Plague, the disease caused by the bacterium Yersinia pestis, is best known for the lethal Black Death plague epidemic that swept across Europe in 1346-1353. Less known is that these plague outbreaks recurred in human populations for much of the 14th to 16th century in Europe, and longer still in other parts of Eurasia and northern Africa. Nor do people realize that the plague bacterium is still commonly found in Africa, the Americas, Asia, and on the edges of Europe in the Caucasus and Kazakhstan. It proliferates in mice, rats, wildlife rodents and shrews in ecosystems as diverse as semi-arid deserts, mountain meadows, tropical forests, as well as urban communities.
We have a rough idea of where the bacterium can still be found in the wild today, based on recorded human cases of plague, and wildlife surveillance programs that screen and test wildlife animals for plague. But exactly what limits the bacteria to those places is unknown: the black rat and its close relatives are a reservoir for plague in Madagascar and south-west China, but not in Europe. Likewise, we have a rough idea of what conditions the plague bacterium needs to thrive, like what temperature ranges the bacterium and infected rodents, shrews and fleas can handle, what salinity of the soil it tolerates, and so on, although we do not always know why these factors matter.
In this project we are combining both data sources: the data of where plague is recorded in humans and in the wild, and the experimental and ecological knowledge of what plague needs to survive. We are now in the final year of the project, and the first two papers - "Plague risk in the western United States over seven decades of environmental change" and "Mapping the plague through natural language processing" are published at Global Change Biology and Epidemics. A third paper is available as a PhD thesis chapter by Samuel Cho at Princeton, fitting a new climate sensitive plague transmission model of the plague outbreaks that hit Hong Kong in 1902 and 1903.
The work on predicting when plague spillovers happen in south-west China will soon be submitted for publication. The results are suprising in that there has to be an epidemic among wildlife rodents before there is an increased risk for humans – just the presence of plague in wildlife is not enough. That highlights the importance of monitoring and understanding the disease dynamics of what happens in the wild - when do wildlife rodents experience epidemics?
The digitization of the data for the global model of plague is complete - we have over 9,000 unique samples of plague across the Americas, Eurasia and Africa, and the models trained on it generalize well to other parts of the world. Together with Colin Carlson, I am now working to improve those models to compensate for biases in the training data, and include the geographic range of key rodent host species.
For the final, and most high-risk part of the research proposal, we selected the suitable climate datasets, and hired Ida Marielle Mienna. She worked with the UiO Natural History Museum and Geosciences to recalibrate a long-term climate model predictions (the IPSLCM6 holocene transient) to recent climate data (ERA20C). These climate simulation models have a high degree of uncertainty, so the predictions of historic plague reservoirs generated using these models will need some rigorous testing. This testing is what we will continue to work on.
