Lyme disease, Virginia, USA, 2000-2011

Abstract

Lyme disease, caused by the bacterium Borrelia burgdorferi and transmitted in the eastern United States by the black-legged tick (Ixodes scapularis), is increasing in incidence and expanding geographically. Recent environmental modeling based on extensive field collections of host-seeking I. scapularis ticks predicted a coastal distribution of ticks in mid-Atlantic states and an elevational limit of 510 m. However, human Lyme disease cases are increasing most dramatically at higher elevations in Virginia, a state where Lyme disease is rapidly emerging. Our goal was to explore the apparent incongruity, during 2000-2011, between human Lyme disease data and predicted and observed I. scapularis distribution. We found significantly higher densities of infected ticks at our highest elevation site than at lower elevation sites. We also found that I. scapularis ticks in Virginia are more closely related to northern than to southern tick populations. Clinicians and epidemiologists should be vigilant in light of the changing spatial distributions of risk.

Figure 1

Locations of 4 field sites at which ticks were sampled, Virginia, May-July 2011. Circles indicate sampling areas. LE, Lesesne State Forest; AB, Appomattox-Buckingham State Forest; GR, University of Richmond–owned field site; CR, Crawfords State Forest. Darker shading represents higher elevation. Scale bar indicates kilometers.

Figure 2

Progressive geographic spread of human Lyme disease across Virginia, 2001–2011. Data were reported by the Virginia Department of Health http://www.vdh.virginia.gov/epidemiology/surveillance/surveillancedata/index.htm. Cases per 100,000 population were calculated by county or city census estimate data published for the year preceding the year of the report. LE, Lesesne State Forest; AB, Appomattox-Buckingham State Forest; GR, University of Richmond–owned field site; CR, Crawfords State Forest.

Figure 3

Variation in estimated prevalence of Borrelia burgdorferi infection in Ixodes scapularis nymphs at 4 field sites in Virginia. Sites are arranged west to east from left to right. LE, Lesesne State Forest; AB, Appomattox-Buckingham State Forest; GR, University of Richmond–owned field site; CR, Crawfords State Forest. Error bars represent 95% CIs.

Figure 4

Maximum-likelihood phylogenetic reconstruction of Ixodes scapularis lineages based on 16S rRNA gene sequences using Tamura 3-parameter model (35). All samples beginning with IS were collected during this study; reference sequence GenBank accession numbers are indicated, as were sampling locations (2-letter state abbreviation). The clade containing samples collected in GA, FL, NC, OK, and SC is known as the Southern clade (sensu Norris et al. [20]); the clade containing all samples from this study, indicated by the prefix IS, represents the American clade (more complete explanation of these terms is provided in the text). Bootstrap values at nodes are based on 500 replicates. Scale bar indicates nucleotide substitutions per site.

Figure 5

Centroids of annual incidence, by county, Virginia, 2000–2011. The size of each circle represents the annual number of cases reported by the Virginia Department of Health and is proportional to annual incidence (cases/100,000 population). Black arrow represents the mean linear direction of annual movement among centroids during 2006–2011 (these years indicate the recent dramatic increase in Lyme disease incidence in Virginia).