Lyme disease-carrying ticks may invade new territory in the Midwest

Black-legged ticks may soon expand their territory across the midwestern U.S., carrying Lyme disease with them, a new study finds. 

The study researchers created a timeline of when and where the black-legged tick (Ixodes scapularis) has spread since the 1960s, when the parasitic arachnid was first detected in Wisconsin and Minnesota. The team pinpointed several environmental factors that help the ticks migrate from place to place, and using this data, they predicted where ticks would go next.

By the end of 2021, the black-legged tick will likely be detected in 42 additional counties in the Midwest, the model predicted. And with them, they'll bring the bacterium Borrelia burgdorferi, the microbe that causes Lyme disease. 

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The bacteria causes a rash that can linger in the skin for about a month. The infection can then spread to the rest of the body, causing aches, fatigue and fever. Without treatment, Lyme disease can cause tingling in the limbs, arthritis and neurological changes, such as memory loss and paralysis.  

"Ticks are thought to be the sole vector of Lyme disease to humans," with I. scapularis being the most significant, study author Allison Gardner, a professor of biology and ecology at the University of Maine, told Live Science. Because some symptoms of Lyme disease overlap with those of other diseases, such as shingles, it's important for doctors to be aware of tick populations nearby to suspect Lyme disease, Gardner said. If identified quickly, most cases of Lyme disease can be treated with a few weeks of antibiotics, according to the Centers of Disease Control and Prevention (CDC).

The team's model used county-level data on reports of ticks and environmental data from the region, such as the number of rivers and amount of forest cover in different counties; they also tracked Lyme disease incidence using CDC databases, journal articles and county-level data.

It turned out that ticks spread in a wave-like pattern. Unsurprisingly, the strongest predictor of spread was whether a tick-free county sat next to one where ticks had been detected within the previous 10 years. Greater forest cover proved to be another key factor, likely because forest animals, such as white-tailed deer and white-footed mice, are hosts for the tick, Gardner said.

Rivers also raised the risk that ticks would spread to a region. "In Illinois, for example, the ticks first arrived along the Illinois River and then spread up and down the river quite quickly," study author Brian Allan, an entomology professor at the University of Illinois Urbana-Champaign, said in a statement. Rivers often act as "wildlife corridors," meaning animals like deer and migratory songbirds, which also carry ticks, frequently travel along their lengths, Gardner said.

For the years 2012 to 2016, the model predicted the appearance of ticks in new counties with greater than 90% accuracy, so the team then used the model to make predictions about 2021. 

As would be expected, once black-legged ticks were found in a county, a huge spike in Lyme disease in that county followed. But the authors also identified 31 counties where Lyme disease cases have spiked, even though the parasite ostensibly doesn't live there. That suggests the black-legged ticks may have already invaded those counties, though they haven't yet been detected. 

Surveying for ticks is laborious and expensive, Gardner noted; it involves trapping and inspecting local wildlife for ticks, as well as inspecting game that people bring to hunting posts. Surveyors may also drag large pieces of cloth through the woods and count all the ticks that stick, she said. Flagging regions as high-risk for invasion could help counties get funding for such efforts.

The findings were published Dec. 22 in the journal Proceedings of the Royal Society B. 

Originally published on Live Science.