I love going for walks in the woods — it’s good for the body and mind. But since Lyme disease has become such a major health issue, I always make sure to check for ticks after I get back to the house.
Plus, now I know to keep an eye out for acorns, mice, chipmunks, opossum, and raccoons — just to name a few. That’s because a new study reveals how the biodiversity of the forest can increase the chances of being bit by a Lyme-infected tick.
Lyme disease baffles modern medicine
Scientists with Centers for Disease Control (CDC) can tell you about the symptoms and mainstream treatments for Lyme disease and what black-legged ticks (the disease carriers) look like. But they knew very little about the root of the problem — or how to control it. They just don’t put a lot of resources into studying it.
On the other hand, environmental scientists like my daughter delve deeply into Lyme disease…and they look at the broader picture. (My daughter actually just completed her Master of Science in Environmental Sciences, which includes the fascinating study of forest ecology.) And they’ve made some excellent progress in understanding why Lyme disease rates have skyrocketed in recent decades in the northeastern U.S.
In fact, over the past 20 years, scientists at the renowned Cary Institute of Ecosystem Studies in Dutchess County, New York, have been studying the spread of Lyme disease. I have passed there many times when driving between Florida and New England (and avoiding New York City traffic).
In the early 1990s at the Walter Reed National Military Medical Center, I also worked very closely with the woman who is today the Chief Medical Examiner of Dutchess County. So, if there were any fatal cases of Lyme disease, they should have been reported to her.
Researchers closely examine ecosystem
For the new study, researchers collected data at six forested field plots (around 5.5 acres each) on the grounds of the Cary Institute — which sits in the beautiful Hudson River Valley — over a 20-year period.
First, they tracked numbers of black-legged ticks, the species that spreads the Borrelia burgdorferi bacterium that causes Lyme disease.
Interestingly, black-legged ticks are actually born free of the bacterium that causes the Lyme disease infection. And they spend 90 percent of their lives freely wandering the forest.
They only acquire the bacterium when they take a blood meal from an infected mammal. Interestingly, they only take three blood meals in their entire lives — one during each of the three-stages of their lives: larva, nymph, and adult. (Each stage lasts about two years.)
For this study, researchers captured free-living ticks using drag cloths at sample sites. Over the 20-year study, they documented 147,238 larvae and 11,115 nymphs. And they tested more than 7,000 nymphs for the presence of Lyme disease.
What makes this a wonderful study is that the researchers took it a step further and looked beyond black-legged ticks…
The surprising role of small and big mammals
Over the 20-year study, the researchers tracked small mammals such as eastern chipmunks and white-foot mice, from which ticks take their blood meals.
They also tracked populations of small predatory animals like bobcats, foxes, and opossums, as well as larger ones like coyotes.
The researchers safely captured all the animals, recorded their tick burdens, and tagged them. The captures occurred over several days every three to four weeks from May to November.
During the study period, they also noted the make-up of the forest — whether it was dense and intact…or sparse and fragmented. Additionally, they noted the daily temperature and humidity. They also made observations regarding the prevalence of acorns on the land plots.
With all that detailed data, they came away with some very interesting conclusions…
1.) Mice and chipmunks are major carriers of Lyme disease
Most people think deer are the biggest carriers of Lyme disease. But this study found eastern chipmunks and white-food mice are the strongest reservoirs. In fact, mice can host up to 50 feeding ticks while still functioning normally.
2.) Small predators help keep infection rates down
The researchers specifically linked high numbers of small predators — including bobcats, foxes, and opossums — with lower infection rates. These predators help keep down populations of disease-carrying mice and chipmunks.
On the other hand, the researchers noticed a link between high numbers of larger predators, like coyotes, with higher infection rates. That’s because in those areas, the smaller predators that did all the “dirty work” were displaced or had been preyed upon.
And once the smaller predators are out of the picture, it increases the likelihood smaller animals will pick up and carry the infection. Overall, sites with more predator diversity had lower infection rates compared to areas dominated by coyotes.
So, if you see lots of chipmunks and mice on your walk in the woods — it’s probably a sign there are lots of Lyme-infected ticks in the area. On the other hand, if you see lots of foxes and opossum, you’re most likely in a less-infected area.
3.) More acorns signify more Lyme disease
As I mentioned earlier, the environmental scientists at Cary observed the prevalence of acorns on the forest floor during the 20-year study.
They noted this detail because when acorn abundance is greater, more rodents feed on them and successfully survive the winter to breed in the spring.
In fact, when they observed an abundance of acorns on the plots, there was an increase in rodent populations the following year. And there was an increase in the number of infected ticks two years after acorn boom years.
So, if you see lots of acorns on your walk, you can bet in another two years the area will be swarming with infected ticks.
4.) Denser forests equate to less Lyme disease
The Cary researchers also noted forest density at each of the six sites.
Why would they do that?
They knew that sparse, patchy forests can’t support small predators like foxes that hold down the mouse and chipmunk populations.
Indeed, they found the most infected ticks at sites with fragmented forests. On the other hand, the sites with the thickest forest cover supported a greater diversity of predators, thereby reducing infected-tick populations.
The take away? Steer clear of small, sparse forests and take walks in areas with dense coverings. Many state parks are ideal locations with safe walking paths.
5.) Weather plays a part too
Temperature, humidity, and precipitation also influence the number of infected ticks in an area. Specifically, the researchers noted an association with warm, dry weather during the spring and winter weather with fewer infected ticks. On the other hand, warm, wet weather during the winter and spring seems to increase the number of infected ticks.
These findings make sense as ticks need moisture to survive. Since they spend at least 95 percent of their time free-living, without feeding on hosts, they can dry out if the climate is too dry.
Holistic, commonsense approach
The problem of Lyme disease is not going to be understood by conducting controlled experiments in comfortable labs at the CDC. You need to look at the whole ecology. And that’s a good lesson for really understanding all health problems as well.
The Cary Institute is currently conducting a smaller, five-year study to test whether neighborhood-based prevention can reduce human exposure to Lyme disease. And I’ll keep you posted on their findings.
In the meantime, I recommend taking Nature walks whenever you can. Just be sure to choose areas with dense, full forests. Also, be sure to take these safety measures:
- Wear long-sleeved pants and shirts on your walks.
- Walk in the center of the trail.
- Bathe or shower as soon as possible after outdoor activities to wash off ticks that may be crawling on you.
- Do a full-body tick check using a mirror. Or better yet, perform it with a partner.
- Look closely at the places ticks commonly attach themselves:
- Around the waist
- Behind the knees
- Between the legs
- In and around the ears
- In the belly button
- In the hair
- Under the arms
- Ticks can also hitch a ride into your house on pets, clothing, or hiking gear. So, examine both!
- Tumble dry clothing and gear in a dryer on high heat for an hour to kill remaining ticks.
Happy trails!
Sources:
“Tick-borne disease risk in a forest food web,” Ecology, 2018; 99 (7): 1562
“Science-Based Responses to Commonly Asked Questions About Tick-Borne Disease Prevention,” Dutchnessny.gov (www.dutchessny.gov)