Borrelia miyamotoi is a newly described emerging pathogen transmitted to people 
by Ixodes species ticks and found in temperate regions of North America, Europe, 
and Asia. There is limited understanding of large scale entomological risk 
patterns of B. miyamotoi and of Borreila burgdorferi sensu stricto (ss), the 
agent of Lyme disease, in western North America. In this study, B. miyamotoi, a 
relapsing fever spirochete, was detected in adult (n=70) and nymphal (n=36) 
Ixodes pacificus ticks collected from 24 of 48 California counties that were 
surveyed over a 13 year period. Statewide prevalence of B. burgdorferi sensu 
lato (sl), which includes B. burgdorferi ss, and B. miyamotoi were similar in 
adult I. pacificus (0.6% and 0.8%, respectively). In contrast, the prevalence of 
B. burgdorferi sl was almost 2.5 times higher than B. miyamotoi in nymphal I. 
pacificus (3.2% versus 1.4%). These results suggest similar risk of exposure to 
B. burgdorferi sl and B. miyamotoi from adult I. pacificus tick bites in 
California, but a higher risk of contracting B. burgdorferi sl than B. miyamotoi 
from nymphal tick bites. While regional risk of exposure to these two 
spirochetes varies, the highest risk for both species is found in north and 
central coastal California and the Sierra Nevada foothill region, and the lowest 
risk is in southern California; nevertheless, tick-bite avoidance measures 
should be implemented in all regions of California. This is the first study to 
comprehensively evaluate entomologic risk for B. miyamotoi and B. burgdorferi 
for both adult and nymphal I. pacificus, an important human biting tick in 
western North America.