Читать книгу Parasitology - Alan Gunn - Страница 75

The name vivax derives from the lively nature of the trophozoites in our red blood cells (vivax is the Latin adjective for vivacious). This stage often has an amoeboid appearance in blood films. Vivax malaria was once the most widespread form of the disease and common as far north as Norway and Siberia. However, as European countries developed, they largely eradicated malaria, and vivax malaria is now mostly restricted to Asia and the countries bordering the Mediterranean. However, it is still the most common Plasmodium species in most countries in which malaria remains endemic. Vivax malaria is rare in West Africa because the merozoites only penetrate red blood cells carrying the Duffy buffer blood group antigens Fy ^a and Fy ^b – and most West Africans do not express these. In addition, the merozoites cannot penetrate mature red blood cells and therefore they invade developing reticulocytes. Consequently, P. vivax cannot form the same high parasitaemias achieved by P. falciparum. Plasmodium vivax is notorious for causing latent infections in which the hypnozoite stage remains quiescent within the liver and then, after years of apparent good health, the patient suddenly develops malarial fevers. The factors determining the length of the latent period probably relate to genetic differences between the parasites causing the initial infection and/or sudden changes in the host’s immune status.