Topic for May. 25. ( Fri )

[hicaru]

Climate change, animal loss, urbanization: How do they affect disease spread? : Goats and Soda : NPR

What's worse for disease spread: animal loss, climate change or urbanization?

What's worse for disease spread: animal loss, climate change or urbanization?

Scientists have long known that human activity often helps pathogens spread. Despite hundreds of studies investigating human influence on infectious diseases, scientists weren't sure whether certain activities matter more than others for increasing risk.

Now, new research clarifies that picture – and suggests that humanity's reshaping of the planet is stoking the spread of dangerous infectious diseases not just for people but also for other animals and plants.

The study found these changes had fairly consistent effects on the spread of infectious diseases regardless of geographic location or species, underlining the need to reduce greenhouse gas emissions and conserve biodiversity, scientists say.

"This is a genuinely monumental paper," said Colin Carlson, a global change biologist at Georgetown University who wasn't involved in the research. While it's important to be conservative about what these kinds of meta-analyses can reveal, he says, "the foundational message here is that global change is completely re-determining disease risk in every system in every form of life on Earth."

While parasites and pathogens can be influenced by a host of different factors, the researchers focused on five key "global change drivers" that are upending ecosystems — biodiversity change (for example, declines in the numbers of species in an ecosystem or their population size), climate change, chemical pollution, invasive species and habitat loss.

"There are limited resources for monitoring, controlling and managing infectious diseases," says Jason Rohr, a biologist at the University of Notre Dame and study co-author. "It's really valuable for policymakers to have a better grasp on which global change drivers increase the risk of outbreaks the most."

To get that broader view, Rohr and his colleagues built a dataset from nearly 1,000 studies across all continents except Antarctica. The team looked at 1,497 host-pathogen/parasite combinations (such as humans and malaria, or plants and fungal diseases). 

By taking such a sweeping view of how diseases impact so many different species, instead of focusing solely on human diseases, the researchers were able to look for general mechanisms that might be acting broadly.

"We look for general patterns because if they hold true, they might apply to humans," said Carlson. "Even if these are findings that apply to bats and rodents and primates, but not necessarily us, it's still bad for us if bats and rodents are sicker," he says, in part because those diseases might jump to us.

Changes in biodiversity can stir up diseases in several ways. 

An overall decline in biodiversity can increase infectious diseases through what ecologists call the dilution effect. Disease-causing parasites and pathogens tend to survive and spread better if they infect animal hosts that are abundant rather than rarer host species simply because there are more hosts for them to infect. And when biodiversity declines because of human activity, rare species tend to be the first to go, Rohr says.

Surprisingly, habitat loss — which is a major cause of biodiversity decline — was associated with a decrease in infectious disease outcomes.

The rapid pace of urbanization likely explains this counterintuitive result, Rohr says. 

Still, the lack of an effect of habitat loss is somewhat surprising, given scientists have drawn clear links between deforestation and increased risk of diseases like Ebola.

The study shows "a really clear effect of biodiversity loss and not a clear effect of deforestation," says Carlson. "That doesn't necessarily mean deforestation doesn't cause disease emergence, particularly given we know that deforestation causes biodiversity loss."

The discrepancy highlights how nuance can get lost in meta-analyses.

Caveats and limitations Viewing this problem from such a high vantage point can make it hard to detect the major role certain forces might play in specific locations, or for certain diseases that pose a greater risk to humans, says Erin Mordecai, a disease ecologist at Stanford University who wasn't involved in the study.

When the researchers focused solely on diseases that spread between animals and humans, for instance, climate change emerged as the clearest driver of worse outcomes, with biodiversity loss playing less of a role. That doesn't mean biodiversity loss isn't increasing infectious disease risk for humans for certain diseases, but that existing evidence suggests it might be a bit more complicated.

Another wrinkle stems from the fact that most of the studies included in the analysis treated biodiversity loss, climate change and invasive species as separate entities.

"Global change drivers are acting concurrently," says Mordecai. "Teasing out their separate contributions and interactions remains very difficult and not something likely to show up easily in a meta-analysis." "I hope this evidence can be used in international policy to spur action on climate change and biodiversity loss due to their negative impacts on disease."