The spread of tickborne diseases to humans, including Lyme disease and anaplasmosis, has been increasing in Vermont and many other northern states. Over 380 confirmed cases of Lyme disease have been reported in Vermont each year since 2011, up from 60 or fewer before 2006. Vermont had the highest incidence of Lyme disease in the U.S. in 2015. Confirmed cases of anaplasmosis reported in Vermont have also steadily increased from three in 2010 to 67 in 2014. Climate change can affect the way tickborne diseases spread.
Warmer weather is one of several factors that have contributed to the spread of ticks in Vermont and the rise in tickborne diseases. Warmer winters make it easier for ticks to survive here year-round, while a longer warm season increases the length of time that ticks are active each year. Other factors contributing to an increase in tickborne diseases include better diagnosis and reporting by physicians, changes in forest cover, and changes in deer and small mammal populations that serve as hosts for tickborne diseases.
In areas where tickborne diseases are present, the likelihood of a person getting a tickborne disease depends on three factors:
- How many ticks are in the area
- How many of those ticks are infected with the pathogen
- How often people come into contact with those ticks.
To a varying degree, climate change can affect all three of these factors:
- Ticks can only live in areas where the climate is suitable for them, having the right temperature and the right amount of moisture. Warming temperatures due to climate change may make Vermont more hospitable to blacklegged ticks. This could result in tick populations increasing in areas where they are already present, and the introduction of ticks to areas that were not previously infested, such as colder, northern areas and areas at higher elevations.
- Work done by our partners at Lyndon State College suggests that areas more densely populated with ticks in Vermont also tend to have higher rates of infection with Borrelia burgdorferi (the bacteria that causes Lyme disease) among these ticks. This suggests that in areas where blacklegged tick populations grow due to climate change, a greater proportion of these ticks may become infected with the bacteria.
- Ticks are typically not active at temperatures below 40oF. Warming temperatures due to climate change mean more days when ticks are active and looking for blood meals, which means a greater risk of ticks biting people.
There are four types of ticks that can transmit diseases that currently exist in Vermont: the blacklegged (deer) tick (Ixodes scapularis), the American dog tick (Dermacentor variabilis), the woodchuck tick (Ixodes cookei), and the lone star tick (Amblyomma americanum). Nearly all (99%) of cases of tickborne diseases in Vermont are from blacklegged (deer) ticks, which are among the most commonly encountered ticks in the state.