For the first time in 18 years, researchers from the U.S. Centers for Disease Control and Prevention tracked tick populations in North America, and the findings are alarming. According to their measurements, populations are on the rise in nearly half of all U.S. counties, and the trend is likely to lead to have a trickle-down effect on human health.

For the study, published online January 18 in the Journal of Medical Entomology, researchers analyzed populations of the blacklegged tick (Ixodes scapularis) and the western black-legged tick (Ixodes pacificus). In order to get an accurate understanding of how much tick populations have changed over time, the team used the same methods to analyze these pest populations as those that were used during the last tick census in 1998.

Overall, the analysis revealed that populations of these two tick species had increased by 3.4 to 3.6 percent in nearly half of all U.S. counties. However, a further breakdown showed that some areas of the country were more heavily affected than others. For example, according to the study, populations in the Northeast saw significant tick population increases, while in counties further South, there was less population growth.

According to a statement made by study researcher Dr. Rebecca Eisen, a CDC biologist, the change in tick populations is substantial and “highlights areas where risk for human exposure to ticks has changed.”

Human exposure to increasing tick populations is of significant concern due to the fact that they are the exclusive vector of the bacterium that causes Lyme disease. According to the CDC, initial signs of Lyme disease include fever, rash, facial paralysis, and arthritis, and can often be mistaken from a flu. The condition is also characterized by intermittent muscle and joint pain, nerve pain and cognition problems. When treated early on, these symptoms will usually go away for most patients. However, when left untreated, symptoms can often persist for many years.

Although it is not entirely clear as to why tick populations are increasing in certain areas, some experts have suggested that warming temperatures may play a role. Warmer winters and longer summers shorten the amount of time ticks need to stay in hibernation and also widen the scope of their habitat.

Source: Eisen RJ, Eisen L, Beard CB. County-Scale Distribution of Ixodes scapularis and Ixodes pacificus (Acari: Ixodidae) in the Continental United States. Journal of Medical Entomology. 2016.