The Plague: It was the Gerbils

Aug. 7, 2015

By Medical Discovery News

In the past 800 years, many things have been blamed for the plague that swept through Europe in the Middle Ages: the alignment of the planets, bad air, lack of proper hygiene, black rats, and their fleas. Now scientists have data that suggests the climate in Central Asia at that time influenced the size of the great gerbil population, which triggered cycles of plague in Europe. These furry little rodents carried the plague bacterium, as did the fleas that fed on them. When the gerbil population shrank, the fleas found alternate hosts like horses, humans, and eventually rats, which then made their way to Europe and triggered the plague pandemics.

The plague was caused by the bacterium Yersinia pestis. It is transmitted to humans through the bite of a flea that has fed on an infected rodent. Plague outbreaks have afflicted humans for thousands of years and changed the course of history. The first recorded plague pandemic began in 541 and was named the Justinian Plague after the 6th century Byzantine emperor. Frequent outbreaks for the next 200 years are likely to have killed over 25 million people. The second pandemic, called the Great Plague or the Black Death, began in China and spread westward along trade routes to Constantinople and into Europe. About 60 percent of Europeans died, eliminating entire towns.

The third pandemic, or Modern Plague, also began in China and spread to Hong Kong by 1894. Rats hitching rides on steamships spread the plague to port cities around the world for the next 20 years, killing about 10 million people. By then scientists were able to identify the bacterium responsible and how it spread. Efforts to control the rat population eventually ended the pandemic. It continued to infect people (although in much smaller numbers than before) during the 20th century, such as in Vietnam during the war. The bacterium is still in the reservoir of wild rodents, and today most cases of plague are in sub-Saharan Africa and Madagascar. The plague can be effectively treated with common antibiotics, but if left untreated it has a high mortality rate.

Since there are still lots of rats in Europe, some wonder, why is there no plague? Researchers proposed that each time, the plague actually started in Asia. To test their theory, they examined climate records using the rings of trees. The incidence of plague did not correlate with climate changes in Europe, but it did with changes in Asia. It was already known that the Asian great gerbil carries Yersinia pestis, and when the weather in Asia was good, gerbils thrived, but when it turned bad, their population would crash. Then their fleas would seek another host such as human traders and their pack animals, who spread the plague to other parts of the world. They found no evidence that rodents in Europe carried Yersinia pestis, so that would explain why cases of the plague disappeared between pandemics.

So don’t worry about the little gerbils in the pet store – they are not carrying the plague.

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The Bright Side of Black Death

April 17, 2015

By Medical Discovery News

Bright Side of Black Death

It’s easy to think that nothing good could come from a disease that killed millions of people. But Dr. Pat Shipman, an anthropologist at Pennsylvania State University, disputed that notion in his recent article in “American Scientist,” where he suggested the Black Death that ravaged Europe in the Middle Ages may have resulted in some positive effects on the human population. Considering that we are in the midst another significant plague (the Ebola virus in West Africa), we could certainly use more information about the role of pandemics on human populations.

The Black Death or Bubonic plague started in the mid-1300s and was caused by a bacterium called Yersinia pestis, which typically enters the body through the bite of a flea. Once inside, the bacterium concentrates in our lymph glands, which swell as the bacteria grow and overwhelm the immune system, and the swollen glands, called buboes, turn black. The bacteria can make their way to the lungs and are then expelled by coughing, which infects others who breathe in the bacteria. The rapid spread of the infection and high mortality rates wiped out whole villages, causing not only death from disease but starvation as crops were not planted or harvested. It killed somewhere between 100 million to 200 million people in Europe alone, which was one-third to one-half of the entire continent’s population at the time. The plague originated in the Far East and spread due to improved trade routes between these two parts of the world.

Today, global travel is easier than ever thanks to extensive international airline networks. Just like with the Black Death, our transportation systems could enhance the spread of a modern plague. Of course, modern healthcare is also more sophisticated and effective, but as the latest Ebola outbreak has reminded us, a pandemic is a realistic possibility.

Dr. Sharon DeWitte, a biological anthropologist at the University of South Carolina, recently made several discoveries from comparing the skeletal remains of those who died from the Black Death and those who died from other causes during the same era. First, she found that older people, who were therefore already frail, died at higher rates. Killing this group at a higher rate created a strong source of natural selection, removing the weakest part of the population.

After the plague years, she found that in general people lived longer. In medieval times, living to 50 was considered old age. But the children and grandchildren of plague survivors lived longer, probably because their predecessors lived long enough to pass on advantageous genes. Today, a genetic variant in European people called the CCR5-D32 allele, which was favored during the natural selection initiated by the plague, is associated with a higher resistance to HIV/AIDS.

Microbes have an intimate relationship with human populations and have shaped human evolution through the ages. We may see survivors of the Ebola virus passing on similarly advantageous genes through natural selection as well.

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Black Death Revealed

By Medical Discovery News

April 14, 2012

The Black Death Revealed?

Underneath the heart of London’s financial district and the Royal Mint lay the remains of East Smithfield Cemetery. There lie some 2,400 bodies, each carefully laid to rest facing east to west with layers of charcoal in between. Around many bodies are trinkets revealing the lives they once had.

Within five years, the Black Death took the lives of these victims, as well as 30 to 60 percent of not only London’s population but Europe’s.

Now East Smithfield, one of only a few known cemeteries to contain victims of the Black Death, has provided scientists with crucial material to confirm the bacterium Yersinia pestis caused the Bubonic Plague, and allowed them to identify the microbe’s genome.

This adds to an old debate among scientists. Doubts over cause of this pandemic have perpetuated because today’s plague infects and kills far fewer people and with different symptoms. Scientists believed having the full genetic sequence of the 14th century strain would help them understand what made it so deadly.

The research team, led by Kirsten I. Bos of McMaster University in Ontario and Johannes Krause of the University of Tübingen in Germany, focused their work on samples of dental pulp, the innermost layer of tissue in teeth, where they were able to extract, purify and enrich the Yersinia genetic information.

Using a technique called targeted capture, they used lab-synthesized “bait” DNA to snag ancient Yersinia DNA strands, leaving soil microbes and other material behind. Then with the newest generation of DNA sequencers, they read snippets of the nearly 700-year-old microbe. It’s the first time an ancient microbe has been reconstructed after taken human remains, opening the way for scientists to identify microbial genomes that caused other major epidemics.

McMaster’s team found the ancient strain to be remarkably similar to today’s plague, which means it’s no deadlier and can be controlled by antibiotics. This leads researchers to conclude that climate, age distribution and living conditions of 14th century Europe contributed to the pandemic.

In 1348, downtown London was densely packed and filthy, the climate was wet and cold, and food was in short supply. People also did not have antibiotics like they do today. Understanding how the microbe exploits different conditions can help preparations for future outbreaks.

Researchers also plan to use the genomic information to recreate the original microbe. While this will help scientists understand the bacterium even further, it’s also a security risk. Yersinia has been weaponized by a number of groups in the past.  Scientists face an ethical dilemma as they consider how their work, if published, may be misused.

Today, Yersinia strains sicken about 3,000 people around the world every year. However, with antibiotics, 85 percent of people recover, reinforcing the power of modern medical capabilities.

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