Valley fever fungus has been on the rise in California in recent years, and new research suggests worsening droughts may be driving the increase.
Valley fever fungus, a fungal disease that causes flu-like symptoms, has been emerging in the western United States for several years now, though its seasonal patterns have been poorly understood — until now.
New research, conducted by researchers from the University of California San Diego and University of California, Berkeley and published in The Lancet Regional Health - Americas, identifies the disease’s seasonal peaks and suggests worsening droughts are boosting cases — highlighting the impact of climate change on infectious diseases.
Valley fever fungus gets its name from California's Central Valley. It’s caused by a fungus, called Coccidioides, that grows in the soil and dirt in the southwestern U.S., infecting people who breathe in the dust from the contaminated earth. Those who are exposed to airborne dust frequently, such as those who work outdoors, are most likely to be infected, but the disease is not contagious.
It often resembles the regular flu, though it can be serious and even life-threatening for some patients. Cases in the U.S. have grown significantly in recent years, tripling from 2014–2018 and again from 2018–2022, according to the California Department of Public Health (CDPH). It’s also often misdiagnosed as it resembles other respiratory viruses. But when left untreated, the fungus can severely damage the respiratory system and spread to other parts of the body, leading to fatal outcomes in rare cases.
“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, in a news release. “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 research
Because it has the potential to cause real harm among some patients and to help healthcare practitioners be more prepared, researchers set out to better understand the disease’s seasonal patterns. To do this, they collaborated with the CDPH to analyze all reported Valley fever cases in California from 2000 to 2021.
The researchers compared these case rates to seasonal climate data to better understand the seasonal cycles of the disease throughout various California counties. They found that while the period from September to November typically sees the highest number of cases, seasonal patterns and timing differed between counties and years.
They also found that counties in the San Joaquin Valley and Central Coast regions had the most pronounced seasonal peaks on average, but the peaks started earlier in the San Joaquin Valley.
Unlike most seasonal infectious diseases, however, which peak each year, the researchers discovered that Valley fever fungus was less consistent. They found that in certain years, few or no counties reported seasonal peaks of cases. Researchers hypothesized that this may be due to droughts.
When comparing case rates to drought periods, the researchers found that seasonal peaks are actually less severe during droughts — but the peaks are particularly high in the one-to-two years post-drought. In other words, while droughts decrease Valley fever cases in the short term, they result in an increase in cases in the long term.
The authors present two possible explanations for why this might be: It’s possible that droughts allow heat-resistant Coccidioides spores to outlive their less-hardy competitors, helping the fungus to proliferate widely with less competition for moisture and nutrients when the rain returns. It’s also possible that because more rodents die during droughts, and dead rodents are believed to be an important source of nutrients for the fungus, the fungus may be able to survive and spread more easily during droughts due to increased nutrients.
“This work is an important example of how infectious diseases are influenced by climate conditions,” said first author Alexandra Heaney, Ph.D., assistant professor at the UC San Diego Herbert Wertheim School of Public Health and Human Longevity Science, in the release. “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.”
Next, researchers plan to analyze other Valley fever hotspots in the U.S., such as Arizona, where about two-thirds of all cases occur.
To protect yourself from Valley fever, try to minimize your time outside during dry and dusty periods, and wear face coverings to block the dust when you are outside.
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