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Emily Guerin | Oct 12, 2012 06:00 AM

All sorts of things have been linked to climate change lately: skin cancer, shrinking leaves, extreme weather and death. This summer, scientists and reporters have been puzzling over a wave of disease outbreaks—hantavirus, valley fever and West Nile virus—and whether they, too, are linked to climate change.

With some of these diseases the climatic connections are too obvious to ignore. The mosquitoes that pick up West Nile from birds, Culex pipiens, breed faster in warm weather and bite more frequently. The virus replicates faster at warmer temperatures, too. Drought also helps the Culex breed by turning streams into cesspools of stinky, stagnant water rich with organic matter, and by making life difficult for predators like dragonflies and frogs. Birds also concentrate around a few remaining water sources during droughts, making transmission easier.

culex mosquito

Not surprisingly, the unusually hot, dry weather we had this past summer aided the spread of West Nile in the United States. As of October 9, 168 deaths and 4,249 cases of the disease have been reported to the Centers for Disease Control and Prevention, mostly from Texas. Most people never develop symptoms, but about 20 percent experience fever, body aches, and nausea, among other signs. The disease will turn serious in about one in 150 people, and symptoms can last weeks.

In California's Central Valley, cases of the aptly-named valley fever, a less-publicized but more common disease, are on the rise. The disease is caused by a soil-dwelling fungus called coccidioides. Fungus spores are carried in wind-blown dust, and once inhaled, take root in the lungs. Once it’s inside you, the fungus never leaves. It can weaken bones, cause muscle wasting and skin pustules. People who work outside, in industries like agriculture and construction, are most likely to get it. Every year, valley fever kills more than 100 people and sickens 150,000.

valley fever

The soil fungus that causes valley fever thrives in drought conditions. Hot, dry weather kills off the organisms that keep the fungus under control, giving coccidioides unlimited access to organic material in the soil. Research by University of Arizona and Kern County, Calif. health officials has shown that valley fever thrives particularly when a rainy year, like last year, is followed by a dry one, like this one, mobilizing the spores that grew during the wet period.

And while the climate connection is much more tenuous, researchers are also debating the role of global warming in a much-publicized hantavirus outbreak in Yosemite National Park this summer, which killed three people out of nine total infected. People get hantavirus through breathing in dust particles from the poop and pee of infected deer mice. Early symptoms include fever, exhaustion and muscle aches, and develop four to 10 days later into coughing and shortness of breath caused by fluid filling the lungs. About 38 percent of those infected die.

deer mouse

A University of Utah study found that increased rainfall from El Niño in the early 1990s precipitated an outbreak of hantavirus in 1993, likely because more rainfall equals more food for deer mice. But others say the climate link to hantavirus is pretty shaky. Janet Foley, an epidemiologist at University of California, Davis, told Mother Jones that the deer mouse is very common regardless of the weather, and the real question is whether (and why) more deer mice are holing up in cabins and shelters, where humans are exposed to them.

It’s difficult to determine how much climate affects the frequency, intensity and location of disease outbreaks, but one thing is clear: the conversation on climate change now concerns public health, not just the environment. And when public health officials talk, the public generally listens. “When we show the evidence that a changing climate does affect their health, people become very concerned and believe it ought to be addressed,” epidemiologist George Luber told NPR. Moreover, people tend to respond positively to a conversation about climate change when it’s framed as a public health issue, even those who are climate skeptics, according to a recent study by researchers at George Mason, American and Yale universities.

It’s certainly not a silver lining to climate-induced spread of disease, but it’s something.

Emily Guerin is an intern at High Country News.

Photo of the Culex pipiens courtesy Flickr user fairfaxcounty; high magnification micrograph of valley fever courtesy Wikimedia user Nephron; photo of deer mouse courtesy Flickr user Tatiana Gettelman.

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