Climate change-induced drought conditions have been increasingly driving the spread of a dangerous airborne fungus across California, according to a recent study.
The study, published in The Lancet Regional Health – Americas, highlights a significant rise in cases of the flu-like disease coccidioidomycosis, commonly known as “Valley fever.”
Over the past two decades, the incidence of Valley fever has surged dramatically, with cases tripling from 2014 to 2018 and again from 2018 to 2022.
Though Valley fever can lead to severe or even fatal complications, the researchers behind the study have identified certain seasonal trends that could assist public health officials in anticipating and preparing for future outbreaks.
The disease is caused by a soil-dwelling fungus called Coccidioides, which was historically concentrated in parts of Arizona and California’s lower San Joaquin Valley.
Valley fever is not transmitted from person to person; instead, it develops when individuals inhale fungal spores directly from the environment.
Those most at risk of exposure to the fungus include farmers, field workers, construction crews, or anyone who frequently interacts with soil outdoors.
The study, led by researchers from the University of California in collaboration with the California Department of Health, analyzed all reported cases of Valley fever in California from 2000 to 2021.
Over these two decades of data, the researchers discovered that variations in the disease’s emergence patterns were often influenced by drought conditions.
While most cases typically occurred between September and November, the researchers observed significant differences in seasonal behavior and timing across various counties and years.
“We were surprised to see that certain years showed few or no counties experiencing a seasonal peak in Valley fever cases,” stated lead author Alexandra Heaney, an assistant professor in climate and health epidemiology at the University of California San Diego.
“This observation led us to question what was causing these seasonal variations from year to year,” she added. “Based on the timing patterns we identified, we hypothesized that drought might be a contributing factor.”
Heaney and her team found that, on average, counties in the San Joaquin Valley and Central Coast regions exhibited the most pronounced seasonal peaks.
However, these peaks occurred earlier in the San Joaquin Valley, which is California’s agricultural hub.
During drought periods, the researchers found that seasonal peaks in Valley fever cases were generally less severe. However, once the rains returned, there was a sharp increase in cases, particularly in the year or two following the end of a drought.
One theory presented in the study suggests that heat-resistant Coccidioides spores might outlast other, less resilient competitors. With the return of rains, the fungus could thrive due to the newly available moisture and nutrients.
Another theory considers the possibility that droughts may lead to increased deaths of rodents, whose decomposing bodies then provide essential nutrients for the fungus, thereby enhancing its survival and proliferation.
“While droughts may appear to reduce Valley fever cases in the short term, the impact is an increase in cases over time, especially as climate change leads to more frequent and severe droughts,” Heaney explained.
Despite the serious health risks associated with the disease — which can infect not only the respiratory system but also the skin, bones, and brain — the researchers suggested proactive measures for at-risk workers.
These measures include limiting time outdoors during dusty periods and wearing face coverings to reduce exposure.
Heaney and her colleagues are now broadening their research to include other Valley fever hotspots, particularly in Arizona, where approximately two-thirds of U.S. Valley fever cases occur.
“Understanding where, when, and under what conditions Valley fever is most prevalent is crucial for public health officials, physicians, and the public to take necessary precautions during high-risk periods,” Heaney concluded.
