Plasmodium
Also known as: malaria parasite, plasmodium spp.
Plasmodium is the genus of protozoan parasites that causes malaria. It is transmitted by the bite of a female Anopheles mosquito and infects liver cells and then red blood cells, producing cyclical fever, hemolytic anemia, and splenomegaly.
Plasmodium species are intracellular protozoa transmitted by the female Anopheles mosquito. Five species infect humans: P. falciparum, P. vivax, P. ovale, P. malariae, and the zoonotic P. knowlesi. The mosquito is the definitive host, because the parasite's sexual reproduction occurs there; humans are the intermediate host.
The life cycle explains the clinical picture. Sporozoites injected during a blood meal travel to the liver and multiply within hepatocytes. Released merozoites then invade red blood cells, mature through ring and schizont stages, and rupture the cell to release a new wave of merozoites. Because these ruptures are roughly synchronized, they produce the characteristic paroxysms of chills and fever. P. vivax and P. ovale cycle every 48 hours (tertian fever), P. malariae every 72 hours (quartan fever), and P. falciparum tends to be irregular. Repeated red cell destruction causes hemolytic anemia, jaundice, and splenomegaly.
Two species-specific points matter most. _P. falciparum_ is the deadliest: it infects red cells of all ages, produces very high parasitemia, and causes infected cells to adhere to capillary endothelium, leading to cerebral malaria and organ failure. _P. vivax_ and _P. ovale_ form hypnozoites, dormant liver stages that reactivate weeks to months later, so eradicating them requires primaquine in addition to the drug that clears the bloodstream. Diagnosis is made with Giemsa-stained thick and thin blood smears or rapid antigen tests. Several inherited red cell traits confer partial protection — sickle cell trait, thalassemia, G6PD deficiency, and Duffy antigen negativity, which specifically resists P. vivax.
Plasmodium is a recurring USMLE Step 1 parasitology topic. Expect vignettes that pair a travel history with a fever pattern and ask you to identify the species, explain the relapse mechanism, name the vector, or select the appropriate treatment.
Key takeaways
- Plasmodium causes malaria and is transmitted by the female Anopheles mosquito, which serves as the definitive host.
- Sporozoites infect hepatocytes first, then merozoites cyclically invade and rupture red blood cells, producing paroxysmal fever and hemolytic anemia.
- P. falciparum is the most severe species, causing high parasitemia, capillary sequestration, and cerebral malaria.
- P. vivax and P. ovale form dormant liver hypnozoites that cause relapse and require primaquine for eradication.
- Giemsa-stained thick and thin blood smears remain the diagnostic standard; sickle cell trait and Duffy negativity confer partial resistance.