Currently, the bulk of the scientific community’s firepower is aimed at Plasmodium falciparum, the deadliest of the four known malaria-causing parasites. But researchers are starting to pay closer attention to Plasmodium vivax, which has heretofore lurked in its cousin’s shadow. Both P. falciparum and P. vivax invade liver cells as part of their life cycle, but P. vivax can lie dormant in the liver for much longer periods of time, potentially causing a relapse of malaria years after the initial infection.
Papers on P. vivax malaria frequently carry the word “neglected” in the title — there’s a lack of understanding of just how widespread the parasite is, which would be key to any sort of control or eradication strategy. In a paper published in PLoS Neglected Tropical Diseases, a team of Oxford scientists offers the most up-to-date picture yet of the global burden of P. vivax. Their maps are based on both incidence of malaria cases known to be caused by P. vivax, and also portray the likely dynamics of parasite transmission, which were calculated using factors like climate, proximity to urban centers and parasite-free areas, and the presence or absence of a certain blood genotype— Duffy negative— that is thought to confer resistance specifically against P. vivax malaria.
They found that, in 2009, up to 2.85 billion people had some risk of P. vivax transmission. Previous estimates from 2005 pegged the at-risk population for P. vivax at about 2.59 billion, and for P. falciparum, 2.51 billion. The burden fell overwhelmingly in Central and Southeast Asia, where over 90% of the at-risk population resides. More than half of the at-risk population lives in areas where P. vivax transmission is low or unstable, suggesting that eradication efforts in these areas could have bright prospects. While P. falciparum remains more deadly, the broader range of P. vivax may place a heavier strain on the health of global populations than previously imagined.
Meanwhile, the author of an essay in PLoS Medicine critiques the messianic single-mindedness of malaria eradication campaigns, such as those funded by the Bill and Melinda Gates Foundation. While acknowledging the good faith on the part of these groups, Naman Shah from the University of North Carolina at Chapel Hill writes that the traditional malaria control measures used in eradication efforts may not work in high-transmission zones, which require new tools and a strategy more suited to their varied on-the-ground conditions.
Malaria map courtesy Guerra et al.