Valley fever, or coccidioidomycosis, is caused by a fungus that thrives in moist soils and becomes airborne during drought. Its spores are easily inhaled, leading to infection. Climate change is creating the perfect conditions for it in the Western United States, with increasing heavy rains followed by prolonged drought. A new study by University of California, Davis, researchers shows that dogs, who are also susceptible to the disease, can help us understand its spread.

“Dogs are sentinels for human infections,” said lead author Jane Sykes, professor of small animal internal medicine at the UC Davis School of Veterinary Medicine. “They can help us understand not just the epidemiology of the disease but they’re also models to help us understand the disease in people.”

Valley fever is common in animals, especially dogs that dig in dirt. The study, published in the Journal of Infectious Diseases, examined nearly 835,000 blood antibody tests from dogs that had been tested for the infection across the country between 2012 and 2022. Nearly 40% of them tested positive.

Valley fever spread

Sykes, along with colleagues at UC Berkeley, also mapped positive results by location and found valley fever in dogs spread from just 2.4% of U.S. counties in 2012 to 12.4% in 2022.

“We were also finding cases in states where valley fever is not considered endemic,” said Sykes. “We should be closely watching those states because there could be under-recognition of the emerging fungal disease in humans.”

The Centers for Disease Control and Prevention receives 10,000 to 20,000 reports of human cases every year, but the actual number of cases may be at least 33-fold higher. Many states do not require the reporting of human cases.  The CDC considers valley fever endemic is parts of six states, including Arizona, California, Nevada, New Mexico, Texas and Utah. The study found valley fever in dogs those states, but also in Oregon, Washington, Idaho, Montana, Wyoming and Colorado.

Sykes said the sheer number of cases cannot be explained by dogs visiting other states, since dogs travel far less frequently than humans. Further, the dog cases correlated with human cases, including in known valley fever “hot spots” in Arizona, California, Texas, New Mexico and Nevada.

States with highest number of cases

Arizona accounted for 91.5% of positive tests, followed by California (3.7%), Nevada, Utah, Colorado, New Mexico, and Texas (2.6% combined), Washington, Oregon and Idaho (0.6% combined). The remaining states (1.3% combined) reported far fewer positive results.

Arizona also had the highest incidence rate of any other state. Rates were 100-fold of those in California, Nevada and New Mexico.

Every state with more than 0.50 tests per 10,000 households a year showed an increasing number of cases of valley fever per 10,000 households in dogs over the study period.

Dogs as models for human disease

Dog breeds that like to dig are more at risk of getting the disease. That includes most medium-to-large dogs as well as terriers. Dogs also exhibit some of the same signs of valley fever as humans. They can have a cough as the infection develops in their lungs. The fungus can also spread to the bones, brain and skin and require lifelong antifungal injections. Dogs can also die from the disease.

Sykes suggested that dogs are an under-recognized model for understanding valley fever. By learning more about valley fever in dogs, scientists may discover new tests or treatments for the disease in humans. They may also help prevent misdiagnosis or undiagnosed disease in humans.

Other authors of the study include George Thompson III of UC Davis School of Medicine and Simon Camponuri, Amanda Weaver and Justin Remais of UC Berkeley’s School of Public Health.

Funding for the research performed at UC Berkeley came from the National Institute of Allergy and Infectious Diseases of the National Institutes of Health.