Link found between malaria parasites, humans, and apes

Scientists have solved a century-old mystery about the evolutionary links between malaria parasites that now infect humans and chimpanzees.

They have discovered that the parasite Plasmodium malariae — one of six species that spreads malaria among humans — originated in African apes before evolving to infect people.

While it is often associated with mild disease, if left untreated Plasmodium malariae can cause long-lasting, chronic infections that may last a lifetime, researchers say.

The evolutionary puzzle has its origins in the 1920s when scientists identified chimpanzees infected by parasites that appeared identical to Plasmodium malariae under a microscope[1].

It was thought both parasites belonged to the same species, but – until now — this could not be verified as the genetic make-up of the chimpanzee strain had never been studied. Recently, scientists have used novel techniques to study the parasites' mitochondrial DNA (mtDNA)[2].

They have found that there are, in fact, three distinct species. One species, Plasmodium malariae, infects mainly humans, while the two others infect apes.

One of the two ape-infecting parasites was found in chimpanzees, gorillas and bonobos across Central and West Africa. This previously unknown species, provisionally called M2, is only distantly related to the human parasite.

The second ape parasite, provisionally called M1-like, is much more closely related to the lineage that infects humans, but exhibits little evidence of genetic exchange with it, and so likely represents a separate species.

This enabled researchers to make detailed comparisons of the genetic diversity of the two species. This revealed that the human malaria parasite population went through some sort of genetic bottleneck, where its population temporarily shrank and most of its genetic variation was lost. A likely explanation for this is that Plasmodium malariae was originally an ape parasite, but a small number of parasites switched hosts to begin infecting humans, the team speculates.

Lead author Dr. Lindsey Plenderleith said: "Among the six parasites that cause malaria in humans, Plasmodium malariae is one of the least well understood. Our findings could provide vital clues on how it became able to infect people, as well as helping scientists gauge if further jumps of ape parasites into humans are likely."

[1] Reichenow: Über das Vorkommen der Malariaparasiten des Menschen bei den Afrikanischen Menschenaffen in Centralblatt für Bakteriologie und Parasitenkunde - 1920
[2] Plenderleith et al: Zoonotic origin of the human malaria parasite Plasmodium malariae from African apes in Nature Communications. 2022. See here.