The argasid tick Ornithodoros coriaceus (Koch) is the only confirmed vector of epizootic bovine abortion (EBA) in the United States. The disease and its tick vector have historically been reported in the foothills of the Sierra Nevada Mountains and coast ranges of California. In the past two decades, the range of EBA has apparently expanded into southern Oregon and northern Nevada. Possible explanations for this expansion include 1) increased recognition and reporting of EBA in these regions; 2) widespread movement of tick-infested and EBA-infected hosts with subsequent colonization of these regions by infected ticks; and 3) widespread movement of the EBA agent, independent of tick movements, into extant tick populations in these new regions. The current study was performed to evaluate these hypotheses by examining patterns of variability in a 420-bp segment of the 16S mitochondrial rDNA gene sequence among 210 O. coriaceus individuals from 14 sites in California, Oregon, and Nevada. Sixty-three unique haplotypes were identified in the ticks tested, with 84% of the sequence variation attributable to among-population variation and 16% to within-population variation. A majority of the haplotypes were unique to their particular collection site, whereas only four collection sites shared haplotypes. Overall, very little evidence of gene flow among tick populations was detected, making it unlikely that widespread tick movement had introduced O. coriaceus and the EBA agent into new regions.
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