Coccidioidomycosis: Increasing Incidence of an “Orphan” Disease in Response to Environmental Changes

Abstract

The number of cases of reported coccidioidomycosis infection, also known as valley fever, has increased significantly in the southwestern USA since the late 1990s. The causative agent of the disease is a dimorphic, soil-dwelling fungus in the genus Coccidioides with two species C. immitis and C. posadasii that persist in dry soils in the form of arthroconidia that can become airborne when soil is disturbed. Excessive disturbance of native soils in Coccidioides endemic areas in California, Arizona, and also Mexico, mainly due to construction and agriculture, has resulted in increased dust emission in these locations. In addition, the prolonged and ongoing drought in the southwestern USA has led to an increased level of PM10 (particulate matter 10 μm or less in diameter) pollution, which resulted in an increase in the amount of airborne arthroconidia of these pathogens which in turn led to the increase in disease incidence. Because coccidioidomycosis is not prevalent in the entire USA, research to elucidate the ecology of the pathogen, medical research to develop a vaccine against the disease, as well as the search for new antifungal drugs with less negative side effects in patients never attracted significant funding in the past. As a result, coccidioidomycosis is often named either an “orphan disease” or a “neglected disease.” In particular, a vaccine to protect humans from the pathogen has been elusive to date, and both accurate diagnosis and treatment of the disease have remained difficult. Therefore, prevention of coccidioidomycosis through reasonable reduction of exposure is likely the best way to reduce disease incidence and the associated human, animal, as well as financial losses. Although the causal relationships between environmental factors and disease incidence are not well understood at this time, it is reasonable to assume that arthroconidia of Coccidioides spp. will be affected by factors that impact the organism’s life cycle as well as human exposure to airborne dust sources. The predicted climate change in the southwestern USA to a drier state in the future and increased soil disturbance due to dramatic population increase in Coccidioides endemic areas of California and Arizona will likely lead to further increase of coccidioidomycosis incidence and may even lead to the establishment of the pathogen in other areas where disease incidence has been traditionally low or nonexistent.