Canine Serology Sheds Light on Underreported Human Coccidioidomycosis Spread

Canine serologic testing data indicate a broader geographic distribution of coccidioidomycosis (Valley fever) in the United States than previously recognized, suggesting that current human surveillance may underestimate the true burden of the fungal disease.

Researchers analyzed 834,899 serologic test results from dogs across the US between 2012 and 2022, using data from all known diagnostic laboratories performing anti-Coccidioides antibody testing in dogs. The overall seropositivity rate was 37.6%, with substantial variation by state. Among states with ≥0.5 tests per year per 10,000 households, average seropositivity ranged from 35.4% in Texas to 74.1% in Montana.

Estimated annual canine incidence per 10,000 households was highest in Arizona (87.8), followed by New Mexico (.89), Nevada (.79), California (.75), and Montana (.63). Positive correlations were observed between canine and human incidence in Arizona (ρ = .75) and California (ρ = .65), based on surveillance data from 2012 to 2022.

The distribution of seropositive dogs expanded significantly over the past decade, increasing from 76 counties in 2012 (2.4% of US counties) to 390 counties in 2022 (12.4%). This trend may reflect both the spread of Coccidioides spp. and improvements in veterinary testing availability.

Due to challenges such as inconsistent soil detection, underdiagnosis, and non-mandatory human case reporting, canine seroprevalence may serve as a valuable proxy for environmental risk. The findings support the need for enhanced human diagnostic capacity and clinician awareness to improve recognition of this emerging fungal infection.

The investigators, including Jane E Sykes, BVSc(Hons), MPH, MBA, PhD, from school of veterinary medicine, University of California-Davis, and Simon K Camponuri, MPH, division of environmental health sciences, School of Public Health at Berkeley, emphasize the value of canine seroprevalence as a proxy for environmental risk, given challenges such as inconsistent soil detection, underdiagnosis, and non-mandatory human case reporting.

"The dog serologic data analysis showed that the incidence in dogs closely follows that in humans, at least in California and Arizona," said Sykes. "We are currently performing additional studies with the data to extrapolate where unrecognized disease might be occurring in humans-both in locations where reporting is required and in other parts of the country."

Sykes further explained that stronger reporting mechanisms for dog cases could improve monitoring of the disease's spread in both dogs and people. "Currently there is no requirement that dog cases be reported and no infrastructure to support such an activity. If there was a requirement for reporting of cases in dogs, we would be better prepared to monitor the spread of the disease in both dogs and people."

Sykes noted that the data could also inform occupational health strategies, "Human health care providers could be educated on the disease, and there may be implications for occupational exposure reduction, such as education of construction workers, military personnel, agricultural workers, or wildfire fighters regarding the need for PPE and dust avoidance," she said.

With the increasing distribution of seropositive dogs, Sykes stressed the importance of enhancing diagnostic infrastructure and provider knowledge in regions with expanding risk for coccidioidomycosis. "Educational materials on Valley fever and risk factors for Valley fever for providers, mandatory training in highly endemic regions for workers with occupational exposure, raising awareness with awareness days/weeks etc.," she added. "Another thing that is badly needed is inclusion of companion animals in census data, because currently we lack a good denominator for studies like this one."

Finally, Sykes highlighted the potential of a One Health approach, integrating animal and human health data, to help address emerging infectious diseases. "When all laboratories are willing to pitch in their results, we can overcome the spatial bias that occurs when only one laboratory participates," she said. "There are so many other emerging diseases where companion animals can be sentinels for human diseases."

This study highlights the need for improved surveillance and diagnostics to better track Valley fever, as canine serologic data reveals the disease is more widespread in the US than previously recognized. Current human surveillance may underestimate the true burden, particularly in the southwestern US.

Reference

Jane E Sykes, Simon K Camponuri, Amanda K Weaver, George R Thompson, Justin V Remais, Use of dog serologic data for improved understanding of coccidioidomycosis: A One Health approach, The Journal of Infectious Diseases, 2025;, jiaf184, https://doi.org/10.1093/infdis/jiaf184https://doi.org/10.1093/infdis/jiaf184