As drought, climate change, and groundwater regulations reshape California’s agricultural landscape, a new study suggests those changes could have consequences for Valley fever risk.
The study, published late last week in Science Advances, found that communities surrounded by dry natural landscapes, certain types of farmland, and recently fallowed fields had higher rates of Valley fever (coccidioidomycosis), a fungal infection caused by inhaling Coccidioides spores.
Coccidioides are common in soil in the southwestern United States, and a specific species, Coccidioides immitis, is endemic to California. Valley fever has increased by over 800% in California in the past two decades.
“California is undergoing a major transformation of its agricultural landscape, and we need to understand how those changes may affect public health,” first author Alexandra Heaney, PhD, assistant professor at the Herbert Wertheim School of Public Health and Human Longevity Science at the University of California (UC) San Diego, said in a UC news release.
Up to 5% of cases severe
Valley fever doesn’t spread from person to person. Rather, it is transmitted when Coccidioides-infested soil is stirred up by wind, farming, or construction and becomes airborne, where it can be inhaled. Most Valley fever infections cause mild respiratory illness, but an estimated 1% to 5% of cases progress to severe disease that can rarely lead to death.
For the study, researchers led by a team at UC San Diego analyzed 65,657 confirmed Valley fever cases across 20 California counties, with a mean annual incidence of five or more cases per 100,000 people and at least 10 cases per year from April 2008 through March 2021.
When the researchers compared patients’ residential locations with crop-specific land-cover data, they found that an increase of one standard deviation in shrubland was associated with a 62% higher Valley fever incidence (incidence rate ratio [IRR], 1.62). Barren land was associated with a 34% higher incidence (IRR, 1.34), and grassland with an 18% higher incidence (IRR, 1.18).
“These landscapes may offer favorable soil conditions (e.g., temperature, moisture, texture, and chemistry) for Coccidioides growth and provide habitat for burrowing mammals, which have been linked to Coccidioides presence in soil,” write the researchers. “Dust emissions from shrubland, grassland, and barren land, due to low vegetation cover and dry conditions, may also shape observed associations between these land cover types and disease incidence.”
Dust from these landscapes may enable transport of Coccidioides spores to nearby communities.
Infections are concentrated in southern inland California and the San Joaquin Valley region, where dust concentrations in the state are highest, “suggesting that dust from these landscapes may enable transport of Coccidioides spores to nearby communities,” write the researchers.
Specific crops were also associated with a higher Valley fever incidence. Areas with greater numbers of grain and/or hay fields (IRR, 1.09), field crops (eg, sorghum, soybean, sugarcane; IRR, 1.09), corn or cotton (IRR, 1.08), and double-harvested fields (those with two crop harvests each year) or grains (IRR for both factors, 1.04) were linked with higher rates of coccidioidomycosis.
“These patterns could reflect dust-generating agricultural activities (e.g., tilling and harvesting) that disturb the soil and aerosolize spores, potentially exposing both farm workers and nearby residents,” the researchers write. “Alternatively, soil under these crop types could have attributes that favor Coccidioides persistence, although direct evidence is limited.”
Newly fallowed land linked to more infections
The team also found that areas with newly fallowed land had slightly higher rates of Valley fever (IRR, 1.03). The positive association was stronger when the land had previously been used to grow a mix of crops (IRR, 1.08). In contrast, newly fallowed land was slightly negatively associated with Valley fever incidence when the land had previously been used to grow fruit and/or nuts (IRR, 0.99) or field crops (IRR, 0.97).
“Land fallowed at any point in the prior 4 years remained associated with elevated incidence, suggesting that once Coccidioides establishes in soil during a fallow period, subsequent soil disturbance could release spores and maintain higher infection rates,” the researchers write.
The observational study can’t prove that particular landscapes cause Valley fever, and patients may have been infected somewhere other than near their homes. Nevertheless, the findings offer insights on environmental risk factors for Valley fever, write the researchers, and point to possible measures that could mitigate infection risk, such as better dust control on fallowed fields and respiratory protection for outdoor workers.
“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,” Justin Remais, PhD, senior study author and professor at the University of California, Berkeley, School of Public Health, says in the release.
