Valley Fever Rising Fastest in Northern Arizona, 2005–2022

Coccidioidomycosis, or Valley fever, is a fungal infection caught by breathing in spores of Coccidioides, a fungus that lives in soil. It usually causes a self-limited illness resembling pneumonia, but about 5% of patients develop disseminated disease with long-term effects and under 1% have severe lung complications. Treatment depends on severity, the form of the disease and the patient's immune status and other conditions; it can mean 3–6 months of an oral azole drug, usually fluconazole or itraconazole, and a year or more — even lifelong — for disseminated or chronic infection.

The disease is endemic to the southwestern United States, and Arizona and California report more than 95% of U.S. cases. Since 2000, incidence has risen sharply across the Southwest along with increasing aridity, warmer temperatures and swings between extremely wet and dry conditions. In Arizona, temperatures peak in July and rain falls mainly in the July–September monsoon and in December–March; Valley fever usually peaks in October–January.

Statewide, Arizona's incidence had risen, but no one had examined how it differed among the state's distinct climate regions. This MMWR report did, using cases reported to the Arizona Department of Health Services.

How the study worked

Laboratory-confirmed cases from 2005 through 2022 were assigned by census tract of residence to six ecological regions; cases without residence information, including those from tribal lands, were left out. Incidence was modeled for 3-year periods to steady the numbers in small regions. Because a major commercial laboratory changed how it tested for and reported Valley fever in 2009 and again in late 2012 — likely inflating cases from 2009 to 2012 — the report compared 2020–2022 with 2005–2007 and 2014–2016, periods unaffected by those changes.

Statewide

Arizona reported 152,446 cases over the period; 126,982 (83.3%) had residence information and were included.

• Incidence roughly doubled. The 2020–2022 average of 145.02 per 100,000 was 2.27 times the 2005–2007 average (63.73) and 1.76 times the 2014–2016 average (82.28).
• It was lowest in 2005 (58.59 per 100,000), climbed to 188.33 in 2011, fell to 62.61 in 2014, and then generally rose, reaching 123.76 in 2022.
• On average, the share of each year's cases peaked in November.

Changes in Valley fever incidence by region, and annual incidence by year and region, Arizona, 2005–2022. CDC figure.

By region

The Sonoran Desert in the southwest — the most populous region and the hottest, with average daily temperatures of 69.71°F–72.43°F — reported more than 95% of cases and had the highest median annual incidence, 112.13 per 100,000. The Plateaus of the northeast, cooler and wetter than the deserts, had the lowest, 19.91 per 100,000, and fewer than 0.5% of cases. The Mojave Desert in the north was the driest region, and the midland Arizona/New Mexico Mountains the wettest.

But the fastest relative growth came in the north:

Every other region also increased. Compared with 2014–2016, all regions' 2020–2022 incidence was 1.40 to 3.13 times higher, with the Mojave Desert highest (3.13) and the Plateaus next (2.16).

Monthly share of annual cases statewide, with regional precipitation and temperature, Arizona, 2005–2022. CDC figure.

Why the north?

The Sonoran Desert, where the disease is hyperendemic, contributed the most new cases. The authors suggested several possible reasons for the steep rise farther north:

• More susceptible people. Migration into Arizona grew most among older adults, who are more likely to have symptoms, seek care and be diagnosed.
• More awareness and reporting.
• More travel from low- to high-endemic areas for work, recreation or temporary moves; a 2022 study found that fungal isolates from northern Arizona patients clustered with those from Maricopa and Pima counties.
• Spread of the fungus into cooler climates, consistent with a California study showing disproportionate increases in cooler, wetter regions outside the hot, dry southern San Joaquin Valley.

More research is needed to sort out the roles of climate and other factors.

Limitations

• Incidence is underestimated because of missed diagnoses, the many mild cases that never see a doctor, and the exclusion of cases from tribal lands; yet the 2009–2013 testing changes may have inflated counts, and rising awareness may explain some of the increase.
• Small populations in some regions make rates unstable.
• Cases were counted where people lived, not where they were exposed.
• Rates were not age-adjusted because patient ages were missing.

What to do

No vaccine exists, and where the fungus lives is poorly understood. Including historically low-incidence regions in public messaging can raise awareness among the public and clinicians, prompting earlier diagnosis and treatment. It may also encourage dust control — planting vegetation or selective irrigation in towns, work sites and homes — and the use of N-95 masks in especially high-risk situations where feasible.

Sources

• Kruger SE, Ruberto I, Williamson T, Remais JV, Heaney AK, Head JR. "Regional Increases in Incidence of Coccidioidomycosis (Valley Fever) — Arizona, 2005–2022." MMWR 2026;75(6).