What is causing eastern blacklegged tick range expansion?

The eastern blacklegged tick, formerly known as the deer tick, is an especially well known species of tick that occurs throughout the southeastern and northeastern United States as well as the upper Midwestern states. This area is referred to as the species range of the eastern blacklegged tick. They are widely known because they can transmit several human pathogens that lead to diseases such as anaplasmosis, babesiosis, and Lyme disease.

Eastern blacklegged tick distribution in the United States. From CDC website: https://www.cdc.gov/ticks/images/map-blacklegged_tick.jpg
Eastern blacklegged tick distribution in the United States. From CDC website: https://www.cdc.gov/ticks/images/map-blacklegged_tick.jpg

Growing up in a family that enjoys the outdoors, I have been aware of what ticks look like since I was very young. However, I was raised in an area outside of this tick’s species range, so I was unfamiliar with this type of tick. I was also not exactly sure what Lyme disease was, let alone the other diseases these ticks can transmit, until I started studying infectious diseases in graduate school. I was especially interested in the organisms that can transmit pathogens to other species. Two examples of these organisms are ticks and mosquitoes, and they are referred to as vectors. Through understanding more about how these vectors move through, and are influenced by, the environment researchers are able to understand more about the pathogens they carry.

The approximate size and coloration of blacklegged ticks. For a better idea of size, larvae are about the size of a poppy seed, nymphs are about the size of a pin-head, and adults are about the size of an apple seed. From CDC website: https://www.cdc.gov/lyme/images/tick_sizes.jpg

For several years there has been increasing evidence that areas outside of the northern portion of the eastern blacklegged tick’s range are becoming more suitable for the establishment and growth of their populations. There is also evidence that tick populations are increasing and the overall range of this tick is expanding north, leading to a larger species range. One of the first questions that come to mind is, “What changed?”

In general, the expansion of a species range occurs when individuals from a population migrate to an area outside of their species range and survive and reproduce. This leads to an established population in this new area. In order for a population to establish, an unsuitable area must change to a suitable area. This occurs through changes in the climate (temperature or precipitation in a region), and changes in the environment.

For ticks, migration mostly occurs through the movements of the animals on which they feed. They can be transported a variety of distances based on what animal they are attached to and how far that animal travels. For example, small mammals would transport them short distances, whereas birds have the potential to transport them long distances.

<strong>White-footed mouse with tick larvae embedded in ear. From: https://lymediseaseguide.org/wp-content/uploads/2014/02/white-footed-mouse-oral-bait-vacinne-lyme-disease-300x199.jpg</strong>
White-footed mouse with tick larvae embedded in ear. From: https://lymediseaseguide.org/wp-content/uploads/2014/02/white-footed-mouse-oral-bait-vacinne-lyme-disease-300×199.jpg


For eastern blacklegged ticks, the contributing climatic and environmental changes include increased temperatures, changes in precipitation, milder winters, and changes in landscape features that have caused many areas in the northern region to be more suitable, not just for this tick, but also for the mammals and birds that this tick depends on as a food source.

Eastern blacklegged ticks are not the only species of tick whose range has been expanding in recent decades. In Sweden, one of the species of tick that transmits Lyme borreliosis (Lyme disease), the castor bean tick, has been expanding its’ range over 30 years into northern Sweden and is still moving north. In fact, the range of this species of tick is increasing throughout most of Europe along with an increase in abundance. The reason for the expansion is similar to the reasons for range expansion in eastern blacklegged ticks. There have been changes in climate leading to shortened or milder winters, and increases in temperatures. This also affects the tick’s food source, mammals and birds, whose ranges are changing in response to climate and environmental factors.

The castor bean tick (Ixodes ricinus) Photo © Biopix: JC Schou (https://www.biopix.com/photos/jcs-ixodes-ricinus-33242.jpg)
The castor bean tick (Ixodes ricinus) Photo © Biopix: JC Schou (https://www.biopix.com/photos/jcs-ixodes-ricinus-33242.jpg)

One of the primary reasons that there is so much interest in this tick and their species range is because of Lyme disease. This disease was discovered in 1975 in Lyme, Connecticut, where many children were suffering from symptoms of arthritis after spending time outdoors. It was later found that the disease is caused by a bacterium that is transmitted through the bite of the eastern blacklegged tick. Since then, mostly because of increases in awareness, but also because of the movement of infected eastern blacklegged ticks, the incidence of Lyme disease started increasing in the United States. Incidence is still increasing in the United States with more than 30,000 cases being reported every year. A tick must be attached for between 36-48 hours (or longer) for the bacterium to infect an individual. Symptoms of Lyme disease include a red, expanding rash called erythema migrans (“bull’s-eye” rash), fatigue, chills, fever, headache, muscle and joint aches, and swollen lymph nodes.

Example of erythema migrans or “bull’s-eye” rash. From CDC website: https://www.cdc.gov/lyme/images/EMforWeb2.jpg
Example of erythema migrans or “bull’s-eye” rash.
From CDC website: https://www.cdc.gov/lyme/images/EMforWeb2.jpg

Through researching and understanding more about the abundance and distribution of eastern blacklegged ticks as well as how they interact with other species and their environment, we are also learning more about the pathogens they carry and can transmit to other organisms. With this increased understanding, public health officials and researchers can develop better strategies to decrease exposure and transmission of pathogens from eastern blacklegged ticks to humans.


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Additional sources:

Diuk-Wasser, M. A., G. Vourc’h, P. Cislo, A. G. Hoen, F. Melton, S. A. Hamer, M. Rowland, R. Cortinas, G. J. Hickling, J. I. Tsao, A. G. Barbour, U. Kitron, J. Piesman, and D. Fish. 2010. Field and climate-based model for predicting the density of host-seeking nymphal Ixodes scapularis, an important vector of tick-borne disease agents in the eastern United States. Global Ecology and Biogeography, 19: 504-514.

Jaenson, T. G. T., David G. E. Jaenson, L. Eisen, E. Petersson, and E. Lindgren. 2012. Changes in the geographical distribution and abundance of the tick Ixodes ricinus during the past 30 years in Sweden. Parasites and Vectors, 5(8).

Jaenson, T. G. T., and E. Lindgren. 2011. The range of Ixodes ricinus and the risk of contracting Lyme borreliosis with increase northwards when the vegetation period becomes longer. Tick and Tick-borne Diseases 2: 44-49.

Khatchikian, C. E., M. Prusinski, M. Stone, P. B. Backenson, I. Wang, M. Levy, and D. Brisson. 2013. Geographical and environmental factors driving the increase in the Lyme disease vector Ixodes scapularis. Ecosphere, 3(10).