Plasmodium vivax

Plasmodium vivax is a protozoal parasite and a human pathogen. The most frequent and widely distributed cause of recurring (Benign tertian) malaria, P. vivax is one of the six species of malaria parasites that commonly infect humans. It is less virulent than Plasmodium falciparum, the deadliest of the six, but vivax malaria can lead to severe disease and death due to splenomegaly (a pathologically enlarged spleen). It afflicted as many as eight U.S. presidents—including George Washington and Abraham Lincoln—and may have helped kill Genghis Khan. P. vivax is carried by the female Anopheles mosquito, since it is only the female of the species that bite.

COURSE OF DISEASE

Unlike P. falciparum, P. vivax can populate the bloodstream with sexual-stage parasites—the form picked up by mosquitoes on their way to the next victim—even before a patient shows symptoms. That means that promptly treating symptomatic patients doesn't necessarily help stop an outbreak, as it does with falciparum malaria, in which fevers occur as sexual stages develop. Even when symptoms appear, because they are usually not immediately fatal, the parasite continues to multiply.

The parasite can go dormant in the liver for days to years, causing no symptoms and remaining undetectable in blood tests. They form what are called hypnozoites (the name derives from "sleeping parasites"), a small form that nestles inside an individual liver cell. The hypnozoites allow the parasite to survive in more temperate zones, where mosquitoes bite only part of the year.

A single infectious bite can trigger six or more relapses a year, leaving sufferers more vulnerable to other diseases. Other infectious diseases, including falciparum malaria, appear to trigger relapses.