Microtus californicus scirpensis is an endangered, isolated subspecies of 
California vole. It requires water pools and riparian bulrush (Schoenoplectus 
americanus) and occupies some of the rarest habitat of any North American 
mammal. The minimally vegetated, extremely arid desert surrounding the pools is 
essentially uninhabitable for Ixodes species ticks. We describe an enzootic 
cycle of Borrelia carolinensis in Ixodes minor ticks at a site 3500 km distant 
from the region in which I. minor is known to occur in Tecopa Host Springs, Inyo 
County, eastern Mojave Desert, California. Voles were live-trapped, and ticks 
and blood samples queried by PCR and DNA sequencing for identification and 
determination of the presence of Borrelia spp. Between 2011-2013, we found 21 
Ixodes minor ticks (prevalence 4-8%) on Amargosa voles and Reithrodontomys 
megalotis. DNA sequencing of 16S rRNA from ticks yielded 99% identity to 
I. minor. There was 92% identity with I. minor in the calreticulin gene 
fragment. Three ticks (23.1%), 15 (24%) voles, three (27%) house mice, and one 
(7%) harvest mice were PCR positive for Borrelia spp. Sequencing of the 5S-23S 
intergenic spacer region and flagellin gene assigned Amargosa vole Borrelia 
strains to B. carolinensis. Ixodes minor, first described in 1902 from a single 
Guatemalan record, reportedly occurs only in the southeast American on small 
mammals and birds. The source of this tick in the Mojave Desert and time scale 
for introduction is not known but likely via migratory birds. Borrelia strains 
in the Amargosa ecosystem most closely resemble B. carolinensis. B. carolinensis 
occurs in a rodent-I. minor enzootic cycle in the southeast U.S. although its 
epidemiological significance for people or rodents is unknown. The presence of a 
tick and Borrelia spp. only known from southeast U.S. in this extremely isolated 
habitat on the other side of the continent is of serious concern because it 
suggests that the animals in the ecosystem could be vulnerable to further 
incursions of pathogens and parasites.