January 12, 2018

Current Trends Leading to an Increase in Tick-Borne Diseases

Tick-borne diseases are on the rise around the world—and for a variety of reasons.

The following article highlights some of the top trends that are contributing to this increase, including environmental and other natural factors as well as shifts in human behavior and advances in clinical diagnosis. Being aware of these trends can help you and your loved ones better prepare for any increased risk of exposure to ticks and tick-borne diseases today and in the future.

Scientists note a steady increase in tick-borne diseases.1https://www.cdc.gov/cdcgrandrounds/pdf/archives/2017/march2017-H.pdf



Warmer Winters Mean More Ticks—and Longer Tick Seasons

Scientists at the National Oceanic and Atmospheric Administration (NOAA) are predicting higher-than-normal temperatures for many regions during the winter months of 2017 to 2018 of the United States, which would make it the third warm winter in a row for many of these areas. This warming trend can affect tick populations and activity levels in a number of ways. As most ticks are dormant during the winter, warmer weather doesn’t have a huge impact on their survival rates. However, milder winters can make it easier for tick hosts, such as deer and mice, to survive winter months, providing ample food sources for ticks to thrive on the following year.

Certain ticks may also be more active during mild winter months. Deer ticks or blacklegged ticks (Ixodes scapularis), for example, are active year-round but tend to hunker down when temperatures are below freezing. These ticks can transmit a number of serious diseases to humans, including Lyme disease, Powassan, Babesiosis and Anaplasmosis. Additionally, warmer weather means they can be on the hunt for hosts more frequently throughout winter months. What’s more, warmer winters and springs can make it possible for tick eggs to hatch a little earlier and nymphs to venture out and start feeding on larger hosts earlier, resulting in a potentially longer tick season.

 

Orange and red colors indicate where the forecast odds favor a much warmer than usual winter. The darker the color, the stronger the chance of that outcome. Although not shown on this map, above-average temperatures are also favored in Hawaii and in western and northern Alaska. NOAA Climate.gov map, based on data from NOAA CPC.2 https://www.climate.gov/news-features/blogs/enso/winter-coming-noaa%E2%80%99s-2017-2018-winter-outlook


An Unusual Season For Trees Helps Tick Populations Flourish

Trees don’t typically produce a lot of acorns each year, but in a “mast year”—which happens only a few times a decade, they can produce up to 100 times their normal amount. 2015 was a mast year for oak trees in the United States, which resulted in a bumper crop of acorns in certain regions of the country, particularly the Northeast. It turns out that all those acorns provided a bounty of food for rodents and pests, including white-footed mice, which are prime carriers of Borrelia burgdorferi, the bacterium that causes Lyme disease.  Over the past two years, all those mice have served as ideal hosts for ticks—including the blacklegged tick. Unlike many other tick hosts such as deer and squirrels, mice don’t typically groom off ticks, which increases the likelihood that the ticks feeding on them will become infected. 3”Mystery of the Mast Year,” AMC Outdoors, August 25, 2017. https://www.outdoors.org/articles/amc-outdoors/mystery-mast-year 4”Increases in acorns, mice mean more disease-transmitting ticks this year, experts say,” April 19, 2017, News at Mason, George Mason University. https://www2.gmu.edu/news/394386

Scientists predict that the growing population of tick larvae that have been feeding on these mice this past year will be mature enough to start feeding on larger hosts, including humans, in the spring and summer of 2018.


As Humidity Levels Rise, So Does the Likelihood For Tick-Human Interactions

Although ticks can live and thrive in a variety of locations and environments, their questing behavior—or their efforts to seek out and attach to hosts—can differ from region to region. A recent study5http://www.climatecentral.org/gallery/graphics/climate-changes-role-in-tick-migration showed that increasingly humid environments in the Northeast region of the United States are making it easier for ticks to venture up from moist ground cover during questing.  This can increase the likelihood that they can latch on to humans and pets as they walk past grassy areas, logs, wood piles or other above-ground perches. This contrasts to dryer environments, where ticks tend to stay close to the ground in leaf litters and around tree roots where moisture levels are higher.


