Plague, leprosy... Researchers examine the microbes of the past

Why did plague epidemics last so long? Was leprosy present in the Americas before the arrival of Europeans? As true microbe archaeologists, researchers have answered these two questions by studying bacteria that are sometimes thousands of years old.

The journal Science published two studies on Thursday on the history of these diseases that have left their mark on the collective imagination: the plague, which caused the terrible Black Death at the end of the Middle Ages, and leprosy, associated over the centuries with images of severely disfigured patients.

"The plague bacterium has a particular importance in human history, so it's important to know how these epidemics spread," microbiologist Javier Pizarro-Cerda, one of the authors of the first study, told AFP.

The researcher works for the French Pasteur Institute, which was involved in both studies. His work, carried out in collaboration with scientists from Canada's McMaster University, explains why each plague epidemic has lasted so long over the centuries.

Over the past two thousand years, the world has experienced three plague pandemics. The first, known as the Plague of Justinian, marked the end of Antiquity and the beginning of the Middle Ages for two hundred years. Nearly a millennium later, the second began with the Black Death, which killed nearly half of Europe's population in the mid-1300s and then gave rise to recurring episodes for centuries.

The third, born in Asia in the mid-19th century, continues today with multiple cases, particularly in sub-Saharan Africa: Democratic Republic of Congo, Madagascar, Uganda....

The study authors examined samples of Yersinia pestis, the bubonic plague bacterium, from each of these outbreaks. They noted one commonality: in all three cases, the microbe underwent genetic evolution that gradually reduced its virulence.

You might think that an epidemic would die down when a microbe becomes less dangerous. But by causing less severe infections, the plague bacteria prolonged their duration, giving it more opportunities to spread from one person to another.

An American leprosy

Pasteur researchers confirmed this hypothesis by infecting groups of rats with certain recent samples: the disease persisted longer when the virulence of the bacteria was reduced.

This is a considerable step forward in our understanding of plague epidemics, even though the current context, where antibiotics are effective in combating the disease, is very different from centuries past.

"This allows us to gain a comprehensive understanding of how pathogens can adapt to different situations," Pizarro Cerda emphasizes. "We can ultimately better understand what the plague is, and how we can develop measures to defend ourselves against it."

The past also illuminates the present in the second study, which looks at the journey of leprosy over the millennia.

Pasteur's teams collaborated here with the University of Colorado to examine hundreds of samples from archaeological digs in North and South America.

They discovered one of the bacteria involved in leprosy, Mycobacterium lepromatosis. However, the samples in question dated back 9,000 years, well before the arrival of the first European settlers.

"We show that there was already a form of leprosy in America that spread across the continent," Nicolas Rascovan, one of the lead authors and a paleogenomics specialist at the Pasteur Institute, told AFP. "It wasn't just a small corner of the continent; it was everywhere."

Be careful, however, not to dismiss colonization as a cause. Europeans did indeed transmit another bacterium, Mycobacterium leprae, to American soil, which was considered until the 2000s to be the sole cause of leprosy.

But this discovery will help guide research into Mycobacterium lepromatosis, which has remained largely unclear since its discovery in 2008.

"We still have a lot of diversity to discover in this pathogen. And we now know that we should look for it in America, not elsewhere," concludes Rascovan. "This helps us consider strategies to combat pathogens and diseases."