The Link Between Lyme Disease and Biodiversity

When it comes to Lyme disease, is it possible deer are not the bad guys?

Exposure to Lyme disease is a fact of life for many gardeners.  Some of us spend a lot of time and money trying to reduce our risk of contracting the disease and it’s possible we’ve been focusing on the wrong variables.

I recently attended a lecture by Dr. Richard S. Ostfeld of the Cary Institute of Ecosystem Studies located in Millbrook, NY about his research into the many inter-related factors that are contributing to the rapid increase in Lyme disease cases. While Ostfeld’s 15+ years of research has been conducted primarily in Hudson Valley region, his findings are transferable to similar ecosystems around the US.

Ostfeld and his team have found that the primary culprit in the skyrocketing increase in Lyme disease cases is the reduction of biodiversity due to habitat fragmentation and destruction and its impact on the number and composition of host species for ticks.

Perhaps deer are not the primary villains in the Lyme disease story as we’ve been lead to believe.

Lyme Disease Basics

Lyme disease is transmitted to humans by the bite of a blacklegged tick (Ixodes scapularis), or on the Pacific coast by the related western blacklegged tick (Ixodes pacificus), that is infected with the Lyme disease bacterium, Borrelia burgdorferi. (For the purpose of this post, I will refer to both the blacklegged tick and the western blacklegged tick simply as ticks.)

 Ticks have a two-year life cycle during which they have distinct life stages – larva, nymph, and adult. During each stage, the tick takes one blood meal from a host mammal and then molts into the next stage. After an adult lays her eggs, she dies.

When larval ticks hatch from their eggs, they are NOT infected with the pathogen that causes Lyme disease. Many ticks will never become infected but others, often depending on which host animal they get their first blood meal from (more on that later), will become infected.

Widespread and Controversial

One of the problems with Lyme disease is that diagnosis can be difficult. Symptoms of Lyme disease vary widely, can mimic those of other illnesses and, at this time, there is no definitive laboratory test for the disease.

Treatment of Lyme disease varies depending on the stage in which it is diagnosed. There’s so much fear over the disease that many patients are given ‘just-in-case’ antibiotics before a diagnosis can be confirmed. There’s also controversy over the existence of chronic Lyme disease and the cost of coverage for associated treatments by healthcare policies.

So it’s easy to see why understanding how we can truly reduce our exposure to Lyme disease, a disease many of us knowingly put ourselves into direct contact with, has widespread implications.

Folk Tales & Misinformation

One of the problems with trying to limit our exposure to Lyme disease is that much of what we think we know about the transmission of the disease is probably wrong.  So many of the ways we’ve been trying to reduce our exposure might not be protecting us at all.

Here are a few folk tales about Lyme Disease you may have heard, and even spread yourself:

◊  A warm winter (or one with lots of snow cover) means there will be an abundance of ticks in the spring. The truth is there have been no rigorous studies that have been able to prove a correlation between winter weather and the number of ticks in the spring.

◊ Bird feeders lead to an increase in the number of ticks. Birdfeeders can attract mice, squirrels and other small mammals, but according to Dr. Ostfeld, there have been no peer-reviewed, rigorous studies that have shown a correlation between bird feeder use and an increase in the number of ticks. If you’re still not sure about using a bird feeder, consider using feeders only when ticks are not active, typically from November – March.

◊ A 3′ wide woodchip barrier between a wooded area and a lawn area can greatly reduce the number of ticks. According to Dr. Ostfeld, this really isn’t true, unless you’re using wood chips from Alaska yellow cedar, which most of us are not. White footed mice and other small rodents are not deterred by the wood chips and are still moving ticks around your garden.

◊ Spraying your yard with a chemical tick spray means you don’t have to worry about exposure to Lyme disease. This is definitely not true. Yes, commercial acaricides work if applied at the proper time but they kill all the good bugs, too. They are harming the environment while giving you a false sense of security.

◊ The more deer you have on your property, the more ticks infected with Lyme disease you will have. The conventional wisdom goes like this…more deer mean more adult ticks which means more eggs which means more larval ticks. Again, Dr. Ostfeld’s research shows the number of deer does not correlate to an increase in Lyme disease infected ticks.

Of Mice and Men

Without getting into too much detail (for that read Dr. Ostfeld’s book or watch a video of Dr. Ostfeld presenting a similar lecture to the one I heard), Dr. Ostfeld has found the number of infected ticks is directly related to the diversity of host mammals, including white-footed mice, chipmunks, squirrels, catbirds, opossum, fox and deer, that the ticks can feed on which is directly related to habitat biodiversity.

The risk of exposure to Lyme disease is lower in a large forested area than it is in a fragmented habitat like a small wooded lot or our own back yards. That’s because small mammals that thrive in large numbers in these fragmented habitats – mainly white-footed mice – seem to infect a large number of ticks with the Lyme disease pathogen.

