Soil ecology of Coccidioides immitis at Amerindian middens in California

Abstract

Outbreaks of coccidioidomycosis and isolation of Coccidioides immitis have been reported from Amerindian middens. This study was undertaken to determine the most important ecological component(s) for the occurrence of C. immitis at archeological sites. Soils from 10 former Indian villages with no prior history of coccidioidal infection were collected and cultured. The physicochemical properties of the midden soils were compared with nonmidden soils and positive soils. The following theories for the sporadic distribution of the pathogen in the soil of the Lower Sonoran Life Zone were considered: (i) the Larrea tridentata (creosote bush) association, (ii) the preference for saline soils, (iii) isolation near rodent burrows, and (iv) animals as possible agents of dispersal. Results showed that a high percentage of the midden soils contained C. immitis, whereas none of the adjacent, nonmidden soils yielded the fungus. Physicochemical analyses revealed that the dark color and alkaline pH of the midden soils were due to past organic contamination. Repeated isolations were made from soils with low to moderate alkalinity. Alkalinity and sandy texture were consistent features of all soils in this study. However, the lack of any reports of nonsandy infested soils possibly indicates that the sandy texture and alkalinity may be factors in the distribution of this fungus. The organic content, soil parent material, and color were not important in the soil ecology. L. tridentata was not significant in the macroflora at the infested sites surveyed. Samples collected without reference to rodent burrows yielded a high percentage of recoveries. Animals, although not the major natural reservoir, cannot be ignored as possible factors in the ecology of C. immitis.