An infectious disease that impacts major agricultural regions in the southwest is predicted to be a risk in a much greater section of the United States as the climate continues to change. Valley fever, which affects people in different ways, comes from airborne spores of a fungus called Coccidioides immitis. When people inhale the microscopic spores, they can contract the illness, which makes some people severely sick while others may never even know they were infected.

While the fungus lives in soil, when conditions are dry and windy, the fungus produces spores that can be picked up and moved, putting outdoor workers at risk of inhaling them. While land in active use, such as agricultural fields, seems to pose less of a threat than disturbing land for the first time, workers could still be exposed if the harmful spores are carried by a breeze from, for example, a site of new construction.  

Valley fever CDC.jpgSoil sampling is a way to determine whether the fungus that causes valley fever is present. (photo: CDC) Morgan Gorris, a postdoctoral researcher at Los Alamos National Laboratory in New Mexico, developed a model using the weather conditions (heat and precipitation) in areas with known cases of valley fever. Then, she applied it to climate predictions to see where the disease might become a threat under different climate scenarios. She and her colleagues used two different climate scenarios to predict how widespread valley fever might become by the end of the century. Under the highest warming scenario, their model predicts Coccidioides will spread into all of Nevada, Arizona, New Mexico and Utah; most of California, Colorado, Wyoming and Montana and parts of the Dakotas, Nebraska, Kansas, Oklahoma and Texas. Counties with higher rainfall or lower temperatures are areas where the fugus is less likely to thrive.

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Gorris said even where climate conditions would seem to favor the fugus spores escaping the soil, that doesn’t necessarily mean the disease will be a threat. The fungus “has to get there somehow,” she said, likely, by hitching a ride on a rodent.

“If the Southwest starts getting too hot, those rodent species will also start traveling further north,” she said, and scientists haven’t yet identified a specific rodent that Coccidioides favors, but with many options “it's not going to have a hard time spreading.”

Gorris said valley fever is interesting in part because although it’s been around since the 1930s, outside of the San Joaquin Valley, where it got its name, it’s not widely known. That contrasts with other diseases that cause more public worry. “West Nile virus was introduced to the United States in 1999 and spread across the United States. And everyone's heard about West Nile virus, but on average more people die each year from valley fever than they do West Nile virus.”

The state of California invested $10 million to improve understanding of valley fever. The money went to the Valley Fever Institute at Kern Medical in Bakersfield, which provides both research and treatment and has found that fewer than half the people who inhale the harmful spores get sick. And among those who do, 60 percent may remain asymptomatic while one percent of those who become infected can develop severe, sometimes life-threatening, conditions.

Because the fungus produces spores that become airborne only when the conditions are hot and dry, periods of drought like the current one raise the level of concern about valley fever. CalOSHA’s Valley Fever website says among outdoor workers, farm laborers may be at less risk than some others such as construction workers or archeologists. “Cultivated, irrigated soil may be less likely to contain the fungus compared to undisturbed soils,” the page says. Preventative measures it recommends include wetting soil before disturbing it, suspending work during windy conditions and providing respirators such as N95 masks to workers who cannot avoid dust exposure.

Gorris said her model showed considerably less spread of the fungus under the climate scenario with more moderate warming, a scenario that accounts for successful efforts to curb greenhouse gas emissions in the coming decades.

“With reduced greenhouse gas emissions, the area that may be endemic to valley fever in the future was more limited,” she said. “So reducing greenhouse gas emissions may reduce the spread of the disease.”

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