Established Population of Blacklegged Ticks with High Infection Prevalence for the Lyme Disease Bacterium, Sensu Lato, on Corkscrew Island, Kenora District, Ontario.

We document an established population of blacklegged ticks, , on Corkscrew Island, Kenora District, Ontario, Canada. Primers of the outer surface protein A () gene, the flagellin () gene, and the flagellin B () gene were used in the PCR assays to detect sensu lato (s.l.), the Lyme disease bacterium. In all, 60 (73%) of 82 adult , were infected with s.l. As well, 6 (43%) of 14 unfed nymphs were positive for s.l. An larva was also collected from a deer mouse, and several unfed larvae were gathered by flagging leaf litter. Based on DNA sequencing of randomly selected amplicons from six nymphal and adult ticks, primers for the flagellin () and flagellin B () genes reveal the presence of sensu stricto (s.s.), a genospecies pathogenic to humans and certain domestic animals. We collected all 3 host-feeding life stages of in a single year, and report the northernmost established population of in Ontario. Corkscrew Island is hyperendemic for Lyme disease and has the highest prevalence of s.l. for any established population in Canada. Because of this very high infection prevalence, this population of has likely been established for decades. Of epidemiological significance, cottage owners, island visitors, outdoors enthusiasts, and medical professionals must be vigilant that s.l.-infected on Corkscrew Island pose a serious public health risk.