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Thursday, April 11, 2024 | Back issues
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Climate change drives another outbreak: In California, it’s a spike in Valley fever cases

Scientists point to Valley fever as the latest disease that could increase in frequency and spread thanks to the worsening impacts of climate change.

BERKELEY, Calif. (CN) — California researchers say climate change is driving a statewide spike in Valley fever throughout the Southwest as the region experiences swings between extreme drought and warming temperatures to unexpectedly high precipitation.

Valley fever, scientifically known as coccidioidomycosis, is an infectious disease that affects residents of the Southwest. People can contract it by breathing in dust that contains spores of the Coccidioides fungus, which grows in soil and can be stirred up by strong winds, digging or other disturbances.

In a study published Wednesday in Lancet Planetary Health, a group of scientists report how California’s recent droughts have helped drive transmissions of the pathogen. California is experiencing the highest recorded level of Valley fever and experts are starting to study how much of a role the state’s changing climate plays in the future spread of this and other diseases.

Scientists analyzed more than 81,000 coccidioidomycosis surveillance records from state and local agencies collected over 20 years. They found multiyear cycles of dry conditions followed by a wet winter can amplify Valley fever transmission. Arid counties like Kern and Kings see the most fluctuation in precipitation, while wetter coastal counties like Monterey and Ventura see the most fluctuations in temperature. Scientists say this could explain why rates have skyrocketed more dramatically in wetter, cooler counties.

 “We know that the extreme precipitation deficit that has plagued California in recent decades is one of the greatest environmental challenges in the western U.S.,” said Jennifer Head, assistant environmental health science researcher at UC Berkeley School of Public Health, who led the research. 

“This research is the first to find evidence that California’s recent droughts are also exacerbating the transmission of an emerging infectious disease in the state. We’ve seen that rates in northern San Joaquin Valley counties are 15 times higher than they were two decades ago. We now understand that transmission within these areas is strongly enhanced by heat.”

Valley fever incidents illustrated by graph.
Incidence rate ratios obtained from distributed-lag non-linear model testing for the association between incidence of coccidioidomycosis during autumn (September to November) and lagged meteorological variables Incidence rate ratios (IRRs) express. From Lancet Public Health 2022.

The researchers estimate the causal effect of two major droughts in California, from 2007 to 2009 and again from 2012 to 2015, on the overall transmission rates of coccidioidomycosis. They found drought conditions initially suppress transmission, but transmission strongly rebounds in the years immediately following drought. 

"During droughts, winter precipitation is too low for proliferation of the organism in the soil, and less fungus in the soil means lower risk of inhaling a pathogenic spore," Head said. She said during these periods other “competitor” bacteria dies off, giving the fungus a chance to spread and prosper when rainfall returns. 

Cases of the disease spike in years that follow drought, such as during the wet season in 2016 through 2017. Researchers estimated nearly 2,500 excess cases of Valley fever, which they said are attributable to the prior drought and more than offset the cases averted during the drought.

For example, Kern County has the highest incidence rates of coccidioidomycosis in California and is among the hottest and driest regions. Over the 47 months of drought from 2012 to 2016, 3,390 cases of coccidioidomycosis were reported among residents of western Kern County. 

The scientists reported that regions that normally are cool and wet see dramatic swings from drought to wetness and seem to experience higher increases in case rates compared with the arid southern San Joaquin Valley counties.

Graph of comparisons by map and color in California valley fever cases
Deviation in observed incidence of coccidioidomycosis during and after drought compared to the incidence expected in the absence of drought. From Lancet Public Health 2022.

“Transmission rebounds sharply when droughts conclude, posing major risks to vulnerable Californians, particularly outdoor workers engaged in construction, farm work, or other trades,” said Justin Remais, professor and chair of environmental health sciences at the UC Berkeley School of Public Health and principal investigator of the research project. “The end of our current drought will be good news for California, but bad news for Valley fever.” 

With the frequency and severity of drought in the state expected to increase with climate change, the researchers say people should be prepared to see the expansion of disease across the West. Climate change is expected to cause increased winter precipitation with less rainfall in autumn and spring, alongside increased duration and intensity of drought conditions — which could enhance favorable conditions for Valley fever to spike. They expect the illness is likely to expand into historically wetter and cooler regions, such as coastal counties in California. 

“Future analyses should consider how the associations resolved in this study can inform projections of the spatiotemporal distribution and seasonality of coccidioidomycosis under anticipated climate regimes in the decades to come,” the researchers wrote.

Head said in an interview that she thinks there is not enough research about Valley fever being endemic to the United States. She said the phenomenon of drought initially suppressing transmission, then driving a spike in fungal cases in later years, is unique.

While Head expects Texas, Utah and Nevada to begin to see a growth in Valley fever cases, she said more research is needed since the pathogen is different in those states. Any increases in reported cases will not likely develop uniformly across those states.

“We should anticipate higher risk of infection in the years that follow future droughts, and we should take extra precautions to educate the public and health care providers,” Head said. “We also need to ensure outdoor workers have access to respiratory protection where appropriate.” 

According to Remais, “We must now dedicate our efforts to detect, treat and prevent Valley fever among those most affected by drought and other climate extremes in the state."

Daniel Swain, Bay Area climate scientist for the Institute of the Environment and Sustainability, said the phenomenon points to an emerging issue from extreme swings in climate — which climate change is intensifying. He said California and much of the Southwest are expected to see greater swings from extreme drought to rainfall, which could point to latent potential for Valley fever to spread even in areas where it was not recorded before. 

Swain cited evidence for wider swings in regional weather extremes from other scientists. While there is lots of research on how climate change is influencing the spread of viruses, he said the Valley fever study is a different illustration of how climate change could spread lesser-known pathogens. He said he is concerned by the correlation between pathogen spread and the combination of warming temperatures and increasing swings between heavy rainfall and drought.

“It isn’t just about dryness or wetness from a soil perspective, it’s the transitions between these states of extreme dryness and wetness,” Swain said. “That is precisely the kind of thing we expect to see increase. We are seeing more variable precipitation in a warming climate.”

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Categories / Environment, Health, Science

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