Warmer Temps Are Fueling Lyme Disease Increases In These Counties

One consequence of a warming planet is an increase in disease-transmitting vectors like mosquitos, ticks, and fleas. Deer ticks are most active when temperatures are above 45°F – a threshold that is becoming more common as winters become shorter and milder. As the range of suitable habitat for ticks expands, the risk of Lyme disease increases.

Over the past 15 years, Lyme disease cases have surged across the Northeast and spread into regions where the illness was once virtually unheard of. Maine’s incidence rate has nearly increased tenfold, adding more than 100 new cases per 100,000 residents. Vermont and West Virginia have seen sharp spikes as well, along with traditionally lower-risk states to the west such as Wisconsin, Iowa, and Michigan. A closing look at the data highlights fast-warming counties where Lyme disease is spreading at an especially rapid pace.

To determine the counties where climate change is contributing to an increase in Lyme disease, Climate Crisis 247 reviewed data on Lyme disease incidence from the Centers for Disease Control and Prevention. Counties were ranked based on the 15-year change in the incidence rate of Lyme disease per 100,000 residents from 2004-2006 to 2019-2021. Supplemental data on average temperature are from the NOAA.

5. Lincoln County, ME

In coastal Lincoln County, where forests, fields, and rocky shoreline support plenty of deer and mice, Lyme disease incidence rose from 40.7 cases per 100,000 residents in 2004-2006 to 253.8 cases per 100,000 residents in 2019-2021 — an increase of 213.1 cases per 100,000. The county’s cool, humid climate was already favorable for blacklegged ticks, and average summer temperatures are now 2.6°F above normal. Warmer, longer seasons mean ticks stay active for more of the year. As a result, residents now face a far higher risk of encountering infected ticks than they did 15 years ago.

4. Dickinson County, MI

In heavily forested Dickinson County in Michigan’s Upper Peninsula, Lyme disease incidence jumped from just 1.3 cases per 100,000 residents in 2004-2006 to 226.0 cases per 100,000 residents in 2019-2021 — an increase of 224.6 per 100,000. This is a striking change for a region once considered too cold for ticks to be a major problem. Average summer temperatures now run 2.4°F above normal, extending the window of time when ticks can feed and reproduce. With dense woods and abundant wildlife hosts, the county has quickly become suitable habitat for expanding tick populations.

3. Waldo County, ME

Waldo County, a rural stretch of central coastal Maine with forests and farmland, saw Lyme disease incidence climb from 9.2 cases per 100,000 residents in 2004-2006 to 291.2 cases per 100,000 residents in 2019-2021 — an increase of 282.0 per 100,000. The landscape already supported deer and rodents that ticks rely on, and summers have warmed by 2.4°F above normal. Those warmer conditions help ticks survive the winter and stay active when temperatures would once have slowed them down. That combination has turned Waldo County into a clear hotspot for Lyme transmission.

2. Hancock County, ME

In Hancock County, home to Acadia National Park’s forests and hiking trails, Lyme disease incidence rose from 10.3 cases per 100,000 residents in 2004-2006 to 300.9 cases per 100,000 residents in 2019-2021 — an increase of 290.6 per 100,000. The county’s mix of woods, brush, and abundant wildlife already made it prime tick country. Average summer temperatures are now 2.5°F above normal, reducing winter die-offs and lengthening the tick season. That shift has made encounters with infected ticks far more common than they were a decade and a half ago.

1. Knox County, ME

Knox County, a coastal region with thick woods and a strong deer population, has the largest increase on the list. Lyme disease incidence rose from 37.7 cases per 100,000 residents in 2004-2006 to 411.1 cases per 100,000 residents in 2019-2021 — an increase of 373.4 cases per 100,000 residents. Summers are now 2.5°F warmer than normal, allowing ticks to stay active later into fall and return earlier in spring. In a landscape already well suited to ticks and their animal hosts, that extra warmth boosts survival and reproduction. The result is one of the most intense concentrations of Lyme disease risk in coastal Maine.