The landscape of human malaria might also be a clue to its age. "Despite the fact that there's so much human malaria, there's a remarkably limited diversity of human malaria parasites," Wolfe says. He suggests that with explosive human population growth, the opportunistic parasite "expanded with us." The picture of malaria species diversity in chimpanzees and gorillas looks very different. "There's a jumble of relationships between these parasites in chimpanzees and gorillas," he says. The new analysis found at least nine closely related Plasmodium species in wild chimpanzee and western gorilla populations. This pattern, he notes, is similar to what researchers are uncovering about various types of HIV, which is now thought to have originated from both chimps and gorillas.
Diversity in droppings
Hahn and some of her co-authors are relatively new to the malaria field. She came to work after studying simian immunodeficiency virus (SIV), the closely related primate version of HIV. Her research showed that contrary to popular assumptions SIV was increasing mortality in many chimpanzees, so she and a team of researchers set out to find any comorbidities that might increase these infected primates' risk of dying. Malaria, although apparently less pathogenic in other primates, seemed an obvious choice.
Once she and her team got wind of a study showing that malaria parasites could be isolated and genetically analyzed from animal droppings, "we knew we were in business," Hahn says.
Stocked with shelves of frozen primate feces, the group is ready to move on to study the origins of Plasmodium vivax, another human malarial parasite, which is found in Latin America and Asia in addition to Africa.
New genetic research will help narrow down genes involved in malarial transmission and allow researchers to compare and contrast genetics in humans and apes. Simple trends of pathology, too, will be important to uncover, Hahn notes, such as whether ape infants are also more susceptible to malaria and whether the parasite causes pregnancy complications, as it does in humans.
The long-sought goal of malaria eradication could have unintended impacts on the parasite's evolution, however. If most current forms of the human malaria parasite are eliminated, "it's possible we open a niche for other parasites to move in," Hahn points out. That type of emergence "can have a major public health impact," she says. And the ever-growing list of known primate parasite species might mean future infection possibilities are greater than we had previously realized. "The diversity of the existing natural reservoirs makes the possibility that there are parasites that can jump into humans" greater, Wolfe says.
But just knowing the number and names of species in the wild will not likely halt future epidemics. "It is probably simplistic to think that describing what is out there in nature will permit disease emergence to be forecast with any accuracy," Edward Holmes, a biologist at the Center for Infectious Disease Dynamics at The Pennsylvania State University, wrote in an essay published in the same issue of Nature. That predictive shortcoming is in large part because "successful emergence also depends on aspects of pathogen genetics and epidemiology," he noted.
Nevertheless, Hahn notes, "It's important to understand how these microbes probe constantly to potentially colonize new hosts." Just like any other species, she says, "they want to survive."
*Correction (9/24/10): This sentence was edited after posting. It originally stated Nathan Wolfe's title as professor of epidemiology.