabolites in the liver. These metabolites are lethal to hypnozoites, preventing relapse. ## Chloroquine Alone Is Insufficient Chloroquine kills erythrocytic schizonts but does NOT eliminate hypnozoites. Recurrence of parasitemia after chloroquine in P. vivax/ovale indicates either recrudescence (inadequate erythrocytic kill) or relapse (hypnozoite reactivation). Primaquine is essential for radical cure. **Mnemonic:** **CHAP** — Chloroquine for Hepatic and Asexual Parasites; Primaquine for Hypnozoites.