Mice encounter lots of ticks. In addition, they do a poor job of grooming off ticks so they have a high rate of permissiveness and a high propensity to infect those ticks that remain with the Lyme disease pathogen, or what Ostfeld calls reservoir competence. As you can see from this table, white-footed mice, and to a lesser degree chipmunks, are apex hosts for Lyme disease.

 

Understanding Alternate Hosts and Acorns are Key

Opossums are tick-eating machines.

Dr. Ostfeld has found that the more diversity of alternate non-mouse hosts there are for ticks to feed on, the lower the number of Lyme disease infected ticks.

This is partly because other host animals don’t infect larval ticks with Lyme disease at the high rate that mice do, partly because the tick burden per white-footed mouse decreases and  partly because some of the alternate hosts, like fox, owls and hawks, do a good job of keeping the mice population in check.

Not only are mice ubiquitous, their numbers are highly cyclical. And it turns out one of the strongest influencers of mice population is the number of acorns in any given autumn. In years when oak trees, particularly red oaks and black oaks, produce an abundance of acorns, mice populations spike the following spring. This leads to an increase in infected nymphal ticks the following summer. So the risk of Lyme disease is higher two years after a bumper crop of acorns. This knowledge can allow you to take extra steps, if needed, during that period of high risk.

Reducing Your Risk of Exposure to Lyme Disease

Hawks are a sign of a diverse ecosystem. Photo © Sue Sweeney

Continue using preventive/barrier methods like wearing light-colored clothing, tucking pant legs into socks and performing daily tick checks.

Clean Up Acorns. While acorns are an important food source for an array of different wildlife, if the primary mammal that is feeding on them in your garden is white-footed mice, and not any mice predators, then cleaning up the acorns will reduce your risk of exposure to an infected tick.

Promote wildlife and habitat diversity. If you’re fortunate enough to live in an area where you see foxes, hawks, and opposums, continue to nurture your garden’s diverse ecosystem. The presence of those alternate hosts means you have the ingredients for a level of biodiversity that can reduce your risk of exposure.

Be an advocate for maintaining expanses of forested land. How many town zoning and planning boards are aware of the relationship of species biodiversity to long-term public health?

Support research into alternate non-toxic tick controls like nootkatone and certain fungi that kill ticks but are harmless to wildlife and people. According to Dr. Ostfeld, the investment needed to fully explore these non-toxic options is minimal compared to their potential payback.

 

Note:  I’d like to thank Dr. Ostfeld for speaking with me and clarifying some of the points he made in his lecture. If you’d like to learn more about the complex interactions between biodiversity and Lyme disease, I encourage you to read his book, Lyme Disease: The Ecology of a Complex System.  

© 2012 – 2013, Debbie Roberts. All rights reserved. This article is the property of Native Plants and Wildlife Gardens. We have received many requests to reprint our work. Our policy is that you are free to use a short excerpt which must give proper credit to the author, and must include a link back to the original post on our site. Please use the contact form above if you have any questions.

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Comments

  1. Ruth Parnall says

    Thank you so much for going to Dr. Ostfeld’s lecture. Very, very useful information. I have forwarded a link to this post to several people who need to know.

  2. says

    Debbie thx for such an incredible article. I have long wondered about this disease and know that we could be prime targets…now that I know mice are the main carrier, I can be on the lookout. We have some but not a lot since we have our fair share of predators and no acorns. I have more voles and rabbits than mice, opossum or fox.
    Donna@Gardens Eye View recently posted..Bird Counting

  3. Ursula Vernon says

    I’ll be curious as to how this translates to STRI (Southern Tick Related Illness.) We do get Lyme down here—my ex-husband picked up a tick and got the bullseye rash and everything—but in the last few years, the black-legged tick has been almost completely replaced locally with the Lone Star tick, which doesn’t carry Lyme, but does carry STRI.

    I average thirty or forty tick bites a season, and while they never stay on for very long and I’ve had no disease scares, it’s still obviously a concern!

  4. Nancy Garden says

    Debbie, Thank you for your post. I really enjoyed the information you so wonderfully condensed from Dr. Ostfield. I am curious about one thing. For years, I assumed that larval ticks were not infected at birth. However, while reading Biography of a Germ by Arno Karlen, I found that some female ticks do actually transfer the bacteria that causes Lyme Disease through the reproductive process. This reminded me that ANY tick bite could be the cause of a hard-to-diagnose disease and should not be left out of your medical conversations. Unless, like so many of us, you get lots of tick bites and your doctor is already thinking that way!

