The ticks that transmit Lyme disease, a debilitating flulike illness caused by Borrelia bacteria, are spreading rapidly across the United States. A new study shows just how rapidly. Over the past 20 years, the two species known to spread the disease to humans have together advanced into half of all the counties in the United States.
Lyme disease cases have tripled in the United States over the last 2 decades, making it the most commonly reported vector-borne disease in the Northern Hemisphere. The disease now affects around 300,000 Americans each year. If diagnosed early—a rash commonly appears around the site of the tick bite—Lyme can be effectively treated with antibiotics, but longer term infections can produce more serious symptoms, including joint stiffness, brain inflammation, and nerve pain.
To get a comprehensive map of where the two species—the blacklegged tick (Ixodes scapularis) and the western blacklegged tick (I. pacificus)—were living, Rebecca Eisen and colleagues from the U.S. Centers for Disease Control and Prevention (CDC) in Fort Collins, Colorado, combined data from published papers with state and county tick surveillance data going back to 1996. They counted reports of tick sightings in each of the 3110 continental U.S. counties to determine whether those counties hosted an established population or just a few individuals. Ticks were considered “established” when sightings of at least six ticks, or two of the three life stages, had been reported in a year.
Their results, published in the Journal of Medical Entomology, show that the blacklegged tick has undergone a population explosion, doubling its established range in less than 2 decades. It is now reported in 45.7% of U.S. counties, up from 30% in 1998. Blacklegged ticks are found in 37 states across the eastern United States. The rarer western blacklegged tick, restricted to just six states, has shown only modest increases in established populations, from 3.4% to 3.6% of counties. Combined, these two Lyme disease vectors are now found in half of all U.S. counties.