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Mapping Lyme: CDC Reveals Distribution of Lyme Disease-Causing Bacteria by County

U.S. distribution of Lyme disease-causing bacteria in ticks

A new study by CDC researchers maps the distribution of Lyme disease spirochetes, Borrelia burgdorferi sensu stricto and Borrelia mayonii, in host-seeking blacklegged ticks (Ixodes scapularis, in the eastern U.S.) or western blacklegged ticks (I. pacificus, in the western U.S.), relative to the previously reported distribution of these vector species. Ticks were considered present in a county if at least one tick was recorded. Counties where ticks have been reported without records of infection may be reported as such either if ticks were not tested or if the pathogen was not detected in tested samples. (Image originally published Fleshman et al 2021, Journal of Medical Entomology)

By Melissa Mayer

Scientists think ticks could be up to 300 million years old. The fossil record even includes amber-encased ticks still engorged with their last dino meal.

Melissa Mayer

Melissa Mayer

Which is amazing. Those blood-sucking arthropods have been doing their thing for longer than humans have existed—and their thing includes carrying disease. Ticks are responsible for more than 75 percent of the vector-borne illnesses reported to the U.S. Centers for Disease Control and Prevention (CDC) every year. The most common of these is Lyme disease with 30,000 to 40,000 annual reported cases—though the CDC estimates the actual number of cases, most of which go unreported, could be more than 400,000 annually.

A team of CDC researchers recently constructed the first map to reveal county-level distribution of host-seeking vector ticks carrying the bacteria that causes Lyme disease. The team from the Bacterial Diseases Branch of the National Center for Emerging and Zoonotic Infectious Diseases reported their findings this month in the Journal of Medical Entomology.

What is Lyme Disease?

In the United States, blacklegged ticks (Ixodes scapularis, present in eastern states) and western blacklegged ticks (Ixodes pacificus, in western states) can carry spirochete bacteria that cause Lyme disease—Borrelia burgdorferi sensu stricto and Borrelia mayonii—and those spirochetes can be transmitted to a human when a tick bites and takes its bloodmeal.

Rebecca Eisen, Ph.D.

Rebecca Eisen, Ph.D.

The bulk of reported Lyme disease cases occur in 14 states in the north central, northeast, and mid-Atlantic regions of the U.S., but tick-borne disease is a moving target. “The distribution of ticks and tickborne pathogens change over time, and, as a result, so does the risk of tickborne diseases,” says Rebecca Eisen, Ph.D., research biologist at the CDC and senior author on the study.

Since the 1990s, scientists have reported a nearly 300 percent increase in U.S. counties classified as high incidence for Lyme disease. Meanwhile, the number of counties where the blacklegged tick has been documented has more than doubled.

Mapping Lyme

In 2018, the CDC increased funding for tick surveillance by state health departments and issued guidance to standardize tick surveillance practices. The following year, they developed the ArboNET data portal so states can report tick data and researchers can monitor trends across the country.

Eisen and her team tapped into that data to construct their county-level map (see map at top of this post). They also looked at the scientific literature and internal pathogen testing databases for records of infected host-seeking ticks that specified the county.

The team limited their search to data reported after 2000 that used advanced detection methods to identify spirochetes, since only B. burgdorferi s.s. or B. mayonii are confirmed to cause Lyme disease in the U.S.—and data that included related species could skew the map.

More Limited Distribution

It turns out that, while vector ticks are distributed broadly, ticks carrying Lyme disease spirochetes have a more limited range. The ticks are established in 41 states (35 eastern and 6 western), but mapping revealed spirochetes in three-quarters of those eastern states and one-third of those western states. At a more local level, about 26 percent of counties where the ticks are established have records for Lyme disease spirochetes.

That news comes with some caveats, the researchers say. These numbers are a baseline and probably low. One reason is that surveillance efforts vary, with more monitoring happening in regions with more reports of Lyme disease cases. And, the team excluded records that didn’t adequately identify the spirochetes or list the county of collection.

Eisen says the map doesn’t yet distinguish between counties where ticks commonly carry Lyme spirochetes (e.g., 10–20 percent of infected nymphs or 50–60 percent infected adults) versus those where Lyme spirochetes are present but at lower levels (e.g., less than 1 percent of ticks infected). The team plans to incorporate this information, plus other metrics—like how abundant infected, host-seeking ticks are in an area—and update the map as new data comes in.

blacklegged tick life stages on U.S. dime

The blacklegged tick (Ixodes scapularis) is the primary vector of the bacteria that cause Lyme disease in the United States. A new study by CDC researchers provides a county-level map of the presence of such ticks confirmed to be carrying the bacteria that cause Lyme disease—though the distribution of ticks that can carry it is much wider. (Image courtesy of U.S. Centers for Disease Control and Prevention)

Going Forward

Of course, Lyme disease isn’t the only tickborne illness in the country. “Tickborne diseases are becoming increasingly more common and widespread in the U.S.,” says Eisen. She points out that blacklegged ticks also carry pathogens that cause the relapsing fever Borrelia miyamotoi disease, anaplasmosis, babesiosis, one form of ehrlichiosis, and Powassan virus.

