Dangerous Airborne Fungus Boosted by California Droughts

“Most seasonal infectious diseases show a peak in cases every year, so we were surprised to see that there were certain years during which few or no counties had a seasonal peak in Valley fever cases,” said first author Alexandra Heaney, Ph.D. , assistant professor at the UC San Diego Herbert Wertheim School of Public Health and Human Longevity Science. “This made us wonder what was driving these differences in seasonality between years, and based on the timing we observed, we hypothesized that drought might be playing a role.”

The researchers found that on average, counties in the San Joaquin Valley and Central Coast regions had the most pronounced seasonal peaks, though the peaks started earlier in San Joaquin Valley.

“This is valuable information to time public health messaging aimed at educating the public about the symptoms of Valley fever and how to protect themselves,” added Heaney.

Valley fever is caused by spores from the soil-dwelling Coccidioides fungus. People contract Valley fever by inhaling infectious spores that become aerosolized when the soil is disturbed by wind or human activity. Valley fever is most likely to affect people who are exposed to airborne dust frequently, including those who work outdoors. However, the disease is not contagious.

Valley fever has long been a problem in the American Southwest, but the number of cases has skyrocketed in recent years, tripling from 2014–2018 and again from 2018–2022, according to the CDPH. However, because it is still relatively rare, and because it causes similar symptoms to other respiratory infections, including COVID-19, Valley fever is often misdiagnosed. When left untreated, the fungus can cause severe damage to the respiratory system and spread to other parts of the body, such as the skin, bones and even the brain, the latter of which can be deadly.

“Knowing when the Valley fever season starts and how intense it will be can help health care practitioners know when they should be on high alert for new cases,” said corresponding author Justin Remais, Ph.D., professor at UC Berkeley School of Public Health. “This is the first study to pin down exactly when disease risk is highest in all of California’s endemic counties, as well as places where the disease is newly emerging.”

The researchers observed that during drought periods, seasonal peaks in Valley fever cases are less severe. However, when the rains return, these peaks are particularly high. One hypothesis to explain this pattern is that droughts allow heat-resistant Coccidioides spores to outlast their less-hardy competitors. When rains return, the fungus is able to proliferate widely with less competition for moisture and nutrients.