Climate change could spark animal-to-human pandemics

• The study, published in the journal Nature, shows how climate change and land use shifts will push pathogens' host animals to novel places where they will mingle with other species for the first time.
• These inter-species connections will expand the reservoirs for viruses, further threatening endangered species and increasing the chances of viruses spilling over to humans.
• Most of these species-to-species jumps could go undetected, the study warns, since current surveillance efforts watch exclusively for viruses jumping from wildlife to humans.
• "This [study] is really stressing that there are lots of cross species transmission events that are happening now, and many more that can happen as environments are changing," says Thomas Gillespie, a disease ecologist at Emory University who was not part of the new study.

How it works: A pathogen's ability to jump from one species to another depends on whether the hosts have a chance to interact and how similar the two host species are to one another.

• There are about 6,500 mammals, including humans, on Earth — all are potential hosts for pathogens. Many are under new pressure due to changes in their habitat driven by warming temperatures.
• Just 7% of mammals have overlapping habitat today, and about 6% of mammals host the same viruses as another mammal species, the researchers write.
• Most jumps to new species are "dead ends" for viruses — they don't make their hosts sick and fail to spread. But those that do can be devastating.

What they found: The new study models how the ranges of mammal species, which are hosts to viruses that are most likely to spill over to humans, might change under different climate and land use scenarios for the year 2070.

• The researchers project that among 3,139 mammal species, there could be hundreds of thousands of first encounters between species in the next five decades if warming is kept to less than 2°C above preindustrial levels.
• Those would lead to more than 4,500 transmissions of viruses from one species to another.

Details: The model also projects where these encounters will happen.

• Most would take place in high-elevation ecosystems with abundant biodiversity in tropical Africa and Asia. As species move up hillsides and mountains in search of cooler temperatures, they will encounter new neighbors. Similar dynamics will play out in the dwindling tracts of species-rich tropical primary forests.
• Many of those places are also where cities are built and crops are planted.
• "Climate change is creating innumerable hotspots of future zoonotic risk or presents a zoonotic risk right now in our backyard," Colin Carlson, a disease ecologist at Georgetown University and co-author of the study, said in a press briefing, using the term "zoonosis" that refers to viruses jumping between species.

Zoom in: Bats will drive many of the virus transmissions because they can quickly fly to cooler climates, which would increase their encounters with other species, according to the modeling.

• For example, several bat species could carry Ebola from West Africa to East Africa, where they would interact with new species and create opportunities for spillover to humans.
• Gillespie cautions that bat species that live in caves may be buffered from some of the effects of climate change. But Carlson says they will still have to deal with shifting food sources, extreme weather and climate events.

Keep in mind: The models didn't take birds, mosquitoes and other animals into account and is likely to be a conservative estimate of the number of new species interactions.

The big picture: Climate change is already significantly ratcheting up the pressure on a wide range of species as their habitat is altered in potentially irreversible ways, the landmark assessment from the U.N. Intergovernmental Panel on Climate Change (IPCC) recently showed.

• Even if carbon emissions are curbed, the models in the new study show the increase in viral sharing events isn't completely preventable. And the study suggests climate change is already driving some of them.
• "The bottom line is the majority of the viral sharing events are going to happen over the next 20 years and going to happen regardless of how well we mitigate climate change within that window," says Gillespie, who adds the focus now should be on addressing land use changes as a way to mitigate these events.
• Climate change and land use are connected, and Carlson said over time climate change will become an even bigger driver of these events.

The bottom line: "Ultimately, this work provides us with more incontrovertible evidence that the coming decades will not only be hotter, but sicker," said study co-author Gregory Albery, who also studies disease ecology at Georgetown.