The parasite Plasmodium vivax is the most frequent and widely distributed cause of benign, but recurring (tertian), malaria. It is one of four species of parasite that commonly cause malaria infection in humans. It is less virulent than Plasmodium falciparum, the deadliest of the four, and seldom fatal. P. vivax is passed on by the female Anopheles mosquito, since it is the only gender that bites.
Microscopically, the parasitised red blood cell is up to twice as large as a normal cell and fine pink Schüffner's stippling are seen on the cell's surface. The parasite within it is often wildly irregular in shape (described as "amoeboid"). Schizonts of P. vivax have up to twenty merozoites within them. It is rare to see cells with more than one parasite within them. Merozoites will only attach to immature blood cell (reticulocytes) and therefore it is unusual to see more than three percent of all circulating erythrocytes parasitised.
The incubation period for the infection usually ranges from ten to seventeen days and sometimes up to a year. Persistent liver stages allow relapse up to five years after elimination of red blood cell stages and clinical cure.
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