The new map, combined with increased tick surveillance efforts, may help bring prevention education and tick control strategies to places that need them most. Right now, Eisen says tick prevention recommendations remain the same: Avoid tick habitats, use EPA-registered repellents, and check for ticks after spending time outdoors.

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.

7 Comments »

  1. I know personally of about 20 people who have been diagnosed with Lyme in a small Mississippi town, as well as myself. Someone at my local clinic said they were diagnosing 13+ plus cases a year. That’s with the cheap, outdated testing being only 50% accurate, at best. This is a small town, population less than 1,800. It also seems that everyone you talk with knows at least one person with it. Yet, Mississippi has no records of infection? This map is totally false. I understand that the research is lacking, but when doctors are diagnosing patients fairly often, reporting these infections should be mandatory. It’s as if making the truth known isn’t so much of a priority after all. My advice to anyone living in a state where Lyme hasn’t been reported.. don’t believe there the map, and if you find a tick on you, save in in a ziplock bag in the freezer to be sent for testing later. Also, go get on antibiotics immediately if you start feeling flu-like symptoms. If you’re lucky, maybe you can find a compassionate doctor who believes Lyme indeed exists in your area. Luckily mine did, because he contracted it himself.

    • Brandon, I thought reporting was already mandatory, but evidently not? We can’t get it eradicated if we don’t know where it is (not that I think anything is being done about it).

      Being a Lyme patient myself (along with two of my horses and one of my dogs), I corresponded with a knowledgeable and kind person at the CDC last year. I asked what they were doing about the astronomical spread of Lyme, and they said all the CDC can do is maintain the data; the USDA is actually the department responsible for taking any action against ticks. As a layperson, I don’t know of any ecological purpose for ticks, so as far as I’m concerned, they can be gotten rid of. Unless Big Pharma is in cahoots with the USDA, there’s no reason to have ticks on the planet.

      You’re extremely lucky to have found a sympathetic doctor. I have several friends in several states who weren’t so fortunate. Because their doctors were uninformed and closed-minded, my friends are now living with Lyme for life. The doctors’ response, “We don’t have Lyme here,” has destroyed many lives. I wish you the very best.

      • Thanks, Robynne. Hope you are doing fine. Not only did I get Lyme, I got babesiosis, which supposedly also doesn’t exist in MS. I have been to plenty of doctors who told me these infections didn’t exist in the South. It’s quite shocking to hear, honestly. The doctor who is treating me told me that he has quite a few veterinarians who he treats, and he made a good point. If animals are getting these diseases, why are doctors and government organizations saying that humans aren’t? It makes absolutely no sense. I feel that there is very little reporting of tick-borne diseases in the South and even many other parts of the country. For one, I’d bet more than half the people probably don’t even know they have it because the tests are so inaccurate. I wasn’t technically “positive” because I was short one band of the CDC’s guidelines, but there was no doubt I had contracted the disease based on my symptoms and my “near” positive result. So.. Testing needs to improve, the guidelines for a positive test needs to improve, and reporting of these infections should be made a priority.

      • Thanks Robynne. Yes, I have had doctors tell me flat out that tick diseases don’t exist here. While in the hospital from the life-threatening symptoms, they denied it had anything to do with a tick bite because I was supposedly cured from the short course of antibiotics I had taken a year prior. After further independent investigation, it turns out I had babesiosis, as well, which also supposedly doesn’t exist. It’s quite scary to be the unfortunate victim of a disease that doesn’t exist. You get ridiculed by doctor and deemed a hypochondriac, while you are dying of some undiagnosed infection. It’s pitiful. Research on effective treatments is lagging so far behind, and I’m sure it’s partially due to the fact that a real crisis in this country is being denied. Now that more and more people are suffering and speaking up, the truth is slowly coming out. I feel if the reporting of these infections were more strictly enforced, this map would change drastically.

  2. I agree with Brandon’s post. I haven’t read the original research, but they must have set very strict criteria for what is considered a reported infection. I personally know many people who were diagnosed in Florida.

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