Between 1347 and 1351 - a period of just five years, Europe's Black Death wiped out about 30 million people - 30 to 50% of the population on the continent. Now geneticists have reconstructed the genome of the bacterium that caused the plague. It is found to be an ancient strain of a bacterium called Yersinia pestis very similar to the strain the exists today.
The discovery, made by scientists at McMaster University in Canada, the University of Tubingen in Germany and collaborators at other institutions, was described in a study published Wednesday in the journal Nature.
For their study the team collected the ancient Y. pestis DNA from 46 teeth and 53 bones excavated from the East Smithfield burial ground in London, a site set aside for burial of plague victims in late 1348 or early 1349. Researchers carefully reconstructed the bacterium's genome and made comparisons to the genomes of existing strains of Y. pestis.
Results showed that the bacterium had not changed much in the more than 600 years since the plague swept Europe. Because estimates have indicated that all existing strains of Y. pestis evolved from a common ancestor sometime between 668 and 729 years ago -- not long before the East Smithfield strain killed its victims - the team concluded that “the medieval plague of the fourteenth century was probably responsible for its introduction and widespread distribution in human populations.”
“We do not see a single position in the ancient genome that cannot be found in modern Y. pestis in the same state,” said researcher Johannes Kraus a professor in the departments of archeological sciences and human genetics at the University of Tubingen in Tubingen, Germany. Researchers used tools and technology recently developed to study Neanderthals to enrich the material they extracted from the teeth to ensure they had authentic ancient bacteria. They analyzed about 99 percent of the genome, Kraus said.
McMaster University issued a statement saying the study marks the first time scientists have published the genome of an ancient pathogen. Having the ability to study the genetic makeup of long-gone pathogens will help researchers track their evolution, said study coauthor Johannes Krause. It could improve scientists' understanding of modern diseases as well.
Bubonic plague still strikes somewhere between 1,000 and 3,000 people each year, according to the World Health Organization. The U.S. Centers for Disease Control report that on average, 10 to 15 Americans get the disease each year, mainly in rural areas.
Modern antibiotics can effectively kill the ancient bacteria, said Hendrik Poinar, a researcher at the departments of ancient DNA, biology and infectious disease at McMaster University in Hamilton, Ontario. The virulence of the outbreak may have occurred because there were other infections circulating at the time, there was a dramatic change in the weather and a less hearty population.
“This is a major technological step forward, a great advance for the entire field of DNA and pathogens,” said Mark Achtman, an expert on ancient plague at University College Cork in Ireland.
But Dr. Achtman disagreed with one issue in Dr. Krause’s findings, that of whether Yersinia pestis also caused the outbreak in the sixth century known as the Justinian plague. When the full genome of the medieval bacterium is compared with DNA recovered from other known human outbreaks, a tree of descent can be constructed. The Black Death genome lies so close to the root of the tree that the human pathogen probably did not exist much earlier or, if it did, has vanished without any descendants, Dr. Krause’s team says. This implies that the Justinian plague was caused by some other agent.
Dr. Achtman said this conclusion was incorrect because the Krause team had omitted DNA from several human cases that place the root of the tree much further back in time. Dr. Krause said he had left these cases out because they seemed unreliable.
Paul Keim, an expert on infectious bacteria at Northern Arizona University, said that work by Dr. Achtman and Dr. Krause had shown that the Black Death “was really a series of epidemics coming out of China and sweeping across the susceptible ecological situation” created by the culture of medieval Europe. The plague in each outbreak probably did not persist very long and was repeatedly re-established by new infections from East Asia, where the bacterium is still endemic in small rodents like marmots.
“We don’t have a human ecological situation comparable today, plus it is really easy to break the transmission cycle with antibiotics and public health,” Dr. Keim said. There are still small outbreaks, like one in Madagascar in the 1990s, but “a multiyear large human outbreak is inconceivable in this day,” he said.
Besides the Justinian plague and the Black Death, a third great wave of plague swept out of China in 1894, causing an epidemic in San Francisco in 1900 and killing millions of people in India.
All the teeth used in the study will be returned to the skulls from which they were taken, now in a London museum whose archaeologists excavated the East Smithfield cemetery in the 1980s. More work is needed to examine small variations to ensure a different layout wouldn’t make it more deadly, according to the researchers.