Plasmodium vivax

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.

Asexual forms:
 * Immature trophozoites (Ring or signet-ring shaped), about 1/3 of the diameter of a RBC.
 * Mature trophozoites: Very irregular and delicate (described as amoeboid); many pseudopodial processes seen.  Presence of fine grains of brown pigment (malarial pigment) or hematin probably derived from the haemoglobin of the infected red blood cell.
 * Schizonts (also called meronts): As large as a normal red cell; thus the parasitized corpuslce becomes distended and larger than normal.  there are about sixteen merozoites.

Sexual forms: Gametocytes:  Round. The gametocytes of P. vivax are commonly found in the peripheral blood at about the end of the first week of parasitemia.

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 3% of all circulating erythrocytes parasitised.

P. vivax and P. ovale that has been sitting in EDTA for more than half-an-hour before the blood film is made will look very similar in appearance to P. malariae, which is an important reason to warn the laboratory immediately when the blood sample is drawn so they can process the sample as soon as it arrives. Blood films are preferably must be made within half-an-hour of the blood being drawn and must certainly be made within an hour of the blood being drawn.

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.

Treatment
Chloroquine remains the treatment of choice for vivax malaria, except in Indonesia's Irian Jaya (Western New Guinea) region and the geographically contiguous Papua New Guinea, where chloroquine resistance is common (up to 20% resistance). When chloroquine resistance is common or when chloroquine is contraindicated, then artesunate is the drug of choice; mefloquine is a good alternative and in some countries is more readily available. Atovaquone-proguanil is an effective alternative in patients unable to tolerate chloroquine. Quinine may be used to treat vivax malaria but is associated with inferior outcomes.

32 to 100% of patients will relapse following successful treatment of P. vivax infection if a radical cure (eradication of liver stages) is not given. Eradication of the liver stages is achieved by giving primaquine, after checking the patients G6PD status to reduce the risk of haemolysis. Recently, this point has took particular importance for the increased incidence of vivax malaria among travelers.