Comment; Models are a good start, they require ongoing validation however.
In the United States, Lyme disease, caused by infection with Borrelia burgdorferi sensu stricto or less commonly by the newly discovered organism Borrelia mayonii, is the most commonly reported vector-borne disease.1,2 An average of 30,000 Lyme disease cases are reported each year, representing a 3-fold increase from 1992 to 2013, and most Lyme disease cases are reported from the Northeast, mid-Atlantic, and upper Midwestern United States.1 Since the mid-1990s, the reported distribution of Ixodes scapularis, the primary vector of Lyme disease spirochetes, has expanded in the United States with the greatest increases observed in the upper Midwest and in the northeastern states.3 In parallel to the observed expansion of the vector’s geographic range, the numbers of counties considered high incidence for Lyme disease by Kugeler et al.4 also expanded in the same geographic regions over a similar time period. Some of the most dramatic increases in the occurrence of Lyme disease and other I. scapularis–borne illnesses have been observed in Minnesota, where, from 1992 to 2006, the percentage of counties reporting at least one human case of Lyme disease increased from 33% to 74%.5 Moreover, from 1996 to 2011, there was a substantial and significant increase in the incidence of Lyme disease, anaplasmosis, and babesiosis.6 Along with a statewide increase in the incidence of I. scapularis–borne diseases, counties reporting human cases of I. scapularis–borne diseases6 and established populations of I. scapularis have expanded geographically.3
In Minnesota, I. scapularis serves as the vector of at least seven human pathogens including B. burgdorferi sensu stricto (henceforth referred to as B. burgdorferi), Anaplasma phagocytophilum, Babesia microti, Powassan virus, Borrelia miyamotoi, and two novel I. scapularis–borne pathogens (B. mayonii and Ehrlichia muris subs. eauclarensis) both of which appear to be restricted to the upper Midwest.2,7–14 To more clearly define the current distribution of areas considered suitable for the establishment of I. scapularis populations in Minnesota, we developed a habitat suitability model for I. scapularis in Minnesota.7 Nymphal ticks pose the greatest risk to humans because they are difficult to detect owing to their small size, and the seasonal timing of peak nymphal questing activity coincides with a period of increased human outdoor activity.1,15–17 However, recognizing that 1) human risk of exposure to pathogens has been correlated with density of host-seeking nymphal I. scapularis, rather than simple measures of presence or absence,18–20 and 2) habitat suitability for ticks is heterogeneous within counties,21 we conducted an extensive field survey of host-seeking I. scapularis nymphs and developed a subcounty (30 × 30 m) resolution model of the DON to better inform estimates of acarological risk of human exposure to I. scapularis.
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