Infections lurking on the margins of civilization are becoming more likely to cause outbreaks as the climate changes, researchers say.

Ravens, rodents and rattlesnakes are moving to new locales as rainfall and temperatures shift over time (ClimateWire, Dec. 14, 2011). The pathogens and parasites that infect these organisms move, as well, creating the risk of these diseases spilling over from one species to another.

This host-parasite relationship is a bellwether for broader changes in the environment, and understanding it could help people anticipate and respond to deadly diseases and economically devastating blights.

"Knowing how parasites are transmitted tells us about the ecological connections within an ecosystem," said Daniel Brooks, an emeritus professor at the University of Toronto. "When a new host is infected, we know something at a larger scale is going on in the ecosystem."

In a paper published this week in the journal Philosophical Transactions of the Royal Society B, Brooks and his co-author, Eric Hoberg, a research zoologist at the U.S. Department of Agriculture, explored how their research upended the conventional wisdom around parasites and how a changing climate would factor into disease transmission.

For a long time, the prevailing idea was that parasites and pathogens evolve to target a particular host and as they get better at infecting a specific animal or plant, they become less effective at infecting other organisms. A mosquito-borne virus that infects crows would be unlikely to infect ravens, for example.

However, in their regions of study—Brooks in the tropics and Hoberg in the Arctic—the scientists found that this pattern didn't hold up.

Parasites attack multiple species
"What we were finding in a huge number of cases was those parasites were not in the original host," Brooks said. "Those parasites are jumping around a lot more than they should be if the way we were trained about coevolution was true."

Upon further study of natural history, the researchers found that parasites have been infecting multiple species for millions of years. "We see these patterns in time going back across glacial ages," Hoberg said.

As the average temperature of the planet goes up and as humanity encroaches on wilderness, pathogens and the organisms they infect are moving into new habitats, increasing the risk of infecting native hosts.

"This is being played out for muskoxen and caribou with geographic expansion on Victoria Island [in Canada]," Hoberg said. "We see that climate and temperature have dramatic effects on their parasites."

The result is often a greater number of infected animals, which in turn leads to mass die-offs. This can spell disaster for communities that hunt these animals for sustenance, Hoberg said.

But the likelier outcome isn't the emergence of a supervirus as much as an overall increase in pathogen pollution—nuisance infections that drain health and economic resources, harming people, crops and livestock.

The West Nile virus is a case in point. The disease emerged in North America in 1999 and circulates between birds and mosquitoes, with humans infected as bystanders. Since its emergence, the disease spread all over the United States and last year, California suffered a record number of infections (ClimateWire, Oct. 9, 2014).

Moving targets
Though the disease ebbs and flows year to year, the West Nile virus is here to stay, barring some drastic eradication effort. Scientists are now tracing climate variables to forecast future outbreaks of the disease (ClimateWire, Sept. 20, 2013).

To anticipate future outbreaks and spillover events, scientists must understand ecosystems at the present, a daunting task even under the best circumstances.

Scott Gardner, curator of parasitology at the University of Nebraska State Museum, studied hosts and parasites in Latin America and found that in some cases, parasites were anchored to a specific geography as hosts moved in and out. "A lot of the parasites we found in the animals down there are restricted to certain parts of the Andes Mountains," he said.

Finding this out required an exhaustive inventory of marsupial hosts in the region. "We worked in Bolivia for 15 years collecting mammals over the whole country," Gardner said. "That was the kind of effort it took for us."

Scientists need to conduct similar assessments in other parts of the world in order to get a better handle on important diseases and prepare for new ones. In 2013, researchers found a new subtype of the dengue virus in Malaysian monkeys, forcing them to design new tests to detect the novel strain (ClimateWire, Nov. 15, 2013).

But as the climate changes, the window for establishing an ecological baseline is closing fast, limiting how much researchers can extrapolate into the future. "Long-term, it's not economically sustainable to respond after-the-fact to a crisis," Brooks said. "We're not winning the war against emerging disease right now, so we need to try something different."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500