Valley fever from desert dust is spreading as the American Southwest dries and warms

A dust cloud kicked up by wind, construction, or an off-road vehicle in the desert Southwest can carry more than grit. It can carry the spores of Coccidioides, a soil-dwelling fungus that causes Valley fever, a lung infection that federal health officials say has been diagnosed far more often over the past quarter-century even as the true number of cases remains badly undercounted. The trend lines up with a region that has grown measurably hotter and drier, conditions researchers say help the fungus disperse into the air people breathe.

Spores From Disturbed Soil Cause an Infection That Mimics Pneumonia

Valley fever, formally called coccidioidomycosis, develops when someone breathes in spores from Coccidioides, a fungus that lives in soil across the southwestern United States, south-central Washington state, and parts of Mexico and Central and South America. Many people who inhale the spores never get sick at all, and those who do typically develop symptoms similar to a lingering case of pneumonia: fatigue, cough, fever, headache, night sweats, muscle or joint aches, and sometimes a rash on the upper body or legs, according to the CDC’s overview of Valley fever. Most people recover on their own within weeks to months without antifungal treatment. Roughly 5 to 10 percent go on to develop serious or long-term lung problems, and in about 1 percent of cases the infection spreads beyond the lungs to the skin, bones, joints, or brain, where it can cause meningitis. Because the symptoms so closely resemble bacterial or viral pneumonia, Valley fever is frequently misdiagnosed, which delays the antifungal treatment that severe cases need. The infection does not spread from person to person or between people and their pets, though dogs and other animals can independently inhale spores from the same contaminated soil and develop their own cases.

Reported Cases Have Multiplied Roughly Tenfold Since the Late 1990s

CDC’s national surveillance data shows just how far the reported case count has climbed. States reported 2,271 cases of Valley fever in 1998; by 2023 that provisional total had reached 21,037, with roughly 20,000 cases now typical in an average year, according to the CDC’s statistics on reported Valley fever cases. Arizona and California account for the large majority of that total, and the rise has not been perfectly linear: Arizona cases spiked past 16,000 in 2011 before falling back and climbing again, a pattern the agency attributes partly to weather swings and partly to changes in how well cases get detected and reported. Even those totals almost certainly understate the real toll. CDC’s own modeling estimates the true annual burden at 206,000 to 360,000 symptomatic cases nationwide, 10 to 18 times the number formally reported, along with 18,000 to 28,000 hospitalizations and 700 to 1,100 deaths a year, several times the roughly 200 coccidioidomycosis-associated deaths that show up on death certificates annually. In the Phoenix and Tucson metropolitan areas, the fungus is estimated to cause 15 to 30 percent of all community-acquired pneumonia cases, a share large enough that local clinicians are trained to consider it routinely.

The Fungus Has Already Confirmed a Foothold Outside Its Traditional Range

Valley fever has historically been described as endemic to Arizona, California, Nevada, and New Mexico, but the fungus’s known footprint has already expanded once in documented, genetically confirmed fashion. In 2013, public health investigators added south-central Washington state to the list of areas with local transmission after several residents developed Valley fever with no recent travel to any previously known endemic area. Whole genome sequencing performed on samples from one patient and from soil at the likely exposure site showed the fungal samples were genetically identical, confirming that Coccidioides was reproducing and spreading in the local environment rather than arriving with a traveler, according to the CDC’s page on areas with Valley fever. The same page notes that outbreaks, while rare overall, have occurred in places scientists did not previously expect the fungus to be able to survive, underscoring how incomplete the current range maps likely are.

A Cycle of Wet Winters and Dry Summers Feeds the Fungus’s Spread

Coccidioides follows a boom-and-bust growth pattern tied directly to weather. The fungus grows in soil after periods of heavy rainfall, then, once conditions turn hot and dry, it disperses into dust and becomes airborne, ready to be inhaled. CDC has documented measurable increases in Valley fever cases in Arizona and California during hot, dry stretches that follow wetter periods, and the agency states plainly that scientists are actively studying how a warming, drying climate is reshaping the fungus’s habitat and its ability to spread into new territory. Soil testing for the fungus remains largely a research tool rather than a predictive one, since Coccidioides can be present in soil without ever becoming airborne, which is part of why public health officials rely on tracking human cases and weather patterns together rather than mapping the fungus directly. Activities that disturb large volumes of soil, including construction, farming, off-road driving, and windstorms, have each been linked to past clusters of cases, which is why outbreaks tend to follow specific disruptive events rather than spreading evenly across an entire endemic region.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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