The origin of malaria: As Dr Livingstone presumed
AGUE, tertian fever, quartan fever, paludism. Malaria has been known about since ancient times and has gone under many names. Today, it kills over a million people a year, most of them young children. Where it originally came from, though, has been a matter of scientific debate for half a century.
In 1958 Frank Livingstone, a noted anthropologist, suggested that Plasmodium falciparum (which is by far the deadliest of the four or five parasites that cause human malaria) had jumped into Homo sapiens from chimpanzees. He speculated that the rise of agriculture had led to human encroachment on wild forests, giving the chimp version of the bug, P. reichenowi, the chance to find a new host. A rival camp, however, argued that P. falciparum was a variant of P. gallinaceum, a parasite found in chickens.
A paper just published in the Proceedings of the National Academy of Science shows that Livingstone got it right. Stephen Rich of the University of Massachusetts at Amherst, Nathan Wolfe of the Global Viral Forecasting Initiative in San Francisco, and their colleagues used genetic evidence to confirm that P. falciparum is, indeed, an offshoot of P. reichenowi. And a recent one at that, as it may have hopped from chimp to human a mere 10,000 years ago—about the time, as Livingstone argued, that human hunter-gatherers were settling down on the farm.
To reach their conclusion, Dr Rich and Dr Wolfe studied the DNA sequences of malarial parasites collected from nearly 100 wild or formerly wild chimpanzees born in central and west Africa. They identified eight distinct versions of P. reichenowi (only one had been known before) and found that the DNA of P. falciparum sat nicely in the middle of all this genetic variation. Moreover, it seems to have made the jump from chimps to people only once. All P. falciparum parasites alive today appear to derive from an individual example of P. reichenowi.
All this is, of course, ancient history, but it is history with a point. Understanding how parasites leap species barriers is crucial to understanding how new infectious diseases start in humans—and an example of a recent leap was also published this week. Nature Medicine carried a report by Jean-Christophe Plantier of the University of Rouen and his colleagues of a new human immunodeficiency virus (HIV) detected in a Cameroonian woman. Unlike the widespread HIV-1 M strain, and the much rarer N and O strains, which jumped to people from chimpanzees, the newly described P strain originated in gorillas.
Dr Wolfe believes this sort of thing happens all the time. He calls it viral chatter. Only occasionally, as with the m strain of HIV-1, does the chatter lead to a serious epidemic. Monitoring the chatter, however, may nip such an epidemic in the bud. The Global Viral Forecasting Initiative is therefore involved in setting up a monitoring system in viral “hot spots”, such as central Africa and South-East Asia, to catch such transfections early.
Dr Rich, meanwhile, observes that because P. reichenowi, the closest relative of P. falciparum, is “of greatly diminished pathogenicity” in its chimpanzee hosts (in other words, it causes them few adverse symptoms), it may provide a basis for the development of new vaccines that are naturally attenuated. And a malaria vaccine would be a good thing indeed.