Study Ties Climate Crisis to Spread of Valley Fever-Causing Fungus in California

Scientists at the University of California noticed that cases of Valley fever, a respiratory infection that is spread only through the inhalation of fungal spores, only peaked in certain parts of the state in recent years—and when they set out to discover why, they found that drought may play a major role in rising cases of the disease.

In a study published in The Lancet Regional Health - Americas on Tuesday, the researchers explained that increased droughts in the state have created conditions for a growing number of people to inhale coccidioides spores, which thrive in soil and can be inhaled through dust.

The scientists analyzed cases of Valley fever from 2000-22, and found that cases have risen dramatically since the turn of the century—particularly from 2014-18 and again from 2018-22, two periods when reported cases tripled.

Cases of the seasonal illness have long been known to occur mostly between September and November, but the researchers found "there were certain years during which few or no counties had a seasonal peak in Valley fever cases," said lead author Alexandra Heaney, an 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."

"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."

During periods of drought, seasonal peaks of Valley fever were less severe, according to the study. It was after the dry periods, when heavy rains returned, that cases spiked.

The researchers suggested that the return of rainy weather could allow the heat-resistant coccidioides fungus to proliferate because of the newly moist soil and the influx of nutrients. Another hypothesis was that droughts cause a decline in the population of rodents that host the fungus.

"Because dead rodents are thought to be an important source of nutrients for the fungus, it may be able to survive and spread more easily in drought conditions," reported UC San Diego Today.

"This work is an important example of how infectious diseases are influenced by climate conditions," Heaney said. "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."

The study comes a month after a music festival in the state's Central Valley was linked to 14 cases of Valley fever, which can cause joint pain and fatigue as well as respiratory symptoms and can spread to the bones or brain in rare, potentially fatal cases.

People who work outside, including farmworkers and construction workers, are most at risk for breathing in the fungus.

The scientists called for more thorough monitoring of the fungus, which can be difficult to detect and whose symptoms are often confused for other respiratory illnesses. They also called for people in the state to wear face coverings when coming into contact with soil and dust and to minimize time outdoors, if possible, during dry and dusty periods.

Heaney said the team is next looking at climate dynamics related to Valley fever in Arizona, where about two-thirds of cases occur in the United States.

"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," said Heaney.