The microbe responsible for plague evolved in or around China more than 2,600 years ago and spread around the globe in the following millennia, according to the most comprehensive genetic study to map out the family tree of the bacterium known as Yersinia pestis.
Y. pestis' pedigree is not only ancient, it is dramatic. Plague has been blamed for decimating societies, notoriously wiping out roughly a third of Europe's population during the Black Death in the mid-14th century.
This pestilence is still alive and well; the youngest strains evolved more than 210 years ago, by the researchers' calculations, and have spread to the United States, South America, Europe, continental Africa, Madagascar, Turkey and Southeast Asia.
Bubonic plague, the most common form, first infects rats or other small animals, and it is spread by the fleas that bite them. Infected humans suffer from swollen lymph glands, called buboes, caused by the plague bacteria, as well as flu-like symptoms. Left untreated, plague can kill 60 percent of its victims, according to the World Health Organization.
This bacterial family tree can shed light on both the past and future of plague, said study co-author Mark Eppinger, a researcher at the Institute for Genomic Sciences at the University of Maryland School of Medicine. While it reflects the spread of plague in ancient times, it can also provide a reference for future outbreaks by helping to identify their origin, he said.
To analyze the relationships among strains of the plague species, the researchers used samples of modern plague bacteria; the first was collected in 1965. They compared the whole genomes of 17 strains to isolate markers called single nucleotide polymorphisms (SNPs, pronounced "snips"), or points where the strains' genetic code was different. They then analyzed 286 additional strains for these markers, and used the results to infer relationships between the strains, and so build a detailed family tree for the bacterium.
Their conclusions are based on the discovery that strains isolated from China showed the most diversity in their SNPs — suggesting they'd been around the longest, to build up so many changes — and appeared throughout the four-branched tree. The data also indicate that the deadly bug spread out of China along historic trade routes.
Prior to the Black Death, plague appears to have spread from China to Western Asian, and then into Europe along the Silk Road trade route. In the 15th century, plague-infected rodents on the ships of the Chinese admiral and explorer Zheng He may have spread the disease to central Africa, according to Eppinger.
The last global pandemic of Black Death occurred in the late 1800s, first appearing in China, then spreading via ship to India, Europe, Africa and the Americas, according to the study published online Oct. 31 by the journal Nature Genetics.
The analysis also shows that the plague bacteria currently found in the United States can be traced to a single import. This supports historical records indicating that plague was brought over from Hong Kong by a ship that docked in Hawaii before arriving in San Francisco.