Climate and Coccidioidomycosis incidence fluctuations in Kern County, California: a review

Coccidioidomycosis (valley fever) is a fungal infection found in the southwestern US, northern Mexico, and some places in central and South America. The fungi which cause it (Coccidioides immitis and Coccidioides posadasii) are normally soil-dwelling but, if disturbed, become air-borne and infect the host when their spores are inhaled. It is thus natural to surmise that weather conditions which foster the growth and dispersal of Coccidioides must have an effect on the number of cases in the endemic areas. This paper reviews our attempts to-date at quantifying this relationship in Kern County, California (where C. immitis is endemic). We have examined the effect on incidence resulting from precipitation, surface temperature, and wind speed. We have performed our studies by means of a simple linear correlation analysis, and by a generalized auto regressive moving average model. Our first analysis suggests that linear correlations between climatic parameters and incidence are weak; our second analysis indicates that incidence can be predicted largely by considering only the previous history of incidence in the county – the inclusion of climateor weather-related time sequences improves the model only to a relatively minor extent. Our work therefore suggests that incidence fluctuations (about a seasonally-varying background value) are related to biological and/or anthropogenic reasons, and not so much to weather or climate anomalies.