(SCIENCE) — 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.