The discovery of two parasites, related to human malaria, which infect African apes has solved the century-old puzzle of how the disease came to plague humans.
The malaria parasite Plasmodium is a strange thing. It's not a virus or even a bacteria, instead being more closely related to plants. Six species infect humans, while many others spend one stage of their life cycle in the blood of mammals, reptiles, or birds, before being transmitted via insects. Since most human-infecting species of Plasmodium do not infect other vertebrates, scientists have been puzzled as to their origins.
Among the human-infecting species, it is P. falciparum that does by far the most damage, being both the most widespread and the most likely to kill. However, the origins of P. malariae have proven more intriguing to parasitologists, to the extent a paper in Nature Communications describes it as “the least well characterized human parasite”. After 100 years of questions, the paper claims to finally have an answer.
In the 1920s, parasites that looked identical to P. malariae were found in the blood of chimpanzees. Did this mean the same parasite was infecting humans and apes? P. knowlesi is known to do this, so it wasn't too hard to imagine. Subsequently, the issue was confused further with the discovery of P. brasilianum, an apparently similar form of malaria that infects monkeys in the Americas.
However, microscopes can only reveal so much. Dr Lindsey Plenderleith of the University of Edinburgh and co-authors compared the DNA of P. malariae with the counterparts in apes and found there are actually three separate species.
One, which the authors call P. celatum is widespread in chimpanzees, gorillas, and bonobos but, despite appearances, is not all that genetically similar to the human varieties. However, the other is a much closer match and thus referred to as P. malariae-like. The comparison between P. malariae-like and P. malariae allowed the authors to explore its genetic history. They conclude that P. malariae evolved from an ape-infecting parasite, and went through a genetic bottle-neck where it became very rare, probably when it was newly colonizing humans.
The process is very similar to the one P. falciparum is thought to have undergone when evolving from a gorilla-only parasite.
On the other hand, P. brasilianum appears to be a break-away from P. malariae that jumped from humans to monkeys after being brought to the Americas, probably with the slave-trade. It's rapidly gone on to infect more than 30 monkey species.
P. malariae has been neglected for study compared to the other malaria parasites because its symptoms tend to be mild. However, the paper notes, “the parasite can also persist chronically and [recur] years or decades after the initial infection.” Consequently, it may be a greater health risk than is usually acknowledged. It may also exacerbate other infections.
"Our findings could provide vital clues on how [P. malariae] became able to infect people, as well as helping scientists gauge if further jumps of ape parasites into humans are likely,” Plenderleith said in a statement.
Moreover, an understanding of other malaria parasites may help improve our capacity to fight P. falciparum, which remains one of the greatest causes of preventable death, particularly in children.