Coughs and sneezes may spread diseases, but transport routes and wet climates may help spread the plague. That’s the conclusion of a new study that uses a novel analytical approach to track how a plague epidemic moved across China beginning in the mid-19th century. The results pinpoint which environmental and social factors are most important for plague transmission and could help scientists refine disease control measures in the future.
Best known as the cause of the Black Death—the epidemic that devastated Asia, Europe, and the Middle East in the 14th century—the plague is an aggressive and deadly disease caused by the bacterium Yersinia pestis. Plague has caused three pandemics to date, which together killed about 200 million people. The disease—typically spread by fleas that carry the bacteria and can infect humans or small rodents, like rats—remains a threat, with minor outbreaks still common in Africa, the Americas, and Asia.
Until now, studies of plague movements have typically focused on the local development of the infection over time, rather than the social and environmental conditions that allow the bacteria to jump between sometimes far-flung geographical locations. In the new study, researchers from China and Norway addressed this underexplored issue. Armed with detailed Chinese historical records, they built a model that traced the origin and geographical spread of the third plague pandemic, which killed at least 10 million people between 1855 and 1959.
The researchers were able to reconstruct the plague’s transmission routes across China, they report online today in the Proceedings of the Royal Society B. After an initial outbreak in Yunnan, the disease advanced in three phases. The first and third began with local transmission events, with new outbreaks occurring close to old ones, but the second appears to have started when the bacteria made long-distance jumps to areas in the north and southeast of the country, hundreds of kilometers away from Yunnan. The team noted that the plague tended to spread along established transportation lines, such as major roads, rivers, and coastlines, and moved fastest on these routes. Furthermore, heavy rains and floods increased the speed at which plague reached new areas. The researchers believe that such flooding may have forced people—and rodents—to move, bringing the disease with them.
Co-author Nils Stenseth, a biologist at the University of Oslo, says that these findings could have implications for future disease control measures, especially because modern-day pandemic diseases have the potential to spread faster than ever over our rapid global transportation networks. Climate change could also increase the kinds of flooding events that have facilitated the spread of plague in the past. Zhibin Zhang, an ecologist at the Chinese Academy of Science’s Institute of Zoology in Beijing and a co-author of the paper, says that in the event of a plague outbreak, “monitoring and prevention measures should be taken in regions facing an obvious increase in precipitation or flooding events, and with close transportation connections with plague [centers].”
Herwig Leirs, a biologist at the University of Antwerp in Belgium who was not involved in the research, commends the study for its use of unusually detailed historical records. The researchers’ approach to estimating how fast the infection spreads under different conditions is novel, he says, and could allow for better identification of long-distance jumps in infection. Viveka Vadyvaloo, a bubonic plague expert at Washington State University, Pullman, who was not involved in the study, concurs, calling the paper “impactful” for providing clear evidence for the long-suspected relationship between climatic and social factors and the spread of disease. Although she agrees that an understanding of plague transmission routes could help limit disease spread, she also cautions that “the need for [a] better understanding of how plague is maintained between pandemics—and what triggers [the] re-emergence of plague—is equally important.”