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

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.

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.

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.