By Linda Weiford, WSU News
PULLMAN, Wash. – A study’s recent finding about a plague that struck 1,500 years ago might seem arbitrary – except that it involves a resurrected pathogen whose secrets, pulled from ancient teeth, can help us understand our world’s emerging diseases.
The pathogen, the oldest to be genetically decoded, caused one of the world’s deadliest scourges. By reconstructing its DNA, scientists were able to reach across the centuries to determine exactly what killed so many people during what’s known as the Justinian plague.
The culprit was Yersinia pestis and it is believed to have contributed to the demise of the Roman Empire.
“Based on odd and sometimes vague descriptions chronicled by a few people who lived during that period, scientists and historians have long suspected that a plague pathogen was responsible, but it wasn’t known for certain,” said Washington State University scientist Viveka Vadyvaloo, who studies the modern-day strain of Y. pestis in fleas.
What we did know – based largely on the writings of Byzantine scholar Procopius in 542 AD – is that a disease tore through Europe with ferocity, she said. Though Procopius described dark swelling at the neck, groin and armpits of victims – suggestive of bubonic plague – smallpox and influenza have also been considered.
However, “just as DNA can be used to solve crimes, in this case it was used to solve a medical mystery reaching back to the sixth century,” she said.
Justinian vs. Black Death
The study that cracked the microbiological whodunit was published last month in The Lancet Infectious Diseases by an international team of scientists that included biologists, geneticists and archeologists. (See http://www.thelancet.com/journals/laninf/article
By reconstructing DNA from the teeth of two corpses buried in Bavaria, Germany, the group concluded that the Justinian plague and the better-known Black Death, which came 800 years later, were caused by different strains of Y. pestis.Both were transmitted to humans by fleas that had bitten infected rodents, primarily rats.
“Our ancient Y. pestis study indicates that the bacterium has emerged from rodents into humans to cause pandemics at multiple time points in history,” said evolutionary biologist David Wagner of Northern Arizona University, lead author of the study. “That two separate strains were able to jump to humans and cause massive pandemics highlights just how virulent the Y. pestis bacterium is,” he said in an interview.
Ancient teeth, modern fleas
After the Justinian plague killed what’s estimated to have been half the world’s population as it spread from Asia to North Africa and Europe, it mysteriously vanished.
Not so with the Black Death, which spawned a third plague epidemic in the late 19th century and led to the strain that currently kills roughly 2,000 people each year, according to the World Health Organization. It is responsible for a recent outbreak on the island of Madagascar that killed 32 and sickened close to 100.
And it is the same strain that Vadyvaloo researches in an inactivated form at WSU’s Paul G. Allen School for Global Animal Health in the College of Veterinary Medicine.
By feeding infected blood to fleas, she analyzes how they pick up the disease and maintain and transmit it. One pin-sized drop of blood can harbor as many as 10,000 bacteria, she said. When a flea bites a human, “all it takes are five to ten to slip inside and infect them.”
Biologically, Y. pestis is as capable of causing sickness and death as it did 1,500 years ago, Vadyvaloo said. Fortunately for humankind, now that’s less likely.
“The Justinian plague wasn’t just about the organism,” she said. “It was also about the poor sanitary conditions that allowed it to thrive, along with climate conditions and the lack of medical knowledge to stop it.”
Most cases of plague can be cured if treated with antibiotics within 24 hours after symptoms appear. Symptoms often include fever and chills, chest pain and swollen, painful lymph nodes.
However, studies are starting to show the emergence of antibiotic-resistant strains. (See http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0000309)
As Vadyvaloo enters her seventh year of studying a relic of the planet’s third pandemic, “it’s my hope that this recent discovery sheds more light on the dynamics of infectious diseases we see today, including plague. I think a lot of people think it’s a disease of the past. Though it’s not the massive killer that has struck three times, it’s still with us.”
Another molecular mystery
One mystery yet to be solved: Why did the Justinian plague disappear while a new one emerged and persists to this day? The Lancet study’s authors speculate that humans’ immune systems became less susceptible to the bacterium or that a climatic shift made the environment less suitable for it to survive.
Meanwhile, the modern day strain continues to circulate, with its most effective mode of transmission the bite of a tiny flea. Maybe, several hundred years from now, scientists will uncover clues to its persistence – lurking in DNA pulled from the tooth of a person who lives among us today.
Viveka Vadyvaloo, WSU molecular bacteriologist, 509-335-6043, Viveka@vetmed.wsu.edu
Linda Weiford, WSU News, 509-335-7209, firstname.lastname@example.org