Almost half of all counties in the U.S. now have established populations of ticks that can carry Lyme disease. The arthropods have dramatically increased their range over the past 20 years, putting many people under threat who historically had little risk of catching the disease. 

The distribution map of the blacklegged tick and eastern blacklegged tick – both known to carry the bacterium that causes Lyme disease – had not been updated since 1998. To rectify this, researchers did a comprehensive analysis of papers with state and county tick surveillance data going back to 1996. The two species were classed as being established in a county if there had been six or more reportings of them in a year. The full study is published in the Journal of Medical Entomology. 

What they found was that the number of counties in the United States with established populations of potentially Lyme disease-carrying ticks has doubled since 1998, when the last map was drawn up. This means that the ticks are now present in almost half of all counties within the country, but these are mainly concentrated in the northeast, where their distribution is near continuous.

Caused by a bacterium living in the mouthparts of the ticks, Lyme disease is spread when the arachnids bite. While the disease is easily treatable with antibiotics, the symptoms can be severe and long-lasting. Many develop as flu-like traits, such as a fever and headache, but an infected bite will also form a distinctive “bull's-eye” rash. Later symptoms can be aching and painful joints, heart palpitations, and problems affecting the nervous system including the paralysis of facial muscles. It’s estimated that around 300,000 people catch Lyme disease every year, with about 30,000 cases reported in the U.S.  

While the ticks have been found to be spreading across the northeastern United States, this expansion might not lead to an increase in Lyme disease due to variations in tick infection and numbers. Despite this, the researchers still warn that if the ticks are present, then the risk of the disease is also there. They recommend that where the ticks are found in specific regions, you should walk in the middle of paths, wear long pants, use insecticides such a DEET, and shower soon after getting home, in order to lower the chance that you’ll get a tick.

^{The forests of New York state are the perfect habitat for the blacklegged tick, which can carry Lyme disease. Zack Frank/Shutterstock}

They think that reforestation and an expansion of their host species, namely deer, is probably behind the spread of the ticks, and could also be linked to an increase in rainfall and temperature. There are, however, natural barriers to their range expansion. While they thrive in forests and along river edges, alpine areas and dry prairies are not their ideal habitat, limiting their spread.

“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,” explains Dr. Rebecca Eisen, who co-authored the study, in a statement. “The observed range expansion of the ticks highlights a need for continuing and enhancing vector surveillance efforts, particularly along the leading edges of range expansion.”  

With the rate of the disease on an upward trajectory within the U.S., it is vital that doctors and researchers have the most up-to-date information on where the ticks that harbor the disease can be found.  

^{Image in text: Map A) is from 1998, and map B) from 2015. Red indicates a county where blacklegged ticks are established, and blue indicates where they have been reported. Green shows a county where eastern blacklegged ticks are established, while yellow shows where they have been reported. Entomological Society of America.}