How Lyme Disease Spreads: Ticks Carrying Illness Populate Half Of US Counties

The two species of ticks that carry and transmit Lyme disease have spread rapidly over the past two decades, populating nearly half of all the counties in the United States, according to a report Monday in Science Magazine. The debilitating illness that features flu-like symptoms now affects some 300,000 people in the United States each year.

Lyme disease, if caught early -- typically through spotting a rash that appears around the tick bite -- can be treated with antibiotics. If it's not detected early, however, the disease can cause serious symptoms like joint stiffness, brain inflammation and nerve pain.

Researchers from the U.S. Centers for Disease Control and Prevention combined data from published papers with state and county tick data going back to 1996 to create a map of where the two species, the blacklegged tick ( Ixodes scapularis ) and the western blacklegged tick ( Ixodes pacificus ), were living, according to Science Magazine. The researchers counted individual tick sighting to determine if they were "established" in the area or not. Finding at least six ticks, or two of the three life stages, qualified as established.

The results, published in the Journal of Medical Entomology, showed that the blacklegged tick has exploded in population and is now in 45.7 percent of U.S. counties, up from 30 percent in 1998. The blacklegged tick is also found in 37 states across the eastern United States. The less-common western blacklegged tick has only shown a modest uptick in prevalence, from 3.4 percent of counties to 3.6 percent.

"Since the late 1990s, the number of counties in the northeastern United States that are considered high-risk for Lyme disease has increased by more than 320 [percent]," said researcher Rebecca J. Eisen, according to Science Magazine. "The tick is now established in areas where it was absent 20 years ago."

While the blacklegged tick is found all over the country, it's highly active in a small area. Ninety-five percent of confirmed cases of Lyme disease came from just 14 states in the northeast and upper Midwest. "Although our map shows a wide distribution … the risk of people getting Lyme disease is not equal across areas of the country," Eisen said, according to Science Magazine.

Should someone contract Lyme disease, the easiest way to spot the illness is often the bulls-eye shaped rash around the tick bite that occurs in approximately 70 to 80 percent of infected people, according to the U.S. Centers for Disease Control and Prevention. The rash typically expands over time and may feel warm to the touch, but is rarely itchy or painful.

To avoid tick bites, the Centers for Disease Control and Prevention recommends avoiding "wooded and brushy areas with high grass and leaf litter" and walking in the center of trails. The CDC also suggests using repellents on your body and clothing.