Researchers from University of California San Diego and University of California, Berkeley have found that certain landscapes surrounding California communities are associated with differing rates of Valley fever, a fungal respiratory infection caused by inhaling spores carried in windblown dust.
Naturally dry, sparsely vegetated landscapes and some types of farmland were associated with higher disease rates, while other agricultural land uses were associated with lower rates. The findings also suggest that fallowing farmland may influence Valley fever rates in some circumstances. The study was published on August 21, 2026, in Science Advances.
"California is undergoing a major transformation of its agricultural landscape, and we need to understand how those changes may affect public health," said Alexandra Heaney, PhD, assistant professor at the Herbert Wertheim School of Public Health and Human Longevity Science at UC San Diego and first author of the study. "Our findings suggest that land management decisions can have consequences that extend beyond agriculture and water use. As more land is taken out of production, we may see health risks and therefore have the opportunity to build prevention measures into how California's landscapes are managed."
Valley fever is caused by Coccidioides, a fungus that grows in soil across parts of California and the Southwestern United States. When soil is disturbed by wind, farming, digging or construction, tiny fungal spores can become airborne and be inhaled, making those that breathe them sick. It's important to note that Valley fever does not spread from person to person. In California, rates of Valley fever have increased by more than 800% over the past two decades. While many people who inhale the spores never develop symptoms, those who do can experience a debilitating respiratory illness lasting weeks to months. A small share of infections become severe, spreading beyond the lungs or causing serious long-term lung disease.
The highest burden of Valley fever in California occurs in the southern San Joaquin Valley – one of the most productive agricultural regions in the world – where drought, water scarcity and groundwater regulations are driving changes in land use, including the fallowing of some farmland.
To investigate how these changes may relate to disease, researchers analyzed 65,657 confirmed Valley fever cases reported to the California Department of Public Health between April 2008 and March 2021. The cases were linked to high-resolution maps showing natural vegetation, crop types and fallowed fields across 20 California counties where Valley fever was more common. Researchers examined how land cover was associated with disease rates while accounting for factors including climate, elevation, socioeconomic conditions and employment in agriculture and construction.
The researchers found that communities surrounded by greater amounts of shrubland, barren land and grassland had higher rates of Valley fever. Some agricultural land uses were also associated with higher disease rates. Greater areas of grain and hay fields, field crops such as sorghum and soybeans, corn and cotton, and fields with multiple harvests were associated with higher Valley fever incidence. In contrast, greater areas of orchards (i.e. deciduous fruit and nut trees), fruits, vegetables, flowers and berries, and rice were associated with lower incidence.
Several factors could help explain these differences. Dry, sparsely vegetated landscapes may provide environmental conditions favorable to Coccidioides and generate dust that can carry fungal spores into nearby communities. Agricultural activities such as tilling, plowing and harvesting can also disturb soil and produce dust. Seasonal patterns of cultivation may play a role, because some crops are harvested during the summer and fall, when infectious spores are thought to be most abundant in air and soil.
Fallowing also emerged as a potential concern. While the total amount of fallow land in an area was not significantly associated with Valley fever incidence, larger areas of newly fallowed land were associated with higher disease rates. However, the relationship varied according to the land's previous cultivation history: newly fallowed land with a mixed history of crops was associated with higher incidence, while newly fallowed land previously used exclusively for some crops showed different associations.
These findings could have implications as California's Sustainable Groundwater Management Act accelerates the fallowing of agricultural land in parts of the San Joaquin Valley. The study does not establish that particular land uses directly cause Valley fever, however. Instead, it identifies associations between land characteristics and disease incidence among people living in surrounding areas. Future research should directly sample soils and fields to determine where Coccidioides grows and how agricultural practices affect its presence and movement.
In the meantime, the findings support consideration of dust-control measures on fallowed and recently retired fields, as well as enhanced surveillance and protective measures for people who work outdoors during dusty periods. These could include respiratory protection such as N95 masks and air quality warnings during high-risk seasons.
"As California adapts to water scarcity and changes in agricultural production, we must weigh the public health consequences of those transitions alongside their environmental and economic impacts," said Justin Remais, PhD, professor at the UC Berkeley School of Public Health and senior author of the study. "If we can determine how decisions like fallowing land may contribute to the release of Coccidioides spores, we can design strategies to inform and protect communities from exposure."
Link to full study: " Impacts of land use and fallowing on coccidioidomycosis incidence in California: a population-based longitudinal study "
Additional co-authors on the study include: Isabel J. Jones in the U.S. Environmental Protection Agency; Simon K. Camponuri, Amanda K. Weaver, Philip A. Collender, John Balmes, Ellen A. Eisen and John Taylor at UC Berkeley; Jennifer R. Head at UC Berkeley and University of Michigan; Gail Sondermeyer Cooksey, Seema Jain and Duc Vugia at the California Department of Public Health; Adeyemi Adebiyi at UC Merced; and Abinash Bhattachan at Texas Tech University.
The study was funded by the National Institutes of Health (grant nos. R01AI148336 and R01AI176770, recipient Remais), and the University of California Multicampus Research Programs and Initiatives (award no. MRP-17-446315, recipient Remais). This work was also partly supported by the Epidemiology and Laboratory Capacity for Infectious Diseases Cooperative Agreement through the Centers for Disease Control and Prevention (5 NU50CK000539-05-00, recipient Jones).
All authors declare they have no competing interests.
Disclaimer: The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views or opinion of the California Department of Public Health or the California Health and Human Services Agency.
