October 2, 2001
(The New York Times News Service) - From 1347 to 1352, a mysterious disease ravaged Europe, killing an estimated 25 million people - 30 percent to 50 percent of the population.
At the time, people said the disease was caused by a peculiar conjunction of the planets, by a miasma stirred up by earthquakes in Central Asia or by a conspiracy of Jews to undermine Christendom.
Many called it the wrath of God and expected the end of the world.
Although the pandemic now called the Black Death lasted no more than six years, according to most medieval historians, the disease behind it erupted periodically in different parts of Europe for the next three centuries, leaving millions more dead in its wake.
Then it largely vanished from the Continent, but questions over its origins remained.
In 1894, two scientists, Alexandre Yersin and Shibasaburo Kitasato, independently identified the rod-shaped bacterium responsible for an epidemic of bubonic plague sweeping out of China.
Yersin soon linked the Black Death to the bacterium, named Yersinia pestis, which is most commonly spread to humans by fleas from rats.
In the past century, many historians and scientists have strengthened the argument that bubonic plague was responsible for the horrors of the Black Death and similar outbreaks in medieval Europe and other parts of the world. But other experts have expressed doubts about that and periodically suggested that other diseases were responsible.
The debate largely stems from the difficulty of identifying a disease based on the few medieval descriptions of the Black Death that have survived.
Now two researchers from the University of Liverpool are presenting a new theory. In "Biology of Plagues: Evidence from Historic Populations," published in March, the authors argue that a hemorrhagic virus, like Ebola, probably caused the Black Death and most of the smaller epidemics that struck Europe for the next three centuries, not bubonic plague.
The authors, Susan Scott, a demographer, and Christopher J. Duncan, a zoologist, say their theory answers many lingering questions about the rapid spread and virulence of the Black Death. Their argument is based in part on reports that the disease was transmitted from person to person.
They also observe that bubonic plague is a disease of rodents but that Europe had no rodent species that could harbor the disease between outbreaks.
The rats that passed the plague through fleas to humans during epidemics all died so the plague would have perished with them, Scott and Duncan say.
Samuel K. Cohn, a professor of medieval history at the University of Glasgow, maintains that many other diseases are better candidates for the Black Death and its subsequent outbreaks through the early modern period than Yersinia pestis.
But Cohn, the author of "The Black Death Transformed: Disease and Culture in Renaissance Europe," due out next year, added that he did not accept Scott and Duncan's suggestion that a tropical Ebola-like virus had caused those epidemics. Such a virus would burn out too quickly to produce the widespread mortality of the Black Death, he said, proposing instead that the Black Death was caused by a highly contagious virus or bacterium that might not exist now.
Most epidemiologists argue that the evidence points most strongly toward the bubonic plague pathogen as the cause of the Black Death.
Bubonic plague - often called just the plague - can take three forms in people, and those forms can account for the descriptions of Black Death symptoms, said David T. Dennis, coordinator of the plague program for the Centers for Disease Control and Prevention in Fort Collins, Colo.
The disease is still a public health problem, with a few thousand cases worldwide, but it can now be successfully treated with antibiotics if it is caught early enough.
Bubonic plague, the most common, is named for its distinctive buboes, or swollen lymph nodes, near the flea bite, usually in the groin and armpits. If patients are untreated, the mortality rate is 50 percent to 60 percent, Dennis said.
Septicemic plague occurs when the infection gets into the bloodstream, causing great damage to organs, hemorrhaging of blood vessels and sometimes gangrene and bleeding from the nose and ears. Although uncommon, septicemic plague is nearly always fatal unless antibiotic treatment is begun quickly.
Pneumonic plague, which usually appears in a small percentage of plague cases, is transmitted from one person to another by blood droplets that are expelled when the victims cough or sneeze, Dennis said, and is generally fatal unless patients are treated with antibiotics.
The few contemporary accounts of the Black Death describe symptoms that included painfully swollen lymph nodes, liquefied organs and hemorrhaging blood vessels, which caused splotches and sometimes widespread discoloration on the skin. Many of the afflicted bled from the nose and coughed up blood. Death often came within days.
