Modeling the Geographic Distribution of Ixodes Scapularis and Ixodes Pacificus (Acari: Ixodidae) in the Continental United States

BACKGROUND:  The black-legged tick, Ixodes scapularis and western black-legged tick, Ixodes pacificus, are the primary vectors of the spirochete that causes Lyme disease, the most common vector-borne disease in the United States, as well as several other human pathogens.  The geographic range of I. pacificus has changed modestly over the last two decades while, in contrast, the I. scapularis range has expanded substantially, which likely contributes to the parallel expansion in the geographic distribution of human Lyme disease cases in some regions.  Identifying counties that contain suitable habitat for these ticks that have not yet reported established vector populations can aid in targeting limited resources allocated for vector surveillance to areas where tick invasion and potential human risk for encounters with ticks are most likely to occur. 

METHODS: We utilized updated county-level vector distribution information and bioclimatic modeling using an ensemble approach to map the potential distribution of I. scapularis and I. pacificus in the continental United States.  We used 19 bioclimatic variables from the WorldClim dataset as well as monthly growing degree days, vapor pressure, and snow cover from the Daymet dataset as predictors.  We also included elevation and percent forest cover from the USGS National Land Cover Database as candidate predictors.

RESULTS: Our results show that I. pacificus is currently present within much of the range classified by our model as suitable for establishment.  In contrast, environmental and climatic conditions are suitable for I. scapularis to continue to expand its range into northwestern Minnesota, central and northern Michigan, within the Ohio River Valley, and inland from the southeastern and Gulf coasts.  Overall, our ensemble models show suitable habitat for I. scapularis in 441 eastern counties and for I. pacificus in 11 western counties where surveillance records have not yet supported classification of the counties as established. 

CONCLUSIONS:  This study identified several counties that were classified as suitable habitat for Lyme disease vectors but where surveillance records do not yet indicate that these tick species are established.  Enhanced tick surveillance coupled with pathogen detection in these regions could aid in identifying emerging areas of risk for exposure to tick-borne pathogens.