Lyme Disease Ticks Double Range Over Past Twenty Years

More on Lyme Disease on MedicalResearch.com

MedicalResearch.com Interview with:
Rebecca Eisen PhD research biologist and
Ben Beard, Ph.D. 
Chief, Bacterial Diseases Branch
Division of Vector-Borne Diseases
Centers for Disease Control and Prevention

Medical Research: What is the background for this study?

Dr. Eisen: Since the late 1990s, the number of reported cases of Lyme disease in the United States has tripled and the number of counties in the northeastern United States that are considered high-risk for Lyme disease has increased by more than >320%.

In 1998, a comprehensive review was published that described the geographic distributions of the blacklegged tick (Ixodes scapularis) and the Western blacklegged tick (Ixodes pacificus). These ticks are responsible for infecting humans with the pathogens that cause Lyme disease, anaplasmosis, and babesiosis.

Medical Research: Would you tell us about the methodology?

Response: CDC researchers recently published an update to the 1998 tick distribution map. The authors reviewed the scientific literature and individual state health department websites for data. Additionally, they contacted public health officials, entomologists, and Lyme disease investigators throughout the United States to assess county-level tick collection data.

Researchers characterized counties with Ixodes scapularis and Ixodes pacificus ticks as “established” if at least 6 individual ticks or at least 2 of the 3 tick life stages had been identified during a collection period. Counties were characterized as “reported” if at least one tick of any life stage had been identified at any time in that county, or if county records did not specify the number of ticks or life stages collected.

Medical Research: What are the main findings?

Dr. Eisen: Established populations of ticks:

  • The number of counties in which the tick is classified as “established” has more than doubled since 1998. Ixodes scapularis now is classified as established in more than a quarter of counties (27.1%) in the contiguous United States. Established populations are found in 35 states.
  • Ixodes pacificusis now classified as “established” in 95 counties that span six states compared to 1998, when it was established in 90 counties and six states. (The six states include primarily California, Oregon, and Washington, but also have populations in Utah, Arizona, and Nevada.)

Tick presence in counties and states:

  • The number of counties in the contiguous United States that have recorded the presence of I. scapularisand I. pacificus has increased by 45% since the previous map was published in 1998.
  • The presence of I. scapularis has now been documented in nearly half (46%) of counties in the contiguous United States (1,420 of 3,110 counties).
  • The presence of I. pacificus has now been documented in fewer than 5% (3.6%) of counties in the contiguous United States (111 of 3,110 counties)

 Other key findings:

  • Kentucky, North Dakota and Ohio now have populations of established I. scapularis, but didn’t in 1998.
  • No single state has records of both tick species.
  • Five states in the Rocky Mountain region lack records for either I. scapularisor I. pacificus: Colorado, Idaho, Montana, New Mexico and Wyoming.

Our study shows that over the past nearly 20 years, the distribution of Lyme disease vectors has changed substantially, with notable increases in the Northeastern and North-Central States.   The tick is now established in areas where it was absent 20 years ago.

As a result, a lot of people are seeing ticks where they didn’t see them 20 years ago.  However, it’s important to remember that although our map shows a wide distribution of the ticks that are capable of transmitting Lyme disease spirochetes, risk of people getting Lyme disease is not equal across areas of the country where the tick is present.  This is largely because of differences in how many ticks are likely to bite people and how many of these ticks are infected with LD spirochetes.

The observed range expansion documented in our study highlights a need for continuing and enhancing vector surveillance efforts, particularly along the leading edges of range expansion.  It’s important to know which ticks are in your area or areas that you visit so that you can take step to protect yourself.

It’s important to remember that each year in the U.S., roughly 300,000 people are diagnosed with Lyme disease.  Taking steps to protect yourself from ticks is always important.  CDC recommends that people:

  • Learn which tickborne diseases are common in their area.
  • Avoid areas with thick vegetation, high grass and leaf litter; walk in the center of trails when hiking.
  • Use repellent that contains 30 percent DEET on exposed skin for protection that lasts several hours. Parents should apply repellent to children; the American Academy of Pediatrics recommends products with up to 30 percent DEET for kids. Always follow product instructions.
  • Use products that contain permethrin to treat clothing and gear, such as boots, pants, socks and tents or look for clothing pre-treated with permethrin.
  • Bathe or shower as soon as possible after coming indoors to wash off and more easily find crawling ticks before they bite you. Remove all attached ticks as soon as possible. Regularly treat dogs with products that kill and/or repel ticks.

Additional Links:

http://www.ncbi.nlm.nih.gov/pubmed/26564403

Medical Research: What should clinicians and patients take away from your report?

Dr. Beard: Healthcare providers should be aware of the risk of Lyme disease and other tick-borne illnesses carried by Ixodes ticks (scapularis and pacificus) in the areas where these ticks occur with the caveat that in the southeastern regions of the U.S., Ixodes ticks prefer non-human hosts and are less likely to bite people. The ticks that bite people in the north are more likely to carry the bacteria that causes Lyme disease.

Medical Research: What recommendations do you have for future research as a result of this study?

Dr. Eisen: Our study sought to assess the geographical distribution of ticks that are capable of transmitting Lyme disease spirochetes, as well as other pathogens that can cause anaplasmosis, babesiosis or other diseases.  However, it’s important to remember that although our map shows a wide distribution of the ticks that are capable of transmitting Lyme disease spirochetes, risk of people getting Lyme disease is not equal across areas of the country where the tick is present.  This is largely because of differences in how many ticks are likely to bite people and how many of these ticks are infected with LD spirochetes.  Additional studies are needed across the counties in which the ticks are established to determine the abundance of ticks, particularly the nymphal life stage which is most often the one that transmits the Lyme disease pathogen to humans, and the infection rates in those ticks.

Additional studies are needed to evaluate the best methods for preventing human encounters with infected ticks.

Medical Research: Is there anything else you would like to add?

Dr. Eisen: These ticks are being found in areas where they weren’t 20 years ago, and we wanted to raise awareness about the fact ticks are spreading. It’s important for people to be aware that there may be ticks in areas where they haven’t seen them previously so that they can take steps to help protect themselves and their families.  CDC recommends that people:

  • Learn which tickborne diseases are common in their area.
  • Avoid areas with thick vegetation, high grass and leaf litter; walk in the center of trails when hiking.
  • Use repellent that contains 30 percent DEET on exposed skin for protection that lasts several hours. Parents should apply repellent to children; the American Academy of Pediatrics recommends products with up to 30 percent DEET for kids. Always follow product instructions.
  • Use products that contain permethrin to treat clothing and gear, such as boots, pants, socks and tents or look for clothing pre-treated with permethrin.
  • Bathe or shower as soon as possible after coming indoors to wash off and more easily find crawling ticks before they bite you. Remove all attached ticks as soon as possible. Regularly treat dogs with products that kill and/or repel ticks.

Citation:

Rebecca J. Eisen, Lars Eisen, Charles B. Beard. County-Scale Distribution of Ixodes scapularis and Ixodes pacificus (Acari: Ixodidae) in the Continental United States. Journal of Medical Entomology, January 2016 DOI: 10.1093/jme/tjv237

Rebecca Eisen PhD, & Ben Beard, Ph.D. (2016). Lyme Disease Ticks Double Range Over Past Twenty Years 

More on Lyme Disease on MedicalResearch.com

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