Several million years ago, Plasmodium falciparum – the parasite that causes most cases of human malaria – jumped into humans from other apes. We’ve known as much for decades but for all this time, we’ve pinned the blame on the wrong species. A new study reveals that malaria is not, as previously thought, a disease that came from chimpanzees; instead it’s an unwanted gift from gorillas.
Swine flu has made the world all too aware of the possibility of diseases making the leap from animal hosts to human ones. Now, we know that another disease made a similar transition from chimpanzees to humans, several thousand years ago. This particular infection is caused by a parasite, and a very familiar and dangerous one – Plasmodium falciparum, the agent responsible for malaria.
Transmitted by the bite of mosquitoes, P.falciparum infects over 500 million people every year. Its closest relative is a related parasite, Plasmodium reichenowi, which infects chimpanzees. Leading an international research team, Stephen Rich from the University of Massachussetts has discovered that P.reichenowi is no mere relative – it’s actually P.falciparum‘s ancestor.
Rich compared the genes of the two species to build a Plasmodium family tree, which showed that all of the 133 known strains of P.falciparum, from all parts of the world, are united one a single branch on the P.reichenowi lineage. The stem of that branch represents a single event where P.reichenowi crossed the species barrier from chimps to humans.
The new study was possible because of eight newly collected samples of P.reichenowi from wild and captive chimps. Until now, only a single sample of this species had ever been isolated. Armed with fresh samples, the team focused their attention on three genes – cytB, clpC and 18s rRNA. They found that those of P.reichenowi are very varied, much more so than its genetically uniform cousin P.falciparum (even though we have over 16 times as many samples of the latter). Chances are that any two samples of P.reichenowi are more genetically distinct that either one is to P.falcarium.