Three major outbreaks of plague between the sixth and 19th centuries claimed millions of lives in Europe. But where was the pathogen hiding between outbreaks? A new study now suggests there was probably no permanent natural reservoir of the plague in Europe. The low diversity of suitable rodent hosts and unfavorable climatic and soil conditions make it more plausible that the plague bacterium did not survive in Europe but was repeatedly introduced and only occasionally used local intermediate hosts.
The "Black Death" is considered one of the most devastating pandemics in world history. At its peak between 1346 and 1353, it cost the lives of an estimated 25 million people in Europe - a third of the population at the time. Centuries earlier, from the sixth to the eighth century, the plague had raged in Europe and helped herald the end of antiquity. A third global plague pandemic occurred at the end of the 19th century. The disease is caused by the bacterium Yersinia pestis. A natural reservoir of the pathogen is rodents, including rats. Transmission to humans usually occurs through parasites such as fleas, which feed on the blood of both rodents and humans.
Comparison with China and the USA
However, whether the plague bacterium survived in natural reservoirs in Europe between the pandemics, or whether it was imported from outside Europe each time, is a matter of scientific debate. A team led by Nils Stenseth from the University of Oslo has now used statistical modeling to investigate the likelihood that Yersinia pestis had long-term natural reservoirs in Europe. As a basis, the researchers used data on the environmental conditions in places in the world where the plague is still present in rodents and repeatedly causes infections in humans.
The starting point was information on soil conditions, altitude, temperature, precipitation and rodent populations in plague-affected regions in China. The researchers validated their model by showing that, based on the Chinese data, it can also correctly identify other existing plague reservoirs around the world, including those in the western United States. In the next step, they calculated the probability of a plague reservoir in Europe. The result: "For only 0.6 percent of Europe, the model predicted conditions that would be suitable for plague reservoirs," according to the researchers.
Europe unfavorable for the plague
Apart from small parts of Spain, Portugal, southern France, Italy and Greece, environmental conditions in Europe make it very unlikely that the plague bacterium can persist there for more than the medium term. The key factors: "The species richness of rodents and presumably also of the flea species associated with them is significantly higher in China and the United States than in Europe," say the researchers. "This suggests that European rodent and ectoparasite communities are unlikely to be favorable for the development of plague reservoirs in Europe." The analysis also showed that soil composition and climatic conditions in most parts of Europe are also unfavorable for Yersinia pestis.
This finding is consistent with the fact that plague no longer occurs naturally in Europe today. Using historical climate data, Stenseth and his colleagues showed that the likelihood of natural plague reservoirs in Europe was even lower in the past than it is today. "Our analyzes strongly suggest that local environmental factors in western and central Europe, including soil chemistry, elevation and climate, do not provide favorable conditions for persistent long-term plague reservoirs maintained by wild rodents and their ectoparasites." , write.
Mechanisms partly unclear
The authors therefore assume that the plague in Europe was only able to survive in local rodent populations in the medium term, but had no long-term reservoir. The large outbreaks were therefore much more likely to be caused by pathogens that were brought in from outside Europe, for example through trade trips.
In some cases, the extent to which the factors included in the statistical model actually influence whether the plague can survive in a region has not yet been clarified. For example, the model predicts that soils high in copper, iron, sodium and magnesium and low in calcium offer a particularly high chance of a natural plague reservoir forming. However, it is not clear from the data why this is the case. The researchers therefore point out that, despite the robustness of their results, field studies are needed to determine the mechanisms underlying the documented patterns.
Source: Nils Stenseth (University of Oslo, Norway) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2209816119