Six independent literature searches were conducted using Scopus and PubMed databases with the following key words “Ixodes scapularis,” “Ixodes pacificus,” and “tick” to identify relevant articles and abstracts published from 1996 through 25 August 2015. We also conducted a search to ensure that papers using the junior synonym “Ixodes dammini” rather than Ixodes scapularis were included. All reports that explicitly presented county-specific tick data were included in our database. In addition, we visited individual state health department Web sites to identify county-level tick surveillance data, and contacted public health officials, acarologists, and Lyme disease investigators throughout the United States to assess county-level tick collection data.