## Distinguishing P. vivax from P. falciparum ### RBC Preference and Relapse Mechanism **Key Point:** P. vivax has a strict preference for young RBCs (reticulocytes), while P. falciparum infects RBCs of all ages. This difference in host cell tropism is the most important epidemiological and clinical discriminator. **High-Yield:** P. vivax forms hypnozoites (dormant liver stage) that can reactivate weeks to months after the primary attack, causing relapses. P. falciparum does not form hypnozoites and thus does not relapse; recrudescence (if it occurs) is due to inadequate treatment of the erythrocytic stage. ### Comparison Table | Feature | P. vivax | P. falciparum | | --- | --- | --- | | **RBC preference** | Young RBCs (reticulocytes) | All RBC ages | | **Parasitemia level** | Low (< 1%) | High (up to 40%) | | **Hypnozoites** | Present → relapses | Absent → no relapses | | **Gametocyte appearance** | Day 3–4 of illness | Day 7–10 of illness | | **Severity** | Benign tertian fever | Malignant tertian fever | | **Cerebral malaria** | Rare | Common | ### Clinical Pearl The presence of hypnozoites in P. vivax is why primaquine (an 8-aminoquinoline) is essential for radical cure—it is the only drug that kills hypnozoites. Chloroquine alone will treat the acute attack but leave dormant parasites in the liver, leading to relapse. This is a critical epidemiological feature: P. vivax can persist in a population even after apparent clinical cure unless hypnozoites are eliminated. **Mnemonic:** **VIVAX = RELAPSE** (hypnozoites cause relapses); **FALCIPARUM = FATAL** (no relapse, but higher mortality if untreated). [cite:Park 26e Ch 7]
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