Active and passive surveillance and phylogenetic analysis of Borrelia burgdorferi elucidate the process of Lyme disease risk emergence in Canada

Abstract

Background: Northward expansion of the tick Ixodes scapularis is driving Lyme disease (LD) emergence in Canada. Information on mechanisms involved is needed to enhance surveillance and identify where LD risk is emerging.

Objectives: We used passive and active surveillance and phylogeographic analysis of Borrelia burgdorferi to investigate LD risk emergence in Quebec.

Methods: In active surveillance, we collected ticks from the environment and from captured rodents. B. burgdorferi transmission was detected by serological analysis of rodents and by polymerase chain reaction assays of ticks. Spatiotemporal trends in passive surveillance data assisted interpretation of active surveillance. Multilocus sequence typing (MLST) of B. burgdorferi in ticks identified likely source locations of B. burgdorferi.

Results: In active surveillance, we found I. scapularis at 55% of sites, and we were more likely to find them at sites with a warmer climate. B. burgdorferi was identified at 13 I. scapularis-positive sites, but infection prevalence in ticks and animal hosts was low. Low infection prevalence in ticks submitted in passive surveillance after 2004-from the tick-positive regions identified in active surveillance-coincided with an exponential increase in tick submissions during this time. MLST analysis suggested recent introduction of B. burgdorferi from the northeastern United States.

Conclusions: These data are consistent with I. scapularis ticks dispersed from the United States by migratory birds, founding populations where the climate is warmest, and then establishment of B. burgdorferi from the United States several years after I. scapularis have established. These observations provide vital information for public health to minimize the impact of LD in Canada.

Figure 1

The number of I. scapularis ticks submitted in passive surveillance in Quebec, number of clinics (mostly veterinary practices) that participated in passive surveillance, and mean number of ticks submitted per clinic, by year.

Figure 2

Locations from which ticks submitted in passive surveillance in Quebec during 1990–2008 were obtained. Also shown is the spatial extent of a cluster of ticks collected during 2004–2008 that had a low probability of being infected with B. burgdorferi. Green circles indicate locations where ticks were evaluated by MLST analysis.

Figure 3

Study sites for active surveillance for I. scapularis establishment. In two cases, circles indicate the overlap of two populations. The size of the circles represents the tick population index for I. scapularis population establishment calculated from the number of ticks and the number of instars collected, as described in the text. Also shown is the spatial extent of a cluster of sites positive for I. scapularis.