Increasing Prevalence of Borrelia Burgdorferi Sensu Stricto–Infected Blacklegged Ticks in Tennessee Valley, Tennessee, USA

Graham J. Hickling; Janetta R. Kelly; Lisa D. Auckland; Sarah A. Hamer

Disclosures

Emerging Infectious Diseases. 2018;24(9):1713-1716. 

In This Article

The Study

In late 2017, we sampled host-seeking I. scapularis ticks at 70 forested sites in 26 low-elevation counties in the upper Tennessee Valley (Figure 1, panel B). To find tick habitats (hardwood or conifer forests <800 m in elevation) accessible for sampling (i.e., trails through public forests or margins of public roads through private forests), we reviewed Google Earth (https://www.google.com/earth/) satellite imagery. We sampled each site once during the peak of adult I. scapularis tick activity (late October–January). We recorded site elevation and geo-coordinates and collected host-seeking ticks using a standardized drag-cloth method; in brief, we dragged a 1-m[2] white corduroy cloth across leaf litter and checked every 10 paces for attached ticks. We dragged cloths 30–60 minutes per site and described tick tallies as number collected per hour to correct for variations in effort per site. We did not conduct drag-cloth collections during periods of rain, strong wind, low air temperatures (<8°C), or low relative humidity (<40%).

We placed ticks in 70% ethanol, identified species using a morphologic key,[6] and tested ticks for Borrelia spirochete infection by DNA extraction and quantitative multiplex real-time PCR using differential probes targeting the 16S rDNA of Lyme group Borrelia and relapsing fever group Borrelia.[7] We then subjected a random subset of negative samples and samples positive by the 16S assay (maximum 6 samples/site) to PCR amplification of the 16S–23S rDNA intergenic spacer region[8] and Sanger sequencing for species-level identification.

No previous tick drag-cloth counts existed for the counties in our survey area, except for a 1,050-m transect of land in a forest in Anderson County, which we have drag-cloth sampled annually each December since 2012. To assess a trend in adult I. scapularis tick abundance, we applied linear regression modeling to the tick tallies from that transect of land.

In late 2017, we collected 479 adult I. scapularis ticks from 49 of 70 sites in the upper Tennessee Valley. Two adult Amblyomma americanum ticks collected during the survey were excluded from analysis. We detected I. scapularis ticks in all 26 counties surveyed, 23 of which met the criterion used by Eisen et al. for established I. scapularis populations (Figure 1, panel B).[3] Site elevations were 210–730 m; the highest elevation at which I. scapularis ticks were found was 570 m. The average number of adult ticks collected per hour during drag-cloth surveys was 8.8 (range 0–48). At the Anderson County site that had been drag-cloth sampled annually, a highly significant increasing trend in I. scapularis ticks was evident (p = 0.003; Figure 2); the count in 2017 (24.8 ticks/hour) was 3.5× higher than that in 2012.

Figure 2.

Six-year trend in adult Ixodes scapularis tick counts at Forest Resources Research and Education Center (36.00°N, 84.22°W; elevation 298 m), Anderson County, Tennessee, USA, 2012–2017. We collected host-seeking I. scapularis adult ticks by drag-cloth sampling vegetation on a 1,050-m transect of mixed hardwood forest once each December.

We tested all I. scapularis ticks collected (N = 479) for Borrelia spp. infection; 46 ticks (9.6%) from 7 sites in 4 counties (Anderson, Claiborne, Hamilton, and Union; Figure 1, panel B) tested positive for Lyme group Borrelia by 16S rDNA PCR screening. We tested 26 samples for the intergenic spacer region by PCR; all were positive for this sequence and identified as B. burgdorferi sensu stricto by sequencing. Most infected ticks came from 2 Union County sites, which had prevalences of 44% (14/32) and 78% (18/23). No ticks were found to be infected with B. miyamotoi or other relapsing fever group borreliae.

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