The appearance of infectious diseases in new places and new hosts, such as West Nile virus and Ebola, is a predictable result of climate change, scientists say.
Humans can expect more such illnesses to emerge in the future, as climate change shifts habitats and brings wildlife, crops, livestock, and humans into contact with pathogens to which they are susceptible but to which they have never been exposed before, researchers said.
"It's not that there's going to be one 'Andromeda Strain' that will wipe everybody out on the planet," said Daniel Brooks, a noted zoologist affiliated with the Harold W Manter Laboratory of Parasitology at the University of Nebraska-Lincoln, while referring to the 1971 science fiction film about a deadly pathogen.
"There are going to be a lot of localised outbreaks putting pressure on medical and veterinary health systems," Brooks said in an article published in the journal Philosophical Transactions of th e Royal Society B.
Brooks and his co-author, Eric Hoberg, a zoologist with the US National Parasite Collection of the the US Department of Agriculture's Agricultural Research Service, have observed how climate change has affected very different ecosystems.
Brooks has focused primarily on parasites in the tropics, while Hoberg has worked in Arctic regions.
Each has observed the arrival of species that hadn't previously lived in that area and the departure of others.
Changes in habitat mean animals are exposed to new parasites and pathogens, researchers said.
For example, Brooks said, after humans hunted capuchin and spider monkeys out of existence in some regions of Costa Rica, their parasites immediately switched to howler monkeys, where they persist today.
Scientists have assumed parasites don't quickly jump from one species to another because of the way parasites and hosts co-evolve.
Brooks calls it the "parasite paradox." Over tim e, hosts and pathogens become more tightly adapted to one another.
According to previous theories, this should make emerging diseases rare, because they have to wait for the right random mutation to occur.
However, such jumps happen more quickly than anticipated. Even pathogens that are highly adapted to one host are able to shift to new ones under the right circumstances.
Brooks and Hoberg call for a "fundamental conceptual shift" recognising that pathogens retain ancestral genetic capabilities allowing them to acquire new hosts quickly.
The new hosts are more susceptible to infection and get sicker from it, Brooks said, because they haven't yet developed resistance.
Though resistance can evolve fairly rapidly, this only changes the emergent pathogen from an acute to a chronic disease problem, Brooks said.
"West Nile Virus is a good example - no longer an acute problem for humans or wildlife in North America, it nonetheless is here to sta y," Brooks said.
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