Guest essay by Eric Worrall
A new study suggests that climate change is increasing the risk of humans contracting the plague. But the risk factors identified, even if the study is right, seem entirely manageable.
The Plague Is More Likely Now Thanks to Climate Change
November 20, 2021 at 8:50 am
The risk of the plague spilling over from humans to animals in the western U.S. has increased since 1950 thanks to climate change, a new study has found. Importantly, the research gives valuable insights into how this deadly disease has historically moved and developed in the U.S., which can help us understand more about its future.
Yersinia pestis is the bacteria that causes plague — including that plague, the medieval Black Death, which killed around 25 million people over the course of four years in the 1300s. The bacteria is spread to humans from animals, most infamously rats, which carry plague-infested fleas on them. Scientists have theorised that the plague, like many other infectious diseases, will probably increase its spread to humans as the planet warms and people come into increasingly closer contact with wild animals.
But there’s not a lot of research out there on what historically are the best conditions for the plague to develop and get out of control. As a result, there are still a lot of big questions about the plague — like why it hasn’t spread to certain geographic areas, or why human cases don’t always overlap with where animals are carrying the disease — that remain unanswered.
The study found that rodent communities in certain areas at higher elevations were up to 40% more likely to harbour the disease, which the researchers say is attributable to warming since 1950. That, in turn, means that the risk of the plague spreading from rodents to humans also increased, albeit more slightly.
“It’s a big, messy, tangled system, and there’s a lot of different levers controlling the ecology of the disease,” Carlson said. “But as we start to identify the big ones, we can look at how the key variables have changed since 1950, and it turns out — more and more of this region is starting to match the conditions that allow plague to hang out in animals, and increasingly, to make the jump into people.”
The abstract of the study;
Plague risk in the western United States over seven decades of environmental change
After several pandemics over the last two millennia, the wildlife reservoirs of plague (Yersinia pestis) now persist around the world, including in the western United States. Routine surveillance in this region has generated comprehensive records of human cases and animal seroprevalence, creating a unique opportunity to test how plague reservoirs are responding to environmental change. Here, we test whether animal and human data suggest that plague reservoirs and spillover risk have shifted since 1950. To do so, we develop a new method for detecting the impact of climate change on infectious disease distributions, capable of disentangling long-term trends (signal) and interannual variation in both weather and sampling (noise). We find that plague foci are associated with high-elevation rodent communities, and soil biochemistry may play a key role in the geography of long-term persistence. In addition, we find that human cases are concentrated only in a small subset of endemic areas, and that spillover events are driven by higher rodent species richness (the amplification hypothesis) and climatic anomalies (the trophic cascade hypothesis). Using our detection model, we find that due to the changing climate, rodent communities at high elevations have become more conducive to the establishment of plague reservoirs—with suitability increasing up to 40% in some places—and that spillover risk to humans at mid-elevations has increased as well, although more gradually. These results highlight opportunities for deeper investigation of plague ecology, the value of integrative surveillance for infectious disease geography, and the need for further research into ongoing climate change impacts.
The plague is treatable with modern antibiotics, though it still kills a handful of people in the USA every year – antibiotic treatment has to be started very quickly when plague is suspected, for a good chance of survival. Regions where plague is endemic maintain vigilance against outbreaks.
I learned something new about plague – the study suggests it can persist in soil, like anthrax, allowing it to re-emerge sporadically and infect local wildlife even if all living carriers are eliminated, though plague is sensitive to soil chemistry – only some soils are suitable.
I think the biggest weakness with the study is in my opinion the study does not delve deeply enough into why modern plague distributions are tied to a small number of species. Plague can famously infect a very broad range of mammals. The study briefly mentions this issue, but I would have like to see more depth on why modern plague is so restricted.
Given some of the species of known reservoir animals are protected species, I strongly suspect the reason why plague is still endemic in some regions of the USA, is nobody has baited and eradicated the reservoir animals. If an unacceptable threat to human life emerged, an intensive culling programme which included protected species which are known carriers would likely quell the threat, regardless of any climatic factors.