A tropical plague is spreading among birds in America's northernmost state in part due to a changing climate, according to new research.
Malaria, a scourge that haunts many parts of humanity, also afflicts our feathered friends. The avian version of the disease does not harm people, but it can serve as an analogue for future infection patterns in humans as the climate changes.
Because mosquitoes transmit the disease, it tends to reside in warmer and wetter parts of the world, areas that are hospitable to these insects. However, migratory birds can carry versions of the Plasmodium parasite, which causes the disease, to new locales, creating test situations that could model how human malaria may behave under similar conditions.
What the scientists have found is that the parasite that infects migratory birds is now infecting local birds. Because the Arctic is the fastest-warming area in the world, Alaska amounts to a vast test tube that helps scientists see the health impacts of rapid change. They published their findings last month in the journal PLOS ONE.
"It's not that we didn't know that avian malaria could exist up there," said Ryan Harrigan, a co-author and a postdoctoral researcher at the Center for Tropical Research at the Institute for the Environment and Sustainability at the University of California, Los Angeles. "This is the first time we had transmission occurring in an arctic area in Alaska."
With help from the Alaska Bird Observatory and the Bureau of Land Management, the researchers captured birds with mist nets and collected blood samples before letting the birds fly away. They tested birds at three locations and found malaria parasites as far north as Fairbanks, at a latitude of 64 degrees north. Out of 676 sampled birds, 7.2 percent were infected with one of seven varieties of Plasmodium.
Looking at the infected animals, the researchers found malaria among birds that summer in the tropics -- which is what they expected -- but they also found malaria in local birds, like chickadees, warblers and thrushes. Even some recently hatched birds carried the parasite, meaning it was infecting birds on their home turf.
Studying an 'arms race' among parasites
This suggests Alaskan mosquitoes picked up the parasite and are spreading it to resident birds. The new transmission cycle could be devastating, according to Harrigan, because immunologically naive birds could suffer greatly from this infection. He cited malaria transmission in Hawaii, which drastically shrank bird populations after the disease emerged there. Isolated birds, like penguins, are especially vulnerable, and birds that survive infection often have a lower birth rate.
Some species have adapted, but the parasite can change, as well, especially with so many versions at play. "It's an arms race," Harrigan said.
In addition, Alaska may not be a unique case. "The Plasmodium we found is a cold-tolerant Plasmodium already found in Europe," said Claire Loiseau, the study's lead author and a postdoctoral fellow in San Francisco State University's biology department. "In the case of Alaska, the probability that a new strain can be transmitted is very low because the parasite needs a good vector to reproduce and be transmitted."
At lower latitudes, there are more varieties of mosquitoes, in greater numbers and for longer portions of the year, Loiseau explained. But as the global average temperature rises, malaria transmission among Alaskan birds may go up as summers get longer, permafrost thaws and snowfall melts into rain, especially because Alaskan temperatures are rising at double the global rate.
"A lot of factors can change the transmission rate. ... Temperature is the main one because the parasite needs a certain temperature to be able to develop in the vector and in the bird," she said. "So with climate change, it's going to be a mix of factors that is going to influence the spread of parasites."
Human illness research may benefit
Harrigan echoed this idea and added that birds may also be migrating earlier, resulting in disease-carrying migrants spending longer summers in the north. "If you get all of those things in the same spot early in the season, you get a good [parasite] amplification cycle," Harrigan said.
Through studying this phenomenon, scientists can open a window into other climate-linked diseases, according to Ravinder Sehgal, another co-author and an associate professor of disease ecology at San Francisco State.
Though humans can't get sick from avian malaria, how it affects geese, puffins and penguins could help scientists develop a useful, albeit simplified, model that gives them a window into human experience with the version of the disease that affects them. "With human malaria, it's really hard to study this," Sehgal said, noting that factors like access to health care, economics and social order are not issues for birds. "This is a good proxy for what could be happening with other diseases in global climate change."
However, even figuring out where malaria is going among birds is challenging. Sehgal said scientists are still trying to grasp which mosquitoes transmit the disease and how far it has spread in Alaska. "We don't know what was the northern extent of transmission," he said.
To fill in the blanks, the researchers said the next step is to continue to survey birds and mosquitoes and collect samples from broader areas. "Your models are only as good as your data," Harrigan said. "I think a constant monitoring of those populations would be key." The team is currently screening Alaskan bird samples collected this summer.
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