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Coccidioidomycosis is an important human fungal infection of American deserts and nearby semi-arid regions with highly endemic areas distributed along the United States-Mexico border. Despite the increasing incidence in the last 20 yr, reports of positive isolations of the causal agent, Coccidioides spp. from environmental samples have been scarce. To resolve this paradox, it is extremely important to first identify the fundamental ecological niche of this fungus. Soil samples (n = 90) including those from heteromyids’ active burrows, latrines and other mammals’ dens were collected using an oriented sampling method from areas of Baja California, Mexico previously predicted as putative endemic "hotspots". The total genomic DNA obtained from the collected samples was subjected to a nested PCR followed by a diagnostic PCR designed to amplify the internal transcribed spacer (ITS) 2 region of Coccidioides spp. From the 42 amplicons obtained and sequenced (37 from Valle de las Palmas (VDP) and five from San Jose de la Zorra (SJZ)), 32 were confirmed to belong to Coccidioides spp. No Coccidioides spp. were found in soils collected in Ensenada.

VDP and SJZ have different soil characteristics but share a Mediterranean climate having less than 250 mm of precipitation per year, as well as a dry period of at least 6 months. The development of Coccidioides spp. is probably related to the structure of the microbial population adapted to these conditions in the semi-arid-mediterranean ecotone. (C) 2011 Elsevier Ltd and The British Mycological Society. All rights reserved.