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 Bad news about ticks: the blacklegged ticks (Ixodes scapularis and the western Ixodes pacificus) that spread Lyme disease, are now reported in almost half of the counties in the U. S. Researchers found blacklegged ticks in 1,420 out of 3,110 counties in the continental U.S., or about 46% of counties, and found western blacklegged ticks in 111 counties, or about 4%. Combined, this is a 45% increase from 1998 when ticks were reported in 1,058 counties.Of course the tick-dense northeast is where Lyme disease is most common. Although the blacklegged tick is found from Florida to Minnesota, 95% of confirmed Lyme disease cases come from just 14 states in the northeast and upper Midwest. 

One interesting study that looked at reasons for these differences was published in PLOS ONE last year by parasitologist Isis Arsnoe and colleagues . They found that populations of blacklegged ticks behave differently in the north and the south United States. Nymphs of the blacklegged tick in the north are bolder and more active in seeking out hosts, a behavior known as questing. Arsnoe found that that tick nymphs originating from Wisconsin and Rhode Island were 20 times more likely to emerge from leaf litter, putting them in the path of passing humans, than nymphs from North or South Carolina. "Questing behavior is a key factor affecting the risk of tick bites." From Science Daily:

Ticks that transmit Lyme disease reported in nearly half of all US counties

Lyme disease is transmitted by the blacklegged tick (Ixodes scapularis) and the western blacklegged tick (Ixodes pacificus), and the range of these ticks is spreading, according to research published in the Journal of Medical Entomology.

Some symptoms of Lyme disease include fever, headache, and fatigue, all of which can be mistaken for the common flu, so medical personnel need to know where these ticks are found in order to make a correct diagnosis. Unfortunately, the range of blacklegged ticks had not been re-evaluated in nearly two decades, until now.

The team used surveillance methods similar to those used in 1998 so that they would be able to accurately judge the degree to which the distribution of these ticks had changed. Using the gathered data, they figured out which counties had established populations, which ones had one or more reports of a blacklegged ticks, and which ones had none.

They found that the blacklegged tick has been reported in more than 45% of  U.S. counties, compared to 30% of counties in 1998. Even more alarming, the blacklegged tick is now considered established in twice the number of counties as in 1998. Most of the geographic expansion of the blacklegged tick appears to be in the northern U.S., while populations in southern states have remained relatively stable. The range of the western blacklegged tick only increased from 3.4% to 3.6% of counties. (The study in J. of Medical Entomology)