September 20th, 2012

A team of biologists has just announced the first documented case of bird-to-bird malaria transmission in Alaska. Writing in the journal PLOS ONE, they’ve shown that this frequently fatal avian illness, which is normally associated with the tropics and temperate areas, may be expanding its range. Fortunately, avian malaria doesn’t affect humans, co-author Ravinder Sehgal of San Francisco State University said, but the findings are particularly significant from a bird conservation as well as a climate change standpoint.

“It also has implications for the spread of vector-borne human diseases as the climate changes,” Sehgal said.

Those implications are straightforward enough: insects that transmit diseases from one human to another are often limited in their geographic ranges, and the limits usually have to do with how cold it gets. As the climate warms up, the insects can expand into new ranges — generally northward in the Northern Hemisphere, and also to higher altitudes — bringing their viral or bacterial passengers along for the ride.

Credit: Claire Loiseau

It was not all that long ago that the evidence for climate change was mostly theoretical, but not any more. Just about every day, it seems, a new piece of hard evidence emerges to confirm that the planet is warming, with all sorts of dire consequences — the greatest meltback of Arctic sea ice on record, a disturbing link between global warming and wildfires in the American West, the fourth warmest August ever measured and more.

Last year, the American Medical Association published a statement warning that dengue fever (carried by mosquitoes) and Lyme disease (carried by ticks) were starting to show up in places where they’ve rarely been in modern times, and this summer, Texas was slammed with an epidemic of West Nile virus, another mosquito-borne illness. In fact, the U.S. has seen the most West Nile cases on record this year.

With illnesses carried by mosquitoes, the range isn’t solely determined by the climate. Malaria used to be widespread in the U.S., for example. It was wiped out, not by cold temperatures, but by massive public health efforts including the draining of swamps and use of pesticides. Dengue fever once plagued the American Southeast, before being beaten back in the early 20th century, although that gain may yet prove temporary.

Nevertheless, there’s always been a northern limit to how far these diseases can thrive, and until now, nobody had seen bird-to-bird malaria transmission in Alaska. “We’ve observed cases of avian malaria,” said Sehgal, “but these could have been brought to Alaska by birds migrating from further south.”

To see whether the disease could be transmitted from one bird to another within Alaska, lead author Claire Loiseau (it's French for “the bird,” believe it or not) of the University of California at Los Angeles and her colleagues took blood samples from non-migrating birds and from young hatchlings that hadn’t yet left Alaska. They found evidence of the Plasmodium parasite that causes malaria in 7.2 percent of them — but no infections north of about 64 degrees north latitude, about where the town of Coldfoot lies. “We found no malaria there, even though there are plenty of mosquitoes and plenty of birds,” said Sehgal. This suggests that Plasmodium can't survive at that latitude in the current climate.

That, however, is likely to change, the scientists said. By 2080, they calculate, based on climate-model projections, the illness will push further north, threatening bird populations that have so far been unaffected.

Since avian malaria doesn’t affect humans at all, the residents of Coldfoot don’t have to worry about that. But the northward push of this one disease, Sehgal said, can foreshadow what might happen with illnesses humans rightly fear.