2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 8
Presentation Time: 10:30 AM


FISHER, F.S. and BULTMAN, M.W., U.S. Geol Survey, 520 N. Park Ave, Suite 355, Tucson, AZ 85719, ffisher@swfo.arizona.edu

Coccidioides immitis is a dimorphic soil-inhabiting fungus and the etiological agent of coccidioidomycosis (Valley Fever). The saprophytic phase of the fungus is characterized by branching segmented hyphae that form a network of mycelium in the upper 5 - 20 cm of Aridisols in the southwestern U.S. and parts of Central and South America. As the fungus matures arthroconidia (spores), 2 to 5 microns in size, are formed as barrel shaped segments of the hyphae that can be easily separated by soil disturbance (natural or anthropogenic) and consequently dispersed by the wind. If an appropriate host inhales airborne arthroconidia, primary infection may occur and the invasive phase of the C. immitis lifecycle will be initiated.

Habitat modeling of the saprophytic phase of the C. immitis life cycle is difficult due to the limited number of known growth sites. This confounds establishment of statistical relationships of the physical, chemical, and biological habitat parameters. Our habitat modeling was accomplished utilizing a spatial fuzzy system with analysis of observed properties from known C. immitis sites. Such properties included the clay content, texture, salinity, and water-holding capacity of soils in addition to the bedrock geology, elevation, slope/aspect, and vegetation density.

The fuzzy system was applied at Organ Pipe Cactus National Monument, Arizona. The result is a map depicting each cell’s favorableness for hosting C. immitis based on a scale of 0 to 1, defined as the fuzzy habitat suitability index. The fuzzy system allows modelers to change and update relationships between the variables as more is learned about C. immitis habitat. It also allows dynamic representation of climate related variables and can be used to predict changes in habitat with changing climate. When combined with USGS dust monitoring efforts in the southwest United States, the fuzzy system has the potential of being a tool for coccidioidomycosis mitigation.