County-Scale Distribution ofIxodes scapularisandIxodes pacificus(Acari: Ixodidae) in the Continental United States

Abstract

The blacklegged tick, Ixodes scapularis Say, is the primary vector to humans in the eastern United States of the Lyme disease spirochete Borrelia burgdorferi, as well as causative agents of anaplasmosis and babesiosis. Its close relative in the far western United States, the western blacklegged tick Ixodes pacificus Cooley and Kohls, is the primary vector to humans in that region of the Lyme disease and anaplasmosis agents. Since 1991, when standardized surveillance and reporting began, Lyme disease case counts have increased steadily in number and in geographical distribution in the eastern United States. Similar trends have been observed for anaplasmosis and babesiosis. To better understand the changing landscape of risk of human exposure to disease agents transmitted by I. scapularis and I. pacificus, and to document changes in their recorded distribution over the past two decades, we updated the distribution of these species from a map published in 1998. The presence of I. scapularis has now been documented from 1,420 (45.7%) of the 3,110 continental United States counties, as compared with 111 (3.6%) counties for I. pacificus. Combined, these vectors of B. burgdorferi and other disease agents now have been identified in a total of 1,531 (49.2%) counties spread across 43 states. This marks a 44.7% increase in the number of counties that have recorded the presence of these ticks since the previous map was presented in 1998, when 1,058 counties in 41 states reported the ticks to be present. Notably, the number of counties in which I. scapularis is considered established (six or more individuals or one or more life stages identified in a single year) has more than doubled since the previous national distribution map was published nearly two decades ago. The majority of county status changes occurred in the North-Central and Northeastern states, whereas the distribution in the South remained fairly stable. Two previously distinct foci for I. scapularis in the Northeast and North-Central states appear to be merging in the Ohio River Valley to form a single contiguous focus. Here we document a shifting landscape of risk for human exposure to medically important ticks and point to areas of re-emergence where enhanced vector surveillance and control may be warranted.

Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis — United States

“…scapularis ( Figure 14) is the vector for A. phagocytophilum in the northeastern and Midwestern United States (68) (Figure 16), whereas the western blacklegged tick, Ixodes pacificus (Figure 17), is the principal vector along the West Coast ( Figure 18). The bites of nymphal and adult ticks can transmit A. phagocytophilum to humans.…”

The Blacklegged Tick, Ixodes scapularis : An Increasing Public Health Concern

“…Lyme disease is caused by the spirochetes Borrelia burgdorferi sensu stricto (herein referred to as B. burgdorferi ) [2], or much less commonly by Borrelia mayonii [3]; both are transmitted by the blacklegged tick, Ixodes scapularis (including the junior synonym, Ixodes dammini ) in the eastern United States where the vast majority of cases occur [4,5]. Over the past two decades, we have seen expansions in both the geographic range of I. scapularis [6] (Figure 1A,B) and the incidence and geographic range of Lyme disease and other I. scapularis -borne diseases [7,8] (Figure 1C,D). In addition, new I. scapularis -borne human pathogens continue to be discovered.…”

“…Tick surveillance is not standardized or routine, thus hampering our ability to monitor changes in the distribution and abundance of I. scapularis [6,30]. Retrospective review of I. scapularis records reveals remarkable range expansion over the past century, particularly in the northern portion of the eastern United States.…”

Lyme Disease Propelled by Borrelia burgdorferi-Infected Blacklegged Ticks, Wild Birds and Public Awareness ─ Not Climate Change

“…In addition, researchers discovered I. scapularis larvae and an I. scapularis nymph on small mammals (i.e. red-backed voles, deer mice) at Marchand, Manitoba in 1991, and these two immature stages constitute an established population [52,53]. Furthermore, adult I. scapularis were collected at the same location and, collectively, all 3 mobile life stages (adult, nymph, larva) represent a reproducing population.…”

“…Likewise, four I. scapularis (2 females, 1 male, 1 nymph) were collected on 12 May and 9 June 1993 from untravelled mammalian hosts (dog, human) residing on the rural property bordering Lake of the Woods north of Rainy River, Ontario; one of the 3 adults was positive for B. burgdorferi [54]. All together, these tick collections constitute an established population of I. scapularis [53]. These epidemiological findings were overlooked on the climate change maps [17,42,45,46,51].…”