Valley Fever Risk Uptick With Drought Conditions

The western United States is experiencing an uptick in Valley fever, an often asymptomatic disease, that may increase in response to climate change-related drought, according to some experts. As dry conditions continue, California residents and travelers to California’s Central Valley or Central Coast regions may need to remain vigilant for an increased risk for the fungal infection through the fall, according to a press release from the California Department of Public Health.

Symptoms of Valley fever can resemble other respiratory infections, so individuals experiencing prolonged episodes of cough, fever, or chest pain, accompanied by fatigue or tiredness, should be checked by a doctor, California public health officials noted. 

Valley fever is a lung infection that is potentially, though rarely, fatal, according to a fact sheet from the Centers for Disease Control and Prevention (CDC). The infection is caused by the Coccidioides fungus, found in the soil in the United States in the southwestern and Pacific northwest regions. The fungus also may be found in parts of Mexico, Central America, and South America.

More than 9000 cases of Valley fever were reported in 2023 in California, and more than 5000 preliminary cases had been reported in 2024 as of July 1, according to the press release. Most cases occur in the southwestern states, primarily California and Arizona, and most patients are aged over 60 years. Many cases are not reported or are misdiagnosed or never diagnosed, according to the CDC.

The increase in cases in California may be linked to drought; research has shown that drought impacts the growth of the Valley fever fungus in the soil, according to a fact sheet from the California Department of Public Health’s Center for Infectious Diseases. 

Valley fever fungus can survive in soil during drought conditions, but fewer infections occur. However, the eventual return of rain brings a rise in infections in the years after a drought and may allow the fungus to grow in more areas. If the changing climate promotes more periods of drought in California, more post-drought Valley fever infections could follow, according to the California Department of Public Health. 

Climate Effects and Clinical Considerations

“Valley fever incidence is variable based upon the amount of precipitation each year as well as factors that affect soil disturbance such as construction,” said Fariba Donovan, MD, PhD, in an interview. Given the expanding endemic area for Valley fever and the increased potential for exposure, it is conceivable to see higher incidence of the disease, said Donovan, an associate professor of medicine at the Valley Fever Center for Excellence at the University of Arizona, Tucson, Arizona. 

“Generally, Valley fever incidence rises the following year after increasing precipitation,” Donovan told Medscape Medical News. “In Arizona, the disease has bimodal incidence,” she said. The highest incidence usually occurs before each monsoon season when the soil is very dry, in mid to late spring or before winter rains, she noted. 

An increased rate of infection is related to both environmental exposure and host immunity, Donovan said. Research has shown that approximately 60% of individuals exposed to the fungus will remain asymptomatic, and 30% could develop pneumonia-like illness. Approximately 10% could develop severe disease requiring hospitalization, and 3% of these patients may go on to develop severe disseminated forms of the disease such as meningitis, she added.

Individuals who are exposed to more fungal spores in settings such as construction sites or during outdoor activities may be at increased risk for infection, as are immunocompromised individuals and those who receive immunomodulators for autoimmune disease, Donovan said.

In clinical practice, providers should be aware of Valley fever as a possible diagnosis, obtain travel history, and consider testing for it, Donovan said.

“Valley fever can mimic community-acquired pneumonia and other upper respiratory tract infections such as influenza or COVID,” she noted. “Our recommendation for clinicians who especially practice in an endemic region for Valley fever is to consider the disease in their differential diagnosis, and, if suspected, to make sure blood testing is performed,” she said. The Valley Fever Center for Excellence offers clear guidelines and flowcharts that can be helpful for providers, Donovan added.

Clinicians and health care providers, especially those in the endemic region for Valley fever, should make patients aware of the various clinical presentations of the disease, said Donovan. If the clinical presentation suggests Valley fever, the providers should consider the disease and order the appropriate tests, she said.

Donovan had no financial conflicts to disclose.