Between 2010 and 2018, the U.S. had approximately 476,000 cases of Lyme disease every year, according to a study out today from the Centers for Disease Control and Prevention (CDC).

That number is substantially higher than the CDC’s previous estimates, of about 300,000 annual Lyme cases, which was based on research from 2010 and earlier that looked at insurance claims and lab test results. The new study used more current information from a large database of commercial insurance claims. 

Despite the large jump, study author Kiersten Kugeler, Ph.D., an epidemiologist with the CDC’s Division of Vector-Borne Diseases, says the growing number of Lyme disease cases isn’t surprising.  

Lyme disease, which is spread by blacklegged and western blacklegged ticks, was once concentrated in just a few states, but is now found throughout the country. It is most common in the Northeast, Mid-Atlantic, and Upper Midwest.

“There are infected ticks in places that there weren’t 20 years ago,” Kugeler says. That means more people are coming into contact with these bugs. “As Lyme is expanding into new areas and becoming more common, [there are] a large number of Americans who are diagnosed and treated for Lyme disease each year in the U.S.,” she says.

Here, what the new numbers mean for you, and how to protect yourself. 

How Researchers Estimated Lyme Cases

To come up with the new estimate, CDC researchers first counted up the number of cases in the database that had been officially coded as Lyme disease in insurance documentation, and for which the person had received an antibiotics prescription—a standard treatment.

The researchers then used statistical tools to estimate Lyme disease cases across the general U.S. population. This included using CDC’s traditional data on Lyme cases to estimate rates among people 65 and older, who aren’t included in commercial insurance claims data.

Previous research has shown that often, cases of Lyme disease are not coded correctly, which means the commercial insurance claims database the researchers used could be missing some cases. The CDC scientists corrected for this by using evidence from other studies that evaluated the scope of the undercounting. That allowed them to estimate the number of yearly Lyme disease cases for the entire U.S.: 476,000.

Linden Hu, M.D., a faculty member at the Tufts Lyme Disease Initiative who wasn’t involved in the new study, notes that the true number of cases of Lyme disease probably varies widely from one year to the next based on changes in tick density in different areas, weather patterns, and more. Still, he says, “there certainly are a lot of cases, and based on the spread of tick range, and things like that, we do anticipate that there’s an upward trend.” 

The Challenges of Counting Lyme Disease

The CDC, via its Nationally Notifiable Disease Surveillance System, keeps its own count of Lyme disease cases that occur across the country every year. But the agency makes it clear that those official surveillance counts significantly underestimate the true amount of Lyme disease in the country. In recent years, the surveillance data has usually found about 30,000 to 40,000 annual cases.

That’s in part because confirming a case through the surveillance system is somewhat labor intensive for local health department staffers, according to Kugeler. “The increasing incidence of the disease and the number of lab reports that come into those health departments has definitely overwhelmed the available resources,” she says.

This highlights a key difference between the CDC’s surveillance data and the new estimate based on insurance claims. In traditional surveillance, potential cases of Lyme are carefully evaluated and confirmed before they’re added to the official count.

But in this new study, researchers didn’t review individual patients’ symptoms or lab tests—which means it’s possible that some cases may have been inaccurately diagnosed as Lyme disease. In the absence of its signature symptom, a target-shaped rash, which may not appear in 20 to 30 percent of cases, Lyme disease can be tricky to diagnose, notes Hu, and doctors are likely to err on the side of treating Lyme if they suspect it as a possibility. If not treated early, Lyme can cause a number of serious health problems, including arthritis, meningitis, and neurological problems.

The authors of the new study note that they can’t tell from the data how much underreporting of Lyme infections or overdiagnosing of cases in clinical settings might be influencing their result, one way or another.

In a way, however, that’s somewhat beside the point, Kugeler explains. Either way, the new estimate does show how many cases are recognized and treated as Lyme disease. “This effort is still, no matter what, a measure of the burden of the disease. It’s people who are seeking healthcare in the name of possible Lyme disease and being prescribed antibiotics in the name of Lyme disease,” she says. 

What This Means for Consumers

One observation the CDC researchers made is that between 2010 and 2018, the rates of Lyme disease diagnoses rose fastest in the states that shared at least one border with states that researchers classified as those with the highest levels of Lyme disease. So while the amount of Lyme in states such as Connecticut, New York, and Minnesota seem to be starting to stabilize, the researchers note, their neighbors are seeing major growth in the number of cases of Lyme.

That group of neighboring states includes Illinois, Indiana, Iowa, Kentucky, Michigan, North Carolina, North Dakota, Ohio, South Dakota, Tennessee, and West Virginia, as well as the District of Columbia. In these areas, awareness of Lyme disease may not be as prevalent as in places where Lyme is a familiar foe. “Clearly risk is spreading into new areas, but sometimes that message isn’t quite reaching the public and providers in those areas,” Kugeler says.

That could mean people in some states where Lyme is historically less common are at risk, because they aren’t as likely to take protective measures against ticks, or to suspect Lyme disease when they develop fever a few weeks after going for a hike.

Lyme disease isn’t the only tick-borne illness that occurs in the U.S., and different kinds of ticks inhabit different regions and cause a variety of diseases. So wherever you live, brush up on what sorts of biting bugs you need to look out for when you’re in tick-friendly habitat like grassy or wooded areas. The CDC provides guidance on the regional spread of ticks, and your local health department is also a good source of information on the ticks in your area.

To protect yourself, whenever you’re spending time in an area where ticks might live, wear long sleeves and pants, and tuck your pants into your socks. Wear clothing treated with the pesticide permethrin, and apply insect repellent (check our ratings for the best products) to any exposed skin.

After you come inside, take a shower and check yourself for ticks. Remove any attached ticks immediately using fine-tipped tweezers.

And for people in areas where Lyme disease is a risk, keep in mind that blacklegged ticks can be active even during winter, in times when the weather is above freezing and there isn’t snow on the ground.