'Valley fever' fungus surging northward in California as climate changes

Workers across California are grappling with yet another climate change-induced threat: a rapidly spreading fungus that can land its unsuspecting victims with prolonged flu-like symptoms, or far worse.

The culprit is a soil-dwelling organism called coccidioides, which is now spreading the disease coccidioidomycosis — known as “Valley fever” — farther and farther north of its southwest origins. Rather than spreading from person to person, Valley fever results from the direct inhalation of fungal spores — spores climate change is now allowing to flourish in new places.

While the disease was previously concentrated in California’s lower San Joaquin Valley and in parts of Arizona, its presence has surged over the past 20 years, according to a report from California’s Office of Environmental Health Hazard Assessment (OEHHA).

“Over the past two decades, California has been seeing dramatically increased incidence in certain areas that are sort of at the fringe of what was historically thought to be the endemic territory,” Jennifer Head, an epidemiologist at the University of California, Berkeley School of Public Health, told The Hill.

The annual incidence of reported cases in 2001 was 4.3 per 100,000 people, according to OEHHA figures. In 2021, it had risen to 20.6 per 100,000 people.

Although most people who contract Valley fever experience self-limited respiratory symptoms that last for a few weeks to months, about 5 to 10 percent exhibit more serious health consequences, according to the Centers for Disease Control and Prevention.

Those complications include long-term problems in the lungs, as well as infection spread to the bones, joints or central nervous system.  

“When an individual inhales the spore, it takes about seven to 21 days to show symptoms,” Head said.

Most likely to inhale the fungus are farmers, field workers and construction crews — or “anyone who is outside and interacting with the soil,” according to Head.

The California Department of Public Health has also identified disproportionate spread among Hispanic and Latino residents, with 47.4 percent of cases occurring in these communities, which only represent 37.3 percent of California’s total population.

Meanwhile, residents of Black or Filipino background, as well as pregnant individuals, older adults and people with weakened immune systems have a greater risk of contracting severe disease, the OEHHA noted.

Two species of soil-dwelling coccidioides can cause Valley fever — one particular to California and the other primarily in Arizona and other parts of the southwestern U.S. and Central and South America, per the OEHHA.

While the “Valley” in Valley fever refers to California’s Central Valley, cases have increasingly been detected in both adjacent counties and in locations farther north over the past decade, according to the OEHHA.

“As the climate is warming and places are getting drier, these areas now have enough heat and enough dryness for the dispersion of the spores to take place,” Head told The Hill.

During periods of drought, dust and particulate matter that contain coccidioides spores can spread deeper into the soil and lay dormant far underground, the OEHHA noted. But when rain returns, that dormant fungus becomes active and plentiful.

“Change is really not about consistently hot or cold or wet and dry,” Shaun Yang, a clinical microbiologist at UCLA Health, told The Hill. “It's really going from one extreme to the other extreme, very rapidly.”

This type of weather shift “creates a perfect moment for cocci,” he added, using another common name for Valley fever.

“Earlier this year, we had so much rain, it just flooded everywhere, and then we really had three or four months of no rain,” Yang said. “And now suddenly, we have this tropical storm that [came] out of nowhere.”

With a few dry months now in the forecast, he said the state is seeing a pattern of “very extreme, hot and cold, wet and dry.” 

Head echoed Yang’s sentiments that when a wet period follows drought, the effect on coccidioides spread is even more powerful.

“We would expect that this wet winter did give the fungus enough moisture to grow,” she said.

Coccidioides can grow in soil — developing long branching chains underground before fragmenting in the soil into individual spores, according to Head.

Those spores can then become airborne either via wind, erosion of the soils or via direct digging in the ground, she explained.

Head and her colleagues showed how drought conditions are exacerbating the transmission of coccidioides in an October 2022 report, analyzing more than 81,000 Valley fever surveillance records amassed by state and local agencies over two decades.

They found that in 2016-2017, two wet years that followed a 2012-2015 drought, there were almost 2,500 excess cases of Valley fever that could be attributed to the impacts of drought on disease spread.

Scientists have two main hypotheses as to why coccidioides can thrive during drought — the first being that potential competitors in the soil die off, according to Head.

“But coccidioides is pretty hardy, so it's maybe just hanging out in the soil and in a spore form and then when the rains do return, it can grow without as much competition,” she said.

A second hypothesis cited by Head is that the fungal growth follows the life cycle of rodents, which provide nutrients to the soil through their skin, hair, nails and dead bodies.

“When wet conditions return following a drought, the rodent population sort of skyrockets back up again,” Head added.

In addition to the impacts of drought, increased winds — driven by climate change — and wildfire could also be inducing spore spread, according to the OEHHA. Outbreaks have occurred among wildland firefighters, particularly those engaged in activities that cause soil disruption, per the research.

With climate change experts also forecasting an easier start to the summer in the years to come, the OEHHA report likewise highlighted “the potential for an extension of Valley fever season, leaving residents and summer visitors in endemic areas more vulnerable to infection for longer periods of time.”

As the coccidioides fungus — and therefore Valley fever — spreads north, an added complication is the fact that physicians outside of California may not be as familiar with the illness, according to Yang.

“Until recently, maybe until five years ago, cocci has been really seen as very endemic only to Southwestern states,” he continued. “It’s very unfamiliar for doctors in other areas such as more northern or more mountain areas.”

But Yang said it’s possible that the fungus could spread to “the whole west side of the United States,” or west of Mississippi.

States like Montana, Idaho and Washington — which has already experienced some incidence of Valley fever — could see increased cases in the future, according to Yang.

Washington “had a very dry year last year and they also have a lot of farms, so all of these are perfect conditions for cocci,” he said.

While elements of coccidioides spore spread are unique among other microorganisms, Yang said he also views the fungus and the disease it causes as representative of other climate change-driven illnesses on the rise.

“For almost 100 years, we didn't really have locally transmitted malaria, and now we are seeing malaria being found in Florida locally transmitted,” he said.

Similarly, he said “it’s probably just a matter of time” until mosquitoes begin spreading Dengue fever — considered a tropical illness — in the U.S. as well.

Yang also identified the bacteria B. pseudomallei — which causes a slow but fatal illness years later called melioidosis, or “the Vietnamese time bomb” — as a type of pathogen that could spread north because of climate change.

This bacterium is typically found only in Southeast Asia and Central and South America, but a few years ago the CDC identified it locally in Texas, Yang noted.

“America has to embrace these kind of changes of dealing with that in what normally would not fall in like the northern hemisphere,” he said.

Though Valley fever may be just one of many climate-driven diseases, Yang described it as “the most striking one” due to its dramatic rise in just the past decade.

“Ten years ago, we'd only see one or two cases a month, and now we see one or two cases a week,” he said of his hospital.

Also problematic is the difficulties doctors face treating Valley fever, particularly in immunocompromised patients, as antifungals effective enough to quash coccidioides don’t really exist, according to Yang.

Cocci patients at his hospital come in “on a daily basis,” and some present with invasive infections like cavitary lung lesions, neuromas or even meningitis, he noted.

While potential vaccines to combat Valley fever are under investigation, Yang said he believes that such a tool would probably only provide a limited response.

More critical, he contended, is in the development of antifungals that are safe and effective against Valley fever — prioritizing strong treatment rather than prevention.

“There's really not much you can [do to] prevent the climate change at this point,” Yang said. “This is a global problem; not even the United States alone can solve this issue — there's no way to stop it.”