The New World fossil anopheline, Anophelesdominicanus, in Dominican amber. This species could have served as a bridge vector to transfer Plasmodium dominicanium from birds to primates. |
One
of the most comprehensive analyses yet done of the ancient history of
insect-borne disease concludes for the first time that malaria is not
only native to the New World, but it has been present long before
humans existed and has evolved through birds and monkeys.
The
findings, presented in a recent issue of American Entomologist by
researchers from Oregon State University, are based on the study of
insect specimens preserved in amber.
The
study outlines the evolution of several human diseases, including
malaria, leishmaniasis and trypanosomiasis. It makes clear that these
pathogens have existed for at least 100 million years, and suggests that
efforts to conquer them will be an uphill battle against such
formidable and adaptive foes.
“Amber
tells us that these diseases have been here for many millions of years,
have co-evolved with their hosts and move readily from one species to
another,” said George Poinar, Jr., a professor of zoology at OSU and one
of the world’s leading experts on the study of fossils in this
semi-precious stone.
“Malaria
is one of the greatest insect-borne killers in human history, and more
than one million people a year are still dying from it,” Poinar said.
“But the evolutionary record suggests it can easily change its protein
coat in response to vertebrate immune reactions. That’s why it’s always
becoming resistant to drugs, and efforts to create vaccines will be very
difficult.”
Insects
preserved for tens of millions of years are offering new clues to the
ancient history of these diseases. Blood-feeding vectors trapped eons
ago in oozing tree sap reveal in near-perfect detail stages of
vertebrate pathogens they were carrying when they became entombed.
“Most
people think of malaria as a tropical disease, which today it primarily
is,” Poinar said. “But historically it occurred in many parts of the
world, including temperate zones.”
“As
recently as 1935 there were 900,000 cases of malaria in the United
States,” he said. “Near Portland, Ore., malaria almost wiped out some
local Indian tribes in the 1830s, and the mosquitoes that carried it are
still prevalent there. In the 1600s it hindered colonization from
Massachusetts to Georgia. And there are 137 million people right now
living in areas of risk in the Americas.
“It’s possible epidemics could explode again, almost anywhere in the world,” he said.
Having traveled much of the world to pursue amber, Poinar knows first-hand the risks involved.
“I
caught malaria in the 1970s in the Ivory Coast in Africa,” he said. “My
arm had bumped up against some mosquito netting while I slept. The
following day, I started shaking with cold, then sweating with a high
fever, thinking I was going to die.”
Millions
have died. Globally, about 300-500 million cases of malaria occur each
year, with more than a million deaths in Africa alone.
Among the points made in this report:
- Discoveries in amber have helped to pin down the minimum ages, origins and early hosts of several insect-borne human diseases.
- An archaic and now extinct malarial parasite was found in 100 million-year-old amber.
- Mosquitoes carrying malaria of the genus Plasmodium,
the type that causes human illness, were established in the New World
at least 15 million years ago, long before modern humans existed. At
that time, the disease infected various types of birds.
- Spaniards
arriving in South America found that when native peoples acquired
fevers, they drank infusions of cinchona bark, which was later found to
contain quinine, an effective anti-malarial drug.
- Malaria apparently first went from birds to monkeys and eventually into humans.
Anatomically
modern humans are only about 200,000 years old, experts say. These
findings indicate they evolved with malaria for their entire existence.
The Origin of Insect-Borne Human Diseases as Revealed in Amber