Plague germs teased from mediaeval cadavers in a London cemetery have shed light on why the bacterium that unleashed the Black Death was so lethal and spawned later waves of epidemics.

The DNA of Yersinia pestis shows, in evolutionary terms, a highly successful germ to which the population of 14th-century Europe had no immune defences, according to a study published Wednesday in the British journal Nature.

Comment: Even if it's true that these cadavers from a London cemetery tested positive for Yersina pestis, keep in mind that the field of DNA analysis has been plagued by problems of contamination from the DNA that is ever present on human hands, bacteria and other sources. Alan Cooper, head of the Ancient Biomolecules Centre at Oxford University has said that Yersinia DNA found in bodies buried in France have been a case of mistaken identification because of accidental contamination of samples. In addition to that, it is not the first time that bodies buried in "medieval" graves actually belonged to an earlier or later period other than that of the Black Death. In Italy, health authorities in the northern states called outbreaks of bubonic plague "minor pests", to distinguish them from the "major pests", which they took much more seriously.

There is compelling evidence that the Black Death was not an outbreak of bubonic plague, but was in fact caused by a hemorrhagic virus. This case is synthesized in the book Return of the Black Death, in which Susan Scott and Christopher Duncan from Liverpool University carefully put all the available clues together, tracking the plague from its first appearance out of nowhere and chronicling its unprecedented catastrophic effects on European civilization.

Studying the parish records and the historical data registered in English provinces, using information about the critical events in the lives of real people and computer modeling, Duncan and Scott were able to not only surmise the amount of time from the appearance of symptoms to death, but also to establish the following about the pandemic:
"It seems that the plague's latent period was 10-12 days", says Chris Duncan, "while the infectious period prior to the appearance of symptoms lasted 20-22 days, giving a total incubation period of about 32 days. This is exceptionally long and it explains why the plague could jump very long distances even in the days of primitive transport: for three weeks, people didn't realise they had been infected. People generally died five days after the first symptoms appeared, so the average time from infection to death was about 37 days. This is an interesting finding, because European health authorities had quickly determined on 40 days as a safe quarantine period for the plague." [From legend to legacy]

It also lays bare a pathogen that has undergone no major genetic change over six centuries.

© AP Photo/Museum of London Archaeology
This undated handout photo provided by Museum of London Archaeology shows skeletons in the East Smithfield Cemetery in London, where Black Death victims were buried in the 13th Century. Scientists used skeletons from this graveyard to decode the genome of the plague.
"The Black Death was the first plague pandemic in human history," said Johannes Krause, lead researcher and a professor at the University of Tuebingen, Germany.

"Humans were (immunologically) naive and not adapted to this disease," he said in an email exchange.

No bug or virus has wiped out a greater proportion of humankind in a single epidemic than the Black Death.

Brought to Europe from China, it scythed through the continent from 1347 to 1351, killing about 30 million people - about one in three of Europe's and nearly one in 12 of the world's population at the time.

Remarkably, more recent variants of the bacterium hardly vary compared to the original microbe, says the paper.

"Based on the reconstructed genome, we can say that the mediaeval plague is close to the root of all modern human pathogenic plague strains," said Krause.

"The ancient plague strain does not carry a single position that cannot be found in the same state in modern strains."This deep similarity between ancient and modern plague calls into question the long-held assumption that virulence-enhancing mutations are what made Y. pestis so deadly to the Middle Ages.

Like Native American Indians who were exposed to smallpox, Europeans had never been exposed to the bacterium, said Krause.

"Plague was among the strongest sources of selection on the human population in the last few thousand years," he added. "People who were less susceptible due to mutations might have survived, and these (beneficial) mutations may have spread." Another likely factor that worsened the Black Death's toll was social conditions, which were far worse compared to the 18th or 19th centuries.

Poverty and malnutrition were rampant, and even the concept of hygiene was non-existent.

The onset of the so-called "Little Ice Age" could also have favoured the spread of the disease which, like many pathogens, travels more quickly in cold climes.

Comment: It is highly unlikely that the cold climate would have favored such a disease. Bubonic plague is a disease carried by rodents and its infection is transmitted to people from rats by fleas. The infectious agent is Yersinia pestis. Some rats are highly vulnerable and die, whereas others are resistant and can survive an infection. This is a key point, since the disease will die out if all rats are highly vulnerable, whereas it will persist in areas where there is a balance between susceptible and resistant rats.