  5. says

    I am so very glad I stopped in on this post. So much misinformation is out there on Lyme disease and you touched on many important points. I work at a nursery that also is a deer farm, and no one has ever gotten Lyme disease from the deer ticks to date. And there are many deer there, but it is always wise to be vigilant to the possibility though.
    Donna recently posted..W4W – Time

  6. Marc Imlay, PhD says

    The more deer you have on your property, the more ticks infected with Lyme disease you will have. The conventional wisdom goes like this…more deer mean more adult ticks which means more eggs which means more larval ticks. Again, Dr. Ostfeld’s research shows the number of deer does not correlate to an increase in Lyme disease infected ticks.

    This is not a folk tale unfortunately. The cause in the loss of biological diversity in natural areas has been found to be equally due to non-native invasive species and due to deer. I reviewed studies sent to me by Scott Bates, a specialist with ticks with the National Park Service. In about half the studies over about 5 states, reducing deer to a natural level of less than 20 per square mile reduced lyme disease to low levels and in about half the studies, including the New York study, it did not. Managed deer hunts is an important tool in the control of Lyme disease even if it did not work in the New York study.

    • says

      Marc, I believe Dr. Ostfeld’s research also found that the deer eat so many plants that all that is left are low understory plants where mice thrive. But his findings do show that the number of ticks does not correlate to the number of deer. I know there have been studies in CT that show the more Japanese barberry there is in an area, a ‘deer-resistant’ plant, the more mice and therefore the more ticks. It seems that all this various research shows that the interactions of a diverse habitat are so complex and interdependent. Given your comments on managed deer hunts, you might be interested in a post Sue Sweeney wrote on that subject on this site. Here’s the link in case you haven’t seen it yet: http://nativeplantwildlifegarden.com/the-cure-for-white-tailed-deer/.
      Debbie recently posted..Lyme Disease, Acorns, Mice & Opposums…Oh, My!

  7. says

    You clarified many points here. We usually see ticks in the spring and try to check ourselves after spending time outdoors. Now that I know opossums are helping with the reduction in tick populations, I will be more welcoming when I see one walk across the front yard at night.

  8. says

    Debbie, Thank you for writing about Lyme disease here and for introducing me and others to the work of Dr. Ostfeld. This is a very hard topic for me to comment on, as I have suffered long from Lyme and can tell you first hand that there is indeed long term chronic lyme. I live in a town with hundreds and hundreds of acres of forest all around and within our community. There is an epidemic here . . . neighbors all around me have had lyme. My habitat here is very diverse with a balance of deer but way too many rabbits that come up and hideout under the barn and shed. I believe the rabbits play a role too and was sorry not to see them mentioned in Dr. O’s study. I read an interesting article on acorns and squirrels recently . . . they too must contribute to the problem. I have always read and believed that mice were helping with the tick population and infections too. I did not know anything about Catbirds specifically and wonder why Robins were not mentioned as they hold the disease within their veins and help spread the disease to ticks. I do have too many catbirds here. They chase other birds away from al the various berries. I love both porcupines and opossums and both call this habitat home too. Both are tick mops and for that I even love them more. I am grateful for your article as it is helping others to appreciate these wild critters more too. It is scandalous that our testing is not better for lyme. I cannot understand why I have so many ticks here with the diversity of plants and wildlife . . . but then after last spring I hardly saw any. The animal diversity had not changed . . . my farming practices had not changed . . . it is all so complex but I am keeping records and tracking the populations of all my insects, arachnids, birds, reptiles and mammals. Other neighbors also reported seeing less. I remember happily the days before I ever even saw dog ticks here. Then the warmer climate and now there are so many of both the dog and lyme ticks. I do feel that our climate is a factor in the increase of ticks and the illnesses they pass on. The mystery of why the deer ticks suddenly disappeared last summer has yet to be solved and if I can figure it out, I will let you know.
    Carol Duke recently posted..Flower Hill Farm BUTTERFLIES OF 2011 ~ Favorite Crescents

    • says

      Carol, I’m so sorry to hear that you’re suffering from Lyme. I also know several people who have it and consider the idea that chronic LD is not real an insult to them. Given your experiences, and your diverse habitat, I think you would probably find Dr. Ostfeld’s book very informative and helpful in managing your garden. My post really boiled down his findings and, to tell you the truth, I’d be surprised if he didn’t look at rabbits and their role in Lyme disease. His book probably gets into that but my post was based on hearing him speak at a 60 minute lecture. Good luck.
      Debbie recently posted..Lyme Disease, Acorns, Mice & Opposums…Oh, My!

    • Kathy says

      I’ve been sick with lyme for over 10 years, since my early 20s. Only recently have I been able to drive a car or walk my dogs again. I’m thrilled to see any new research about lyme – it is such a loaded and highly political subject in the medical community that it does not get the attention it (or those ill with it) deserves.
      I have a whole new respect for opossums. If one chooses to move into my yard, it will now be an honored guest.

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