Phylogeography of Borrelia spirochetes in Ixodes pacificus and Ixodes spinipalpis ticks highlights differential acarological risk of tick-borne disease transmission in northern versus southern California

The common human-biting tick, Ixodes pacificus , is the primary vector of the Lyme disease spirochete, Borrelia burgdorferi sensu stricto (ss) in western North America and has been found to harbor other closely-related spirochetes in the Borrelia burgdorferi sensu lato (sl) complex. Between 2008–2015, 11,066 adult and 3,815 nymphal I . pacificus and five adult and 144 nymphal Ixodes spinpalpis , a commonly collected wildlife tick, were collected from 42 California counties. Borrelia burgdorferi sl was detected in 1.2% and 3.8% I . pacificus adults and nymphs, respectively. Results from this study indicate genetic diversity and geographic structure of B . burgdorferi sl in California I . pacificus ticks, by sequence comparison of the16S rRNA gene, with B . burgdorferi ss, the agent of Lyme disease, found only in I . pacificus collected from the north and central coastal and Sierra Nevada foothill regions; B . burgdorferi ss was not detected in ticks tested from southern California. In contrast, Borrelia bissettiae , a member of the B . burgdorferi sl complex, was detected in both I . pacificus and I . spinipalpis , in the coastal region of both northern and southern California, but was absent from ticks in the Sierra Nevada foothills. In a similar pattern to B . bissettiae , Borrelia americana (a member of the B . burgdorferi sl complex) was detected in a single adult I . pacificus from the north coast and two I . spinipalpis nymphs from south-coastal California. This study highlights that the geographic area of Lyme disease acarological risk in California is the north-central and Sierra Nevada foothill regions of the state with little to no risk in the southern regions of the state.