Geographic Expansion of Lyme Disease in the Southeastern United States, 2000-2014

Abstract

Background. The majority of Lyme disease cases in the United States are acquired on the east coast between northern Virginia and New England. In recent years the geographic extent of Lyme disease has been expanding, raising the prospect of Lyme disease becoming endemic in the southeast. Methods. We collected confirmed and probable cases of Lyme disease from 2000 through 2014 from the Virginia Department of Health and North Carolina Department of Public Health and entered them in a geographic information system. We performed spatial and spatiotemporal cluster analyses to characterize Lyme disease expansion. Results. There was a marked increase in Lyme disease cases in Virginia, particularly from 2007 onwards. Northern Virginia experienced intensification and geographic expansion of Lyme disease cases. The most notable area of expansion was to the southwest along the Appalachian Mountains with development of a new disease cluster in the southern Virginia mountain region. Conclusions. The geographic distribution of Lyme disease cases significantly expanded in Virginia between 2000 and 2014, particularly southward in the Virginia mountain ranges. If these trends continue, North Carolina can expect autochthonous Lyme disease transmission in its mountain region in the coming years.

Keywords: GIS; Lyme disease; North Carolina; Virginia; epidemiology.

Figure 1.

(A) Confirmed and probable Lyme disease cases in Virginia and North Carolina, 2000–2014. A marked and sustained increase in cases occurred in Virginia after 2006, although no such trend was observed in North Carolina. Note surveillance definition changes in 2008 and 2011. (B) Average annual increase in cases. Year-to-year differences in cases were calculated for each 2-year interval and at each zip code location. The average of these interval differences was then computed at each zip code, then interpolated using empirical Bayesian kriging to produce a continuous surface. The greatest average year-to-year increase in Lyme disease cases occurred in southwestern Virginia.

Figure 2.

Scanning statistic local cluster analysis. The primary cluster of Lyme disease in northern Virginia was present throughout the study period. Clustering in southwest Virginia was first detected in 2005–2009. In both of these regions, the geographic extent and the log likelihood ratio both increased throughout the study period. No significant clustering was detected in North Carolina except for a small coastal region during 2000–2004.

Figure 3.

Spatiotemporal scanning statistic cluster analysis. This analysis was limited to spatial clusters of at least 3 years duration and a P value of ≤.0001, ensuring that only significant and durable trends were identified. Clusters of longest duration were in northern and eastern Virginia, and the most recently emergent clusters extended along the Appalachian Mountains towards southwest Virginia.