Understanding the spread of infectious illnesses is vital in preventing and controlling future pandemics but there are few studies which measure how social and environmental factors could influence the speed of disease transmission. An international team of scientists set about tracking the spread of plague in China to shed light on how the deadly disease took hold.
Plague has killed 200 million people worldwide and outbreaks currently occur frequently in the Americas, Asia and Africa, remaining a threat to human health. In 1772 plague struck in a third lethal pandemic, eventually killing 2.2 million people in China and spreading worldwide from Hong Kong. The well documented outbreaks of the disease in China made it an ideal case study for the researchers who followed its spread across China for nearly 200 years from its starting point in Yunnan province until 1964.
To investigate disease transmission scientists often use ‘trend surface analysis’ or TSA. The method approximates the speed of a wave of disease spreading over an area from a central point. The technique works well for illness spreading through contact in small areas but is less reliable or detailed over large areas where disease jumps from one region to another before then starting to spread locally again. These kinds of leaps across wide areas have become more commonplace with advancement in human transportation, so a technique with increased accuracy over longer distances was ‘urgently needed’ say the team.
The team behind this study, led by Lei Xu, designed a simple new technique which they call the ‘nearest neighbour approach’ or NNA. They measured the distance from a ‘sink site’ infected by the disease, to the ‘source site’ which was the origin of the infection, and divided the distance by the length of time from the first recorded case of plague at the source site. This gave them a velocity to tell them how rapidly the disease was spreading. By using the closest neighbouring infected site the team built up a more detailed picture of how and why the plague might have spread over short and long distances.
Using NNA the team found that both climate conditions and transportation had an important impact on how the disease spread across China. The presence of rivers, roads and coastlines all contributed to a faster spread of plague. In fact, using NNA the team found that many long-distance transmissions closely followed human transportation routes. The team suggested that infected humans, rodents or fleas could have carried and transmitted the disease but that trading on ancient routes like the Tea Road and the Silk Road would probably have caused much of the long-distance spread.
The researchers also found that during the 200-year pandemic in China, in regions where there had been flooding the plague spread rapidly whilst in areas of drought its spread was more sluggish. The team suggested that the accelerated transmission of plague in flooded areas could have been be due to increased human contact with disease-carrying rodents who might thrive after flooding when lush vegetation could flourish. The team also suggested that flooding may have forced people (and plague-infected rodents) to leave their homes, increasing the spatial spread of the disease.
The team’s novel nearest neighbour approach let them look in finer detail at the transmission of the epidemic, revealing links to flooding and transport routes as plague spread across China during the third pandemic. The researchers say that the links they found suggest that ‘accelerated climate change and globalisation is likely increasing risk of infectious diseases’ and suggest that we should pay special attention to transportation routes and regions experiencing sustained humidity and flooding to help prevent and control the deadly spreading of future pandemics.