The black legged tick population is booming. Is climate change to blame? And
will tick-borne diseases increase?
By Kevin Ambrose
The U.S. Centers for Disease Control and Prevention reports the number of tick-borne diseases is increasing at
a record pace while the geographic range of ticks continues to expand. Lyme disease is the most commonly
known tick-borne disease, but other diseases, such as ehrlichiosis and STARI, have been discovered and the list
of tick-related illnesses continues to grow.
Tick bites have even been found to cause allergic reactions to red meat in some people.
Is a warming climate responsible for this growing tick threat? Should we expect more ticks and tick-related
diseases this year because this past winter was warm and wet? And how do we protect ourselves from ticks?
I interviewed Matthias Leu, an ecologist and assistant professor at the College of William & Mary, to find
answers to these questions and to learn more about tick prevention. The interview is below.
Does weather influence tick populations? For example, does a warm winter lead to more
ticks, or does a cold winter lead to less ticks?
So far, we have not found an association with weather and tick populations. Many people think cold winters kill
ticks. If that is true, why do ticks live in northern states, like Minnesota and Wisconsin? What does influence
the tick population is the amount of deer and mice available to serve as hosts for the ticks.
The Washington area has experienced a growing deer population over recent decades. Has
the tick population increased in direct proportion?
Yes, urbanization has led to a growing population of deer and mice, which are used by the ticks for blood meals,
and that in turn increases the tick population. Flowers and bushes planted by homeowners are feeding the deer
Have tick-borne diseases increased, and does weather influence the spread of these diseases?
Yes, tick diseases have increased and one tick-borne disease in particular, ehrlichiosis, is impacted by the
weather. Ehrlichiosis, which produces symptoms much like Lyme disease, is passed to humans by Lone Star
ticks that feed on fawns or other hosts, such as rabbits and squirrels. Adult deer have stronger immune
systems, which keeps the ehrlichia bacteria in check, but fawns carry much more of the bacteria. During coldwinters, when the number of fawns is likely lower, the bacteria is not as widespread, which lowers the disease
rate. The opposite is true during warm winters.
Is the spread of Lyme disease influenced by the weather?
The bacteria that causes Lyme disease is passed on by black-legged ticks that feed on white-footed mice, which
carry the bacteria. The bacteria is the causative agent of Lyme disease. It has been found that the bacteria is
more prevalent after warm winters and springs, and also during mast years, when acorns are plentiful. Acorns
are a food source for the mice, which helps to increase their numbers and thus increase the disease rate.
I understand you harvest ticks each year in southeastern Virginia. Have you found the
numbers of ticks have increased from year-to-year?
We found the numbers of ticks fluctuate greatly from year-to-year, and there’s a lot of movement of the ticks
related to the location of their primary food source, deer and mice. We’re just about to do our 2019 tick harvest,
so I’ll have those results soon.
I’ve heard about people developing meat allergies from tick bites. Can you explain?
Yes, alpha-gal syndrome occurs when a tick bites a human and transmits a carbohydrate that it received while
feeding on another mammal host. In some people, the alpha-gel carbohydrates trigger an immune system
reaction when they eat red meat, which can be quite severe. There is no treatment for alpha-gal syndrome
except avoiding red meat.
How do you recommend keeping tick-free when walking through woods or fields?
I have found DEET doesn’t work well for ticks, but permethrin works extremely well. I spray my clothes with
permethrin and let them dry for 24 hours. Ticks will die on contact with the sprayed clothing. Clothes can now
be purchased pre-treated with permethrin.
How do you recommend removing ticks after they have bitten?
I use tweezers to grab the tick’s head as close to the skin a possible. Once I have a firm grip, I pull the tick
straight out and avoid twisting the tick’s body. This should ensure the tick head doesn’t remain embedded in
While a warming climate will provide favorable living conditions for ticks, it’s also the population explosion of
deer and other mammals that live around us that influences the spread of tick-borne diseases.
Urbanization and the fragmentation of forests has brought many of these animals and their hosted ticks
directly into our backyards. Ticks are found near their hosts, and the spread of tick-borne diseases is happening
in many areas that have both warm and cold climates.
Some tick-borne diseases, including Lyme disease, are more prevalent in warm conditions. A study published
in the fall found for a future warming of 3.6 degrees (2 degrees Celsius), “the number of [Lyme disease] cases in
the United States will increase by over 20 percent in the coming decades.”
More research is needed to understand fully the interaction of weather and tick/host distributions.
I contracted Lyme disease in 2003 from a deer tick bite I received while mowing grass in my backyard. The tick
bite produced a bull’s eye rash and flulike symptoms, including extreme fatigue and painful, swollen joints. I
received prompt treatment with antibiotics (doxycycline) and made a full recovery, but it took almost nine
months to feel normal again.
Unfortunately, Lyme disease can become chronic if not treated soon after the bite. It’s important to check for
ticks after venturing outside, particularly in areas with tall grass or brush. Here are tips for preventing tick bites.
"Thanks to Kevin Ambrose and the Washington Post as well as Leu and his research team at the College of William & Mary for the interview. The team
Matthias Leu, ecologist, assistant professor
Oliver Kerscher, molecular biologist, assistant professor
Brent Kaup, sociologists, assistant professor
Dylan Simpson, graduate student biology, William & Mary, now Rutgers University
Olivia Spencer, undergraduate student biology, William & Mary .
Thanks to University of Manitoba for the Video
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