Rise of the Tick
If you are in Connecticut and enjoy the outdoors, then you are no doubt familiar with ticks and the risk of diseases spread by them. When you find a tick in our state, it is most likely the blacklegged “deer” tick (Ixodes scapularis), the American dog tick (Dermacentor variabilis), or even the lone star tick (Amblyomma americanum). Tick-borne diseases are caused by bacteria, parasites, or viruses spread through tick bites. Though the infamous Lyme disease (from Borrelia burgdorferi or B. mayonii) is the most commonly reported tick-borne disease in the United States, you may have also heard of anaplasmosis (from Anaplasma phagocytophilum), babesiosis (from Babesia microti), ehrlichiosis (from certain Ehrlichia spp.) – and the list goes on.
Unfortunately, ticks and tick-borne diseases are increasing in prevalence and range. The blacklegged tick we are so familiar with in Connecticut is expanding its range northward into Canada. Lyme disease in the United States has more than doubled in the last 20 years. The Gulf Coast tick (Amblyomma maculatum), which can transmit a disease called rickettsiosis, was originally only found in our southeastern states but can now be found in Connecticut and also as far west as Arizona.
Why are we finding more ticks and more tick-related illnesses?
Tick and tick-borne disease spread is a multifaceted issue, but it is ultimately caused by human activity and development. Human-driven climate change as well as man-made disruptions to native ecosystems are creating better environments for ticks to thrive in.
The increasing temperatures associated with climate change are creating more favorable habitats for tick populations to spread and thrive. We are experiencing shorter winters with milder temperatures. Warmer weather is allowing ticks to increase their range into areas they were previously not found in at all or where their population was not able to thrive and grow.
For example, blacklegged ticks are most active when temperatures are above 45°F and over 85% humidity. The blacklegged ticks will die starting at temperatures of 14°F and below (though even when winter temperatures drop that low, only 20% of the tick population will die off since ticks will burrow to find warmer temperatures). With historically harsh winter temperatures giving way to a milder season due to climate change, more ticks survive the winter allowing their population to continue to grow at higher rates. With a longer warm season, ticks are emerging earlier in the spring and stick around later into the fall, and can also become active in the winter during warm spells. All of this increases their likelihood of finding hosts and spreading disease.
On top of the risk of our current tick populations being active for longer periods in the year, we also need to be on the lookout for new types of ticks and their diseases, such as the Gulf Coast tick, mentioned earlier. Dr. Goudarz Molaei of the Connecticut Agricultural Experiment Station (CAES) says “It is anticipated that warming temperatures related to climate change may lead to the continued range expansion and abundance of several tick species, increasing their importance as emerging threats to humans, domesticated animals and wildlife.”
The tick population is also on the rise due to the changes humans create in our native ecosystems.
Forest fragmentation and the introduction and spread of invasive plant species lead to a lack of biodiversity. The removal of apex predators and our impact on mesopredators (such as foxes and bobcats) leads to an overpopulation of their natural prey. In Connecticut, the primary reservoir for many common tick-borne diseases, such as Lyme and babesiosis, is the white-footed mouse – a favored host for ticks in their larva and nymph stage. The white-tailed deer is the primary blood meal host for ticks in their adult stage. These hosts for ticks are proliferating (even in urban and suburban areas) since there are less predators and competition to keep their populations in check.
Habitats with few native plants and a dominance of invasive plant species result in a higher population of ticks and tick-hosts, posing an increased risk of spreading tick-borne diseases. Uncontrolled invasive species significantly increase tick and tick pathogen densities. The trails, roads, disturbed riparian zones, and increased forest edges we’ve created are habitats where many of these invasive plants prefer and start to sneak into our natural ecosystems. Since these invasives are not a major food source for any of our native species, their growth and spread is unhindered. For example, Japanese barberry (Berberis thunbergii) has become the dominant understory shrub in many of our forested lands in Connecticut. Its dense foliage provides the shady, moist conditions the blacklegged tick thrives in. Deer will sleep alongside it as cover and it is a favorite choice for the white-footed mouse to call home.
Understanding the factors involved in the spread of tick-borne diseases allows us to find possible strategies and solutions to address the problem.
Check back in next month’s newsletter for the next article on what we can do about this issue!
Resources and for more information:
CAES Press Release: Populations of Gulf Coast Tick
NIH: Range Expansion of Tick disease Vectors in North America
PA DCNR: Will a Harsh PA Winter Kill Ticks?
EPA: Climate Change Indicators: Lyme Disease
UCONN: The Barberry - Lyme Disease Connection
ISAC: The interface between invasives and the increased incidence of tick-borne diseases…
Climate Atlas of Canada: Lyme Disease Under Climate Change
Jessica Kurose is editor of The Quinnehtukqut and part of the communications team for Sierra Club Connecticut.