Climate changed-induced drought conditions have been driving the dispersal of a dangerous airborne fungus across California in recent years, a new study has found.
Cases of the flu-like disease coccidioidomycosis — also known as “Valley fever” — have risen dramatically over the past two decades, tripling from 2014 to 2018 and again from 2018 to 2022, according to the study, published Tuesday in The Lancet Regional Health – Americas.
While the disease can cause grave or even deadly complications, the authors said they have identified certain seasonal patterns that could help public health officials prepare for future surges.
Valley fever spreads via a soil-dwelling organism called coccidioides, which was previously concentrated in parts of Arizona and California’s lower San Joaquin Valley. Rather than passing from person to person, the disease develops from the direct inhalation of these fungal spores.
Those most likely to breathe in the fungus are farmers, field workers, construction crews or anyone who is in close interaction with soil outdoors.
Collaborating with the California Department of Health, a University of California-led research team analyzed all California cases of Valley fever reported from 2000 to 2021. Via these two decades of data, the researchers were able to determine how Valley fever noticed that variances in the disease’s emergence patterns were often driven by drought.
Although most cases tended to occur between September and November, seasonal behavior and timing showed marked differences among counties and years, they found.
“We were surprised to see that there were certain years during which few or no counties had a seasonal peak in Valley fever cases,” lead author Alexandra Heaney, an assistant professor in climate and health epidemiology at the University of California San Diego, said in a statement.
“This made us wonder what was driving these differences in seasonality between years,” she continued. “Based on the timing we observed, we hypothesized that drought might be playing a role.”
Heaney and her colleagues found that on average, counties in the San Joaquin Valley and Central Coast regions exhibited the most pronounced seasonal peaks. Those peaks arrived earlier, however, in the former region, which is California’s agricultural hub.
During the drought periods themselves, the scientists found that seasonal peaks in Valley fever cases were less severe. Yet once the rains returned, cases skyrocketed — particularly in the year or two after a drought ended.
One hypothesis for this pattern, according to the study, is that heat-resistant coccidioides spores may be able to outlast their less hardy competitors. This would mean that with the resumption of rains, the fungus could proliferate with widely available moisture and nutrients, the authors explained.
Another hypothesis the researchers considered is the possibility drought is increasing the deaths of rodents, whose bodies in turn provide critical nutrients for the fungus — strengthening its survival.
“Even though droughts appear to decrease Valley fever cases in the short term, the net effect is an increase in cases over time, particularly as we experience more frequent and severe droughts due to climate change,” Heaney said.
Despite the dangers of the disease — which can severely infect not only the respiratory system but also the skin, bones and brain — the researchers identified proactive ways that workers can protect themselves, such as minimizing time outdoors during dusty periods and wearing face coverings.
Heaney and her colleagues are now expanding their research to include other Valley fever hotbeds, with a particular focus on Arizona, home to about two thirds of U.S. Valley fever cases.
“Understanding where, when, and in what conditions Valley fever is most prevalent is critical for public health officials, physicians, and the public to take precautions during periods of increased risk,” Heaney added.