Doctors depend on scientific research to inform the diagnoses they make and the treatments they recommend. When that information is out of date, the doctors and their patients are at a real disadvantage. That’s certainly the case with Lyme disease research. A new report released this morning shows that Lyme disease-spreading ticks can now be found in nearly half of all U.S. counties—a 50 percent increase since the last prevalence study in 1998.

Rebecca Eisen is a biologist at the U.S. Centers for Disease Control and Prevention (CDC). When she realized that the last national survey of tick distribution had been completed nearly 20 years ago, she decided it was time for an update. To ensure that they could compare their results to those from 1998, Eisen and her colleagues used the same techniques employed in the earlier study. They tabulated reported sightings of the blacklegged tick (Ixodes scapularis) and the less-common western blacklegged tick (Ixodes pacificus), both commonly known as deer ticks.

Blacklegged tick. Image credit: Gary Alpert, Harvard University, Bugwood.org.

The results were alarming. The range of I. scapularis has expanded into 45 percent of U.S. counties. That’s a 50 percent increase from 1998, when the tick could only be found in 30 percent of counties. The tick’s territory increased most dramatically in northern states and remained fairly stable in the South. Eisen and her colleagues also tracked the range of the less-common I. pacificus, which seems to have remained relatively steady. In 1998, western blacklegged ticks were reported in 3.4 percent of counties; by 2015, that number had only risen to 3.6 percent.

The map on top is from 1998, and the one below it is from 2015. Red indicates a county where I. scapular is is established, and blue indicates that it has been reported. Green indicates a county where I. pacificus is established, and yellow indicates that is has been reported. Image credit: Entomological Society of America.

While the majority of patients with Lyme disease can make a complete recovery if they’re treated soon after infection, the research is still vital. As it is now, diagnosing the disease can be lengthy and complicated. A bullseye rash is a pretty good sign that a person has been infected, but many people never develop a rash. The most common symptoms of Lyme—fever, headache, and fatigue—are easily mistaken for symptoms of the flu or a viral infection, and blood tests for Lyme disease are notoriously imprecise.

This research could help doctors spot the disease faster, by knowing if their patients have been exposed to Lyme disease-spreading ticks.

“This study shows that the distribution of Lyme disease vectors has changed substantially over the last nearly two decades and highlights areas where risk for human exposure to ticks has changed during that time,” Eisen said in a press release.