A study published in The Lancet Regional Health describes coccidioidomycosis seasonality in endemic regions of California and characterizes important drivers of disease transmission.
Study: Coccidioidomycosis seasonality in California: a longitudinal surveillance study of the climate determinants and spatiotemporal variability of seasonal dynamics, 2000–2021. Image Credit: Kateryna Kon/Shutterstock.com
Background
Coccidioidomycosis is a respiratory disease predominantly caused by the fungal species Coccidioides immitis and Coccidioides posadasii. The disease is rapidly emerging in the southwestern US, as evidenced by a nearly twofold increase in annual reported cases across the United States and a more than threefold increase in California since 2014.
A few previous studies suggest that coccidioidomycosis incidence has an unimodal seasonal pattern, with a peak mostly observed in October. Some epidemiological studies suggest that wet, followed by dry climate conditions facilitate the transmission of coccidioidomycosis.
In this study, scientists have characterized the seasonality of coccidioidomycosis across endemic counties in California and determined the association between drought conditions and coccidioidomycosis seasonal periodicity and timing.
The scientists characterized seasonal patterns of coccidioidomycosis incidence by analyzing disease surveillance data in California between 2000 and 2021.
They determined the effect of drought conditions on county-level seasonality by analyzing high-resolution temperature and precipitation anomaly data.
Important observations
A total of 89,281 cases of coccidioidomycosis were reported across 17 counties from 2000 to 2021.
The annual incidence in 2,000 was 2.58 cases per 100,000 population across counties, which increased to 32.08 cases per 100,000 population in 2018. This indicated a 12 times higher incidence in 2018 compared to that in 2000.
The highest proportion of annual cases was reported in September, October, and November across counties during the study period. However, a significant variation in the monthly distribution of annual cases was observed between counties and years.
In some years, including 2010 and 2016, a stronger seasonal pattern characterized by a peak in reported cases during fall months was observed. In some years, including 2007 and 2014, an even distribution of cases was observed across the year.
Regarding annual periodicity, the study found significant 12-month periodicity across counties. This indicates sharp seasonal increases in incidence once every 12 months. However, a significant heterogeneity in seasonal strength and timing was observed across counties and years.
On average, the strongest 12-month periodicity was observed in the Southern San Joaquin Valley and Central Coast. The annual seasonal cycles in the Southern San Joaquin Valley and the Southern Inland regions occurred earlier than those in coastal and northern counties. However, the county-level timing of annual cycles became more aligned by the end of the study period.
Regarding potential drivers of disease transmission, the study found that drought conditions are significantly associated with a strong attenuation of the annual seasonal cycle. A marked increase in seasonal peaks was observed 1 – 2 years after a drought.
Study significance
The study finds that the monthly incidence of coccidioidomycosis peaks across California counties between September and November, at the end of the dry season. In contrast, the incidence remains the lowest between April and June, at the end of the wet season.
Existing evidence indicates that the incubation period of coccidioidomycosis is about 7 – 12 days, and the duration between symptom onset and diagnosis is about 2 – 5 weeks. Considering these estimates, the current study findings suggest that the highest exposures to coccidioidomycosis-causing fungal species are likely to occur between July and September, and the lowest exposures are likely to occur between February and April.
The study finds that climatic conditions significantly contribute to coccidioidomycosis transmission. The findings indicate that unusually wet conditions in the prior wet season (November to March), as well as unusually dry conditions during the concurrent dry season (April to October), are associated with more pronounced seasonal peaks of coccidioidomycosis incidence.
Epidemiological and experimental studies have shown that wet conditions can promote the growth of Coccidioides immitis in soils, and concurrent dry conditions can facilitate fungal dispersion with dust emissions. These observations support the current study findings that unusually wet conditions at the end of the transmission year are associated with more pronounced seasonal peaks.
Conclusion
Overall, the study indicates that drought conditions suppress coccidioidomycosis seasonality. However, a transition from drought to wet conditions can lead to more pronounced seasonal peaks in California.
Due to global climate change, California is anticipated to experience more frequent droughts followed by heavy rainfall in the coming years. This can potentially affect both incidence rates and seasonal patterns of coccidioidomycosis.
As the scientists mentioned, further research is essential to understanding the mechanistic connections between extreme climate events and coccidioidomycosis transmission, which will inform accurate disease prediction systems and mitigate future disease burdens.
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