Bird migration Expands Tick Populations, Varieties and Habitat 

Every year, migratory birds transport as many as 39 million different neotropical ticks from Central and South America to different regions of the United States. A recent study to determine the effect of bird migration on the spread of ticks across America found that this trend is not only increasing tick populations nationwide, it’s also introducing ticks to new regions and habitats.  In addition, scientists found that some of these ticks were infected with spotted fever bacteria, Rickettsiae. Since many of the transported ticks were in the larvae or nymph stage, they could still become infected with pathogens for other tick-borne diseases.6”Lyme Disease Spread by Migratory Birds,” Global Lyme Alliance, Blog, July 13, 2016. https://globallymealliance.org/lyme-disease-spread-migratory-birds/ 7”The Postential Role of Migratory Birds in the Spread of Tick-borne Infections in Siberia and the Russian Far East,” Achievements in Life Sciences, Far Eastern Federal University, Vol. 8, Issue 2, October 2014.  https://www.sciencedirect.com/science/article/pii/S2078152015000061

Similar findings have been reported in studies on the potential role of migratory birds in the spread of tick-borne diseases in Europe and Asia. In one study, for example, researchers found ticks on migratory birds in Siberia and Far East Russia were infected with eight species of pathogens, including those that cause Lyme disease, Spotted Fever, Babesiosis, Bartonellosis, Ehrlichiosis, and Anaplasmosis.


Homes Encroaching On Tick Habitat

Another important trend that has contributed to the higher incidence of tick-borne diseases is suburbanization. Most ticks live in wooded areas, meadows and grasslands where hosts, such as deer, squirrels, mice, birds and other rodents thrive. In the past, people encountered ticks only occasionally from recreational activities, hunting or other outdoor activities. Suburbanization, however, has significantly changed the frequency of human exposure to tick habitats, as homes and neighborhoods increasingly expand into environments where tick populations thrive.  In a recent publication by the World Health Organization, researchers state8”Ticks,” Public Health Significance of Urban Pests, World Health Organization, Regional Office for Europe, 2008.   https://pubs.er.usgs.gov/publication/5211434:  “… suburbanization has contributed substantially to the increase in tick-borne disease transmission in North America by fostering increased exposure of humans to tick habitat. The current trend toward suburbanization in Europe could potentially result in similar increases in transmission of tick-borne diseases.”

The publication goes on to advise that consulting with vector-borne disease experts during planning stages of new developments can help minimize the potential for residents to encounter infected ticks by identifying more effective dwelling and landscape designs. Moreover, it suggests that public education programs aimed at those living in higher risk areas for tick encounters coupled with ongoing surveillance to monitor the distribution and spread of ticks can help lower the incidence of tick-borne diseases.


Improved Diagnostics and Clinical Recognition

In addition to trends that affect tick populations and proximity to humans, the increase in tick-borne diseases is also due in part to better clinical recognition and more accurate methods of diagnoses. In short, these advancements mean more cases of tick-borne diseases are being identified and reported over time.  New and more advanced laboratory methods are not only making it possible to diagnose certain tick-borne illnesses earlier, they are also helping researchers identify new disease pathogens, including two new species of the Borrelia bacteria: B. mayonii, which can causes Lyme disease in humans, and B. miyamotoi, which can cause Tick-Borne Relapsing Fever (TBRF) in humans.9https://www.cdc.gov/media/releases/2016/p0208-lyme-disease.html 10https://www.cdc.gov/ticks/diseases/trends.html 11https://www.igenex.com/tick-borne-diseases/borrelliosis-relapsing-fever-disease/


Additional Resources

References   [ + ]

1. https://www.cdc.gov/cdcgrandrounds/pdf/archives/2017/march2017-H.pdf
2. https://www.climate.gov/news-features/blogs/enso/winter-coming-noaa%E2%80%99s-2017-2018-winter-outlook
3. ”Mystery of the Mast Year,” AMC Outdoors, August 25, 2017. https://www.outdoors.org/articles/amc-outdoors/mystery-mast-year
4. ”Increases in acorns, mice mean more disease-transmitting ticks this year, experts say,” April 19, 2017, News at Mason, George Mason University. https://www2.gmu.edu/news/394386
5. http://www.climatecentral.org/gallery/graphics/climate-changes-role-in-tick-migration
6. ”Lyme Disease Spread by Migratory Birds,” Global Lyme Alliance, Blog, July 13, 2016. https://globallymealliance.org/lyme-disease-spread-migratory-birds/
7. ”The Postential Role of Migratory Birds in the Spread of Tick-borne Infections in Siberia and the Russian Far East,” Achievements in Life Sciences, Far Eastern Federal University, Vol. 8, Issue 2, October 2014.  https://www.sciencedirect.com/science/article/pii/S2078152015000061
8. ”Ticks,” Public Health Significance of Urban Pests, World Health Organization, Regional Office for Europe, 2008.   https://pubs.er.usgs.gov/publication/5211434
9. https://www.cdc.gov/media/releases/2016/p0208-lyme-disease.html
10. https://www.cdc.gov/ticks/diseases/trends.html
11. https://www.igenex.com/tick-borne-diseases/borrelliosis-relapsing-fever-disease/

Share