Posted on behalf of Joseph Milton.
Which of our primate relatives spread malaria to humans? It’s a question with many of the elements of a murder mystery; the story involves a series of unfortunate victims, a long list of likely perpetrators, many of whom have been the prime suspect at some point, and a string of red herrings and plot twists as the body of evidence builds up.
Twice now, scientists have believed they had the culprit nailed, only for new evidence to point the finger of blame elsewhere.
The latest suspect who may now be exonerated is the gorilla, as a new paper published in the Proceedings of the National Academy of Sciences suggests malaria is just as likely to have spread to humans from monkeys.
The idea that gorillas were responsible comes from a Nature paper published last September that identified the closest relative of Plasmodium falciparum, the parasite that causes the bulk of malaria cases in humans, as a parasite found in gorillas – evidence that effectively absolved the previous suspects, chimpanzees, of blame.
The DNA data in the Nature study also suggested that malaria passed to humans from a reservoir in gorillas just once, but both of that paper’s assertions have now been called into question by the discovery of a strain of P. falciparum in monkeys.
An international team led by Franck Prugnolle from the Institut de Recherche pour le Développement, France sampled the blood of 338 monkeys in Gabon, representing 10 different species. In doing so, they identified a single individual of the spot-nosed guenon (Cercopithecus nictitans, pictured), born wild but kept as a pet, that was infected with what appeared at first glance to be human P. falciparum.
Investigating the parasite infecting the monkey, the researchers found its DNA differed significantly from that of P. falciparum infecting local humans and was more closely related to malarial parasites found in gorillas, suggesting the monkey had not caught the disease from humans while living as a pet, but must instead have been infected in the wild before its capture.
Scientists had assumed that P. falciparum was a parasite specific to gorillas and humans, but this study suggests the parasite exists in a wider range of primate species, throwing open the field of suspects in the mystery of its spread to humans.
The findings also call into question the idea that malaria made the jump to humans just once, say the study’s authors. However, they add, extensive further sampling in monkeys and apes would be required to shed conclusive light on the origins of human disease, particularly as their work is based on a single case.
For now then, we have no clear suspect in the ongoing whodunit concerning the origins of human malaria. And given past form, we might expect the next line of enquiry to lead somewhere entirely new.
Image courtesy of donjd2 via Flickr under Creative Commons.