Communities surrounded by dry natural landscapes and certain kinds of farmland recorded higher rates of Valley fever than communities that were not, and newly fallowed fields showed the same pattern, according to a study published in Science Advances by researchers at UC San Diego and UC Berkeley.
The finding lands as California reshapes its agricultural map. Drought, warming, and groundwater regulations are pushing land out of production, and the study suggests that these transitions carry a public health cost that has not been accounted for alongside the environmental and economic costs.
The distinction that matters for readers is narrow but important. The study documents a higher incidence of disease in communities near these land types. It does not show that any specific land use change caused a measured increase in cases.
Landscapes That Line Up with Higher Incidence
Researchers analyzed 65,657 confirmed Valley fever cases reported to the California Department of Public Health between April 2008 and March 2021, across 20 counties where the disease is more common. They linked patients' residential locations to high-resolution maps of natural vegetation, crop types, and fallowed fields, accounting for climate, elevation, socioeconomic conditions, and employment in agriculture and construction.
Communities surrounded by greater amounts of shrubland, barren land and grassland had higher rates of Valley fever, with shrubland showing the strongest association.
The researchers offer two possible explanations, and they are not mutually exclusive. These landscapes may provide soil conditions favorable to Coccidioides growth. They may also simply generate more dust, because low vegetation cover and dry conditions make it easier for soil to be lifted into the air and carried into nearby communities.
Crop types split in both directions, a detail most often lost in summaries. Greater areas of grain and hay fields, field crops such as sorghum, soybeans, corn, and cotton, and fields with multiple harvests were associated with a higher incidence. Greater areas of orchards, fruits, vegetables, flowers, berries, and rice were associated with a lower incidence. The researchers point to soil-disturbing activities such as tilling, plowing,, and harvesting as a likely pathway and note that some crops are harvested in summer and fall, when spores are thought to be most abundant.
Fallowed Ground Carries the Clearest Policy Signal
The following result is the most directly relevant, because fallowing occurs when water becomes scarce.
The total amount of fallow land in an area was not significantly associated with the incidence of Valley fever. Larger areas of newly fallowed land were. And the relationship varied with what the land had grown before: newly fallowed land with a mixed cropping history was associated with higher incidence, while land previously used exclusively for certain crops showed different associations.
"California is undergoing a major transformation of its agricultural landscape," said first author Alexandra Heaney, an assistant professor at the Herbert Wertheim School of Public Health and Human Longevity Science at UC San Diego, in a university release. She said land management decisions can have consequences beyond agriculture and water use, and that recognizing them creates an opportunity to build prevention into landscape management.
Senior author Justin Remais, a professor at the UC Berkeley School of Public Health, put the tradeoff directly. "We must weigh the public health consequences of those transitions," he said, alongside their environmental and economic impacts.
That matters because California's Sustainable Groundwater Management Act is accelerating the fallowing of farmland in parts of the San Joaquin Valley.
Evidence Limits Worth Stating Plainly
This is an observational study and cannot establish that particular landscapes cause Valley fever. The authors say so themselves.
A second limitation is specific and significant. Cases were identified using patients' residential addresses, but people can be infected anywhere they breathe in contaminated dust, including at work, in transit, or while traveling. Someone who lives in a low-risk area and works in a high-risk one would be misclassified.
The study also does not directly measure Coccidioides in soil. It infers exposure from land cover, which is a reasonable approach at this scale but leaves the biological step unverified for any specific field. The researchers say future work should sample soils and fields directly.
None of this makes the findings unusable. They identify environmental risk factors and point to measures that could reduce infection risk, including dust control on fallowed and recently retired fields, enhanced surveillance, and respiratory protection, such as N95 masks, for outdoor workers during dusty periods.
Practical Reading for Households in Endemic Areas
Valley fever does not spread person to person. It is contracted by inhaling Coccidioides spores stirred into the air by wind, farming, digging, or construction. Many people who inhale the spores never develop symptoms. Those who do can experience a debilitating respiratory illness lasting weeks to months, and a small share of infections become severe, spreading beyond the lungs or causing long-term lung disease.
Valley fever incidence in California has risen more than 800 percent over the past two decades. The state health department reports that on average more than 1,000 Californians are hospitalized with it each year, of whom about one in ten die.
Risk is not evenly shared. Outdoor workers in agriculture and construction face the highest exposure, followed by residents of the San Joaquin Valley and southern inland counties. Severe disease is more common among people with weakened immune systems, people with diabetes, pregnant people, and adults over 60. Filipino and Black Californians have historically shown higher rates of disseminated disease.
Practical steps are modest and worth doing anyway. On windy days in endemic areas, closing windows and running air conditioning on recirculate reduces dust exposure. Wetting soil before digging cuts airborne spore release, one of several practical environmental health precautions worth building into a routine. Outdoor workers should ask employers about respiratory protection, which California workplace rules address for agricultural and construction settings.
The most useful action is clinical rather than environmental. Anyone in or returning from an endemic area who develops a cough, fever, fatigue, and chest discomfort lasting more than a week should ask a clinician directly whether Valley fever has been considered. It is frequently mistaken for community-acquired pneumonia, and testing is generally required. MedicalDaily has reported on how often that testing is skipped at hospitals in endemic areas.
Key Questions Answered
What did the study actually find? Communities surrounded by shrubland, barren land, grassland, and certain crop types had higher Valley fever incidence across 20 California counties, with shrubland showing the strongest association. Orchards, fruits, vegetables, flowers, berries, and rice were associated with lower incidence.
Does this prove farmland changes are causing Valley fever cases? No. The study is observational. It documents associations between land cover and disease incidence and cannot establish causation, and patients may have been infected away from home.
Why does fallowed land matter so much? Because water scarcity is pushing California land out of production. Total fallow area was not significantly associated with incidence, but larger areas of newly fallowed land were, particularly where the land had a mixed cropping history.
How is Valley fever spread? By inhaling Coccidioides spores from disturbed soil. It does not spread from person to person.
How serious is it? Many infections cause no symptoms. Those that do can produce illness lasting weeks to months, and a small share become severe. California hospitalizes more than 1,000 people a year on average, and about one in ten of those die.
Who faces the highest risk? Outdoor workers in agriculture and construction, and residents of the San Joaquin Valley and southern inland California. Severe illness is more common in people with weakened immune systems, diabetes, pregnancy, and adults over 60.
What should someone with symptoms do? Persistent cough, fever, fatigue, and chest discomfort lasting more than a week in or after travel to an endemic area warrants asking a clinician specifically whether Valley fever has been ruled out. Testing usually must be requested.