Those symptoms, Dennis said, match the various forms of plague. Standard histories say this about the Black Death: It began in Central Asia and came to Europe after Mongol soldiers catapulted the bodies of plague victims into the Genoese fort at Kaffa on the Black Sea. The retreating Genoese then carried the disease to Messina, Sicily, in November 1347. It moved quickly to other Mediterranean ports and the rest of Europe, spread by fleas from the black rat and people with pneumonic plague.
Human fleas found on dogs and cats may also have played a role in the spread, Dennis said, although no one is certain to what degree.
Looking at the speed with which the Black Death struck Europe, a number of scholars suggested over the years that the cause was not plague, which usually spreads relatively slowly, but a form of anthrax, typhus, tuberculosis or viral hemorrhagic fever.
Rebutting those theories, a team of French researchers reported in November 2000 in The Proceedings of the National Academy of Sciences that they had found Yersinia pestis in the dental pulp of three people buried in Montpellier, France, in the 14th century. "Medieval Black Death was the plague," they declared.
But Duncan and Scott argue that when bubonic plague occurred in medieval Europe, it was generally confined to Mediterranean port cities and southern France. The French team's results, the authors say, merely showed that two plague organisms occasionally existed simultaneously in Europe.
"You can forget about bubonic plague as the cause of the Black Death," Duncan said in a telephone interview from Liverpool.
Fleas infected with Yersinia pestis move to people only after their host rats have died, he said, yet there are no reports from the time of a huge number of dead rats.
Duncan added that because there was no native rodent population in England or northern Europe that could carry plague without dying, it could not have become established there between outbreaks.
A viral hemorrhagic fever, on the other hand, would always be transmitted person to person, Duncan said.
After the Black Death subsided in 1352, the virus became endemic among people in central and southern France, he contended, erupting periodically over the next 300 years as human carriers moved along trade routes and conditions became ripe for outbreaks.
The most significant symptom of the pestilence, Duncan said, was not buboes but red spots - often called God's tokens - on the chest. The spots, caused by hemorrhaging blood vessels, were considered harbingers of death.
The hemorrhagic virus had a latency period of about 10 days after exposure, Duncan and Scott calculate, followed by a 22-day infectious period, during which victims could spread the disease but had no symptoms, and a final five-day period of symptoms.
The 37-day course of the disease is around four weeks longer than that of bubonic plague and corresponds almost exactly with the 40-day quarantine period that officials in Italy established during the Black Death, the authors add.
The month between exposure and the appearance of symptoms meant that people were able to travel by land or by sea a rather considerable distance, the authors write, thereby hastening the spread of the disease.
Outbreaks of the disease faded in Europe during the height of a little ice age, in the late 17th and early 18th centuries, probably because the cold made the hemorrhagic virus less infectious, Duncan suggested, and the virus itself may have mutated.
Scott and Duncan also argue that a genetic mutation in humans, perhaps found in 40 percent to 50 percent of the European population by the end of the 17th century, made epidemics of hemorrhagic fever less likely.
Dennis of the CDC said that while he had not yet read Scott and Duncan's book, he was familiar with the argument but saw no link to a hemorrhagic virus.
Although questions remain about things like the rapid movement and high mortality rate of the Black Death, as well as the size of the rodent population in Europe, Dennis said, most epidemiologists agree that it and subsequent, smaller outbreaks were caused by the plague bacterium.
But some scientists suggest that more concrete evidence - for example, finding Yersinia pestis in the remains of victims from northern Europe and England - is needed to settle the question of what caused the Black Death.
"Yersinia still seems to me the most reasonable assignment as the cause of the Black Death," said Dr. Joshua Lederberg, an emeritus professor of microbiology and a Nobel laureate in medicine at Rockefeller University, "but I say that with less than unshakable conviction."
Copyright 2001 The New York Times News Service. All rights reserved.