Lyme Disease Update
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, 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.
The blacklegged tick has undergone a population explosion, doubling its established range in less than 20 years. 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 uncommon 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.
“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%,” Eisen says. “The tick is now established in areas where it was absent.”
Perhaps most worrying, the tick-dense northeast is where Lyme disease is most common. Although the blacklegged tick is found from Florida to Minnesota, 95% of confirmed cases come from just 14 states in the northeast and upper Midwest.
A study published in PLOS ONE last year might hold the answer. Parasitologist Isis Arsnoe from Michigan State University and colleagues found that populations of blacklegged ticks behave differently in the north and the south of the 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 and his team 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 Tennessee and Florida. “Questing behavior is a key factor affecting the risk of tick bites,” Arsnoe explains. “Ticks that stay buried in the leaves are not likely to have an opportunity to bite passing humans – and unless they bite they cannot transmit disease.” Arsnoe is concerned that the ticks found in the north may also expand into southern states, taking their questing behavior with them.
Avoiding areas of thick vegetation, using a strong repellent, and bathing after hiking are usually enough to avoid contact, CDC says. Eisen says that the most important thing now will be to carefully monitor the spread of the blacklegged tick, so that that people can educate themselves about the potential disease vectors in their area and take steps to protect themselves.
Tick Management: PDF Source Book TickManagementHandbookCDC