Knowing when the valley fever season starts can help public health agencies target their warnings and identify when physicians should be on alert for new cases, says Justin Remais, an environmental health researcher at the University of California, Berkeley.

“Some infectious diseases are known to be amplified by drought conditions,” Remais says. To pin down whether the same is true for valley fever, Remais and colleagues analyzed climate data and all reported cases in California from 2011 to 2021.

Cases tend to peak between September and November, the end of California’s dry season, the team found. Smaller seasonal peaks occurred during drought, but those spiked a year or two after returning rains ended the dry spell.  

The reason for the pattern is unknown. Coccidioides populations might crash during droughts, helping surviving spores thrive with less competition in wet weather. The lack of rain may also kill off fungus-carrying rodents, providing fungal nutrients from decomposing animals, or boost amounts of dust, spreading spores that moisture brings back to life.

It’s unclear whether droughts affect case spikes in other states such as Arizona, the U.S. hotspot for valley fever. And as droughts worsen and rains get heavier with climate change, more people could be at risk (SN: 1/10/23).

“We need to better understand how these pathogens are adapting to new environments in order to better prevent infection and protect those at risk in the decades to come,” Remais says.