California droughts may help valley fever spread

Long dry spells can give a lethal fungal disease a lift.

While California droughts can temporarily keep cases of valley fever — a sometimes deadly illness caused by Coccidioides fungi — relatively low, cases skyrocket when rain clouds move back in, researchers report in the October Lancet Regional Health — Americas.

Valley fever is on the rise across parts of the western United States, spreading through fungal spores kicked up in airborne dust (SN: 1/4/23). Though most people who breathe in spores won’t get sick, those that do develop respiratory symptoms such as cough and shortness of breath. In severe cases, the fungus can cause long-term lung problems or invade other parts of the body.

Estimated areas for Coccidioides in the United States

The fungus that causes valley fever is typically found throughout the western United States. Coccidioides is not distributed evenly and may not be present everywhere within the shaded areas. It may also be present outside of the areas indicated.

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.