The Origin of Plasmodium falciparum. Where Did Malignant Malaria Come From?

Posted by Rosemary Drisdelle Aug 5, 2009

In his book Who Gave Pinta to the Santa Maria (Norton, 1997) tropical disease specialist Robert Desowitz wrote that study of the DNA of Plasmodium falciparum proved the malaria parasite had leapt from birds to humans (p.61 – 2). I thought this a fascinating discovery, and one that answered several questions for me: Why does P. falciparum look so different from the three other species of Plasmodium that infect humans? And why is P. falciparum so much more dangerous than the other species?

That a bird parasite looks different from a monkey parasite is not too surprising. And because we are very different from birds, their Plasmodium sp. might have to co-evolve with humans for a long time before it became less dangerous to us.

I should, perhaps, have been more skeptical—it’s a big leap from a bird to a human, and its less likely P. falciparum in the wrong host—the human—would spread from person to person with such efficiency.

Now, twelve years later, we’re told that, based on genetic studies, P. falciparum came from chimpanzees, and it did so relatively recently—perhaps 5000 to 10000 years ago. Is that short time span an explanation for the severity of malignant malaria, infection with P. falciparum? Not necessarily. Scholars like Christopher Wills and Paul W. Ewald write that disease causing organisms only become less deadly (virulent) if that makes it easier for them to get from one person to another. In the case of P. falciparum, there’s another possibility: the parasite may have become more virulent when humans settled down in permanent settlements because the parasite spread faster among relative crowds when its victims were sicker and less able to ward off hungry mosquitoes.

I wonder, though, why the same thing didn’t happen with the other three Plasmodium spp. of humans?