Scott and Duncan explain how Yersinia pestis has never persisted in any European rodents because they are not resistant. In addition to that, the only species of rats in Europe came either some 60 years after the last European plague or could not survive without a warm climate, making it impossible to spread infection rapidly and wildly in winter. They argue that:
... it is known that the Black Death was carried across the sea to Iceland and that there were two severe and well-authenticated epidemics in the fifteenth century. [...] Yet it is known that no rats were present on the island during the three centuries of the Black Death. Infections continued through the winter when the average temperature was below -3 degrees Celsius, where transmission by fleas is impossible. It is also agreed that there is no mention in any of the accounts of rat mortality during the Black Death. A temperature of between 18 degrees and 27 degrees Celsius and relative humidity of 70% are ideal for flea egg-laying, whereas temperatures below 18 degrees inhibit it. Researchers had collected all the available climatological data for central England during the Black Death and at no time was the average July-August temperature above 18.5 degrees Celsius.
Britain, much less Iceland or Sweden, did not have a climate capable of sustaining regular seasonal outbreaks of flea-borne bubonic plague. Right from the beginning, the people of medieval Europe realized that it was an infectious disease spread directly from one person to another, not a disease associated with, or coming from, rats.

There are two forms of bubonic plague in humans: bubonic and pneumonic. Patients with bubonic plague are not contagious to other people. Pneumonic plague is contagious, appearing in about 5% of cases of bubonic plague; that is, it cannot occur in the absence of the bubonic form and it cannot persist independently. It happens when the bacterium reaches the lungs, and the time from infection to death of bubonic/pneumonic plague is 5 days, not 37 days. For more information read New Light on the Black Death: The Viral and Cosmic Connection

The same goes for the rats that carried the blood-sucking insects - fleas or lice, perhaps both - that transmit the disease.

Indeed, the species of rodent, Ratus ratus, that sowed terror across a continent in the 14th century is not the same as the one that transports plague today, Ratus norvegicus, Krause said.

Comment: By ignoring the experience of Britain, a piece of a puzzle was missed by Krause:
For the past century, historians have assumed that the Black Death was synonymous with bubonic plague, since many people afflicted by the Black Death developed the swollen glands known as 'buboes'. Alexandre Yersin had shown that the infectious agent in bubonic plague is a bacterium, now known as Yersinia pestis, which afflicts rodents and can be transmitted by their fleas - so historians also assumed that rats and/or their fleas were responsible for transmitting the Black Death.

"Many historians continue to argue along these lines", comments Sue Scott, "and that's probably because they find the science inaccessible. The key facts are as follows. The incubation period for bubonic plague is just 2-6 days, not 32 days - and the disease spreads quite slowly, at around 8-12 miles per year, whereas the Black Death could cover this distance in a single day.

"Once people exhibited symptoms of the Black Death, death was sure to follow, whereas human mortality from bubonic plague is lower, and the disease is not particularly infectious: today people with bubonic plague are nursed in open wards.

"Then there's the issue of transmission ... bubonic plague is transmitted by rat fleas but, as Graham Twigg pointed out in 1984, rats were unknown in Britain's rural areas until the 1720s when the brown rat first arrived in Britain - 50 years after the Black Death vanished; Britain's only other rat species, the black rat, never strays far from ports. At the time, people assumed transmission was person to person - and this would explain why the Black Death was able to reach Iceland in the 15th century - yet there were no indigenous rats at the time and rats were not introduced to the island until hundreds of years later."

"The Black Death was quite simply a different disease", asserts Chris Duncan, "and the main questions to consider are - what was it, and could it come back? Our best guess at the moment is that it was some kind of emergent haemhorrhagic fever, rather like Marburg and Ebola, and the infectious agent was probably a virus. Could it come back? Possibly, but we're more likely to encounter a completely new disease - like SARS." [From legend to legacy]

The 1918-19 Spanish flu pandemic killed by some estimates 50 million people. In absolute terms, this was the deadliest pandemic in human history.

But with a world population that was close to two billion, the toll in relative terms was far smaller than that of the Black Death, when the number of humans was in the hundreds of millions.

The first outbreak of plague occurred in China more than 2,600 years ago before reaching Europe via Central Asia's "Silk Road" trade route, according to a molecular "family tree" - mapped out last year - of 17 Y. pestis strains.

It then spread to Africa, probably by an expedition led by Chinese seafarer Zhang He in the 15th century.

In the late 19th century, plague came to the United States from China, arriving in the ports of California via Hawaii, according to this evidence.

Source: Agence France-Presse