Please enjoy this guest post from Matthew Yoskowitz! Matt, aka "Yosko" to his students, is a high school biology and environmental science teacher in Vermont, and he's also my brother in law! He's a consulting author for SquidBooks (an online science textbook) and an expert on all things bird watching. In his spare time, you can find him enjoying the outdoors with his wife and kids, brewing beer (and cider), or smoking all the meats.
Was your NCAA bracket busted? No problem, there is another tournament that you should be following. March Madness segues into Spring Sadness as those of us in the Northeast begin to turn our thoughts away from winter and back towards warmer weather. “Wait a minute,” you’re thinking to yourself, “isn’t this a good thing?” Let us not forget that this increase in temperature will once again awaken those who have been lying dormant during the winter months. Folks, the ticks of the Northeast, and all across the northern United States are waking up, and they are ready to play. Tick species, and the illnesses they carry, are about to start their own tournament of champions, but who will win?
WEST SOUTH
Western Black-legged Tick Lone Star Tick
EAST MIDWEST
Black-legged Tick (Deer Tick) Brown Dog Tick
If we were to actually play this tournament out, the Black-legged, or Deer Tick, as it is more commonly known, would win hands down. It is a perpetual champion for a variety of reasons. Deer ticks have one the largest geographical distributions of the various species, and are notorious for spreading Lyme disease. This illness is actually caused by a bacteria species that is passed from an infected tick to the organism it is feeding on, leading to a myriad of symptoms that can range from mild to severe. According to the CDC (2021), Lyme disease is also the most common bite-transmitted disease in the United States. However, one might not think this would be the case after a quick glance at the disease distribution map below. Despite the Deer tick’s large distribution, Lyme disease seems to be confined to the Northeast and Northern Midwest when compared to the rest of the country. Why is that?
There are two major factors contributing to this, starting with the geographical distribution of the tick species that carry the disease. Again, this notion might sound contradicting, because Lyme is not prevalent in the entirety of the range. Regardless, it is fair to say that there is an extremely low chance of the disease being present in areas where the Blacklegged and Western Blacklegged ticks do not live. Looking at the CDC (2020) and Geology.com maps below help explain why Lyme disease is rare in parts of the Midwest and Western United States. However, this does not explain the lack of Lyme in the rest of the Midwest and the South. For that we turn to temperature and humidity.
The difference in regional climates is what answers this question, and backs up the previous factor. These two species of tick prefer mild temperatures coupled with adequate humidity. Researchers also found that climate is a driving factor of tick behavior, which can be correlated to infection rates.
Howard Ginsberg and colleagues at the University of Rhode Island and Michigan State University have been studying eastern Deer ticks since 2014, and have compiled evidence to back up three major findings. The first is that the larval stage of these ticks live longer in relatively cool temperatures. The take away from this is that the longer the larvae can stay alive, the more chances it will have at attaching to a host, perpetuating the spread of Lyme disease.
In 2015, the second of their findings showed that ticks in the wild will often climb plant stems or grass, allowing them to wait for a host to pass by, but southern ticks usually stay hidden under the leaves on the forest floor. Think about it like this; if it is extremely hot and humid outside, where do you go to avoid being uncomfortable? Perhaps you take a dip in a pool, go inside, maybe under a fan, or do you crank the AC? Ticks are no different; environmental conditions dictate behavior. In the Northeast and upper Midwest, ticks have the luxury of hanging out wherever they feel like, waiting for their next host to saunter by, versus having to hide under the shade of leaf litter in the South. People are more likely to brush up against a bush with a tick on it up north as opposed to walking right over the top of it down south.
In 2017, Ginsberg conducted another study looking into the cause behind these different behaviors; the third major finding. Temperature and humidity are key factors in determining the life span of a tick. If it is too hot and too dry, ticks in all life stages will dry up and die quickly. This also backs up why they are not found in some of the midwestern plains or western desert regions. In the study, the ticks made to experience the climate conditions of the south, hot and humid, can live longer, but ultimately succumb. Ticks that experienced a northern climate simulation thrived. This suggests that ticks from northern climates are best suited to openly go about their business with little hindrance from their environment, leading to more contact with humans, and ultimately more cases of Lyme disease
To sum up, there seems to be a combination of factors that lead to higher incidences of Lyme disease in the Northeast and upper Midwest; the geographic distribution of the two species that carry the disease coupled with the regional climates in these areas of the United States. With global climate change occurring, we can expect to see a migration of Lyme disease northward as ticks behaviors are partially driven by temperature and humidity.
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