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
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
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.
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