Malaria is a scourge on humanity, afflicting more than 200 million people worldwide annually. Now scientists find bats may have been the first mammals to host the group of parasites that include the germs behind malaria, report findings detailed in the Proceedings of the National Academy of Sciences.
In humans, malaria is caused by at least six different species of parasites belonging to the genus Plasmodium. These germs have more than 550 related species that infect other animals. Altogether, this family of parasites are known as hemosporidians.
To learn more about the history of malaria, scientists investigated bats, the second largest order of mammals after the rodents, comprising more than 1,200 living species, representing about a fifth of all living mammal species. Recent studies have found bats are also important reservoir hosts for numerous emerging and dangerous infectious viruses, such as SARS, Ebola, Nipah, Hendra, Marburg and MERS. However, little was known about the hemosporidian parasites of bats.
“It’s a bit challenging working on bats in general because of their hosting some of the more virulent viral pathogens,” says researcher Susan Perkins, a microbial systematist at the Sackler Institute for Comparative Genomics at the American Museum of Natural History in New York. “Any whole blood samples from bats that come into the U.S. are screened by the Centers for Disease Control first.”
Researchers captured 274 bats belonging to 31 different species in West Africa.
“I was part of two bat surveys in Guinea and Liberia,” says researcher Juliane Schaer, an evolutionary biologist at the Max Planck Institute for Infection Biology in Berlin. “To assess different bat species, we had to hike deep into the rainforest, always with the help of local guides, who knew the areas. Little research had been done in those West African countries in the past due to civil wars and politically unstable times. We camped out in very remote places for many days, encountering safari ants, snakes, heavy rains.”
Roughly 40 percent of these bats, belonging to 13 species, were infected with hemosporidians. Analysis of these parasites revealed they belonged to four of the 17 known hemosporidian genera.
One of the four bat hemosporidian genera was Plasmodium, the same hemosporidian genus that infects humans. Bat hemosporidians were closely related with rodent malaria parasites, suggesting the germs had switched multiple times between bat and rodent hosts — perhaps rodents such as tree-dwelling African thicket rats or scaly-tailed flying squirrels.
The genetic complexity of the parasites seen in the bats also suggests they may have been the first mammal hosts of Plasmodium, contracting it from bird or reptile hosts before the parasites began infecting other mammals, including humans and other primates.
“These results underscore that it is important to continue to survey wild hosts for malaria parasites in order to better understand their history,” Perkins says.
This discovery concerning the evolution of malaria could help combat the disease.
“It provides an opportunity to look at closely related parasites of the main species used to study malaria in the lab in a very different host system, bats, which are now thought to respond very differently to pathogens,” Perkins says. “This can be informative for dissecting host-parasite interactions.”
The researchers note it would make no sense to cull bats just because they host parasites related to human malaria, since they are extremely important within ecosystems when it comes to pollinating flowers and devouring insects. “Bats even have a larger impact on insect populations than birds,” Schaer says.
In addition, “we’re not in any danger of getting malaria from these bats,” Perkins cautions. “There are lots of animals that are hosts to their own species of malaria parasites and they aren’t easily transferred.” Moreover, “the local bats in the U.S. are not known to be hosts of malaria parasites at all.”
As for why bats seem to host so many diseases, “the current thinking is that the transition to a lifestyle of flying was somehow linked to changes in the immune system that make them more resistant to diseases that are very pathogenic to humans, though how that happened is still unknown,” Perkins says.