Lyme Disease and Forest Ecology

Date: May 20, 1998
From: rec.backcountry newsgroup

The predictions of a bad year for Lyme disease are based on several factors, one of which is El Nino. Lyme disease is part of a complex cycle of forest ecology which goes something like this:

1) White-footed mice are the principal reservoir. They like to feed on acorns, so a big crop of acorns ( a so-called "mast year") causes a boom in the mouse population in areas with a lot of hardwoods. The deer tick feeds on these mice.

2) In a typical large forest, hardwoods will be in certain areas, and species like pine will predominate in other areas. Deer range through both types of areas, along with their associated ticks. Deer do not serve as good reservoir hosts for Lyme disease, so if they are far from the oak forests where the mice live, they are less likely to be infected.

3) Mast years occur every 3-4 years. For poorly understood reasons, oaks in an entire region will collectively "decide" to produce a large crop of acorns that year. Winter temperatures may have some effect, but other factors also appear to be at work. The evolutionary benefit of this strategy is obvious to most ecologists, but the mechanism is not.

4) When there is a mast year, the mouse population increases dramatically, and the deer also move into the hardwood groves to eat the acorns, whereas they might ordinarily be in other parts of the forest. That means lots of mice, lots of deer, and lots of ticks converge in hardwood groves during mast years.

5) When deer congregate in an area, so do deer hunters. If it's not a mast year, the deer might be found in low-lying pine forests with few mice. In mast years, hunters need to go to the oak forests, where they encounter the deer, but also the mice and ticks. As you can see, the population of susceptible hosts increases dramatically in oak forests during a mast year. This leads to an increase in human cases of Lyme disease. Deer are also more likely than mice to wander into outlying suburbs and other poorly-planned living areas, where they encounter humans and their pets. Thus, when deer are more likely to be infected, everyone living near them (hunter or not) is in danger.

6) This year, we're due for a mast year in many parts of the U.S. In addition, a lot of areas had highly atypical weather because of El Nino. In areas where mice and deer are expected to profit from the disruption and the large concentration of acorns, expect lots of Lyme disease in humans.

I don't know what the situation is in California, but would expect it to vary from one part of the state to another. The Northeast certainly had a highly variable winter, with downstate New York through the Mid-Atlantic region experiencing a sort of extended Autumn with rain and a couple of light flurries, while upstate New York, New England, and parts of Canada got flattened by ice storms.

Best advice I've heard is to assume that every year and every area in the forest is a bad one for Lyme disease, and take prudent precautions. Permethrin spray on your lower pants legs, pants either tucked into boots or tied shut at the ankles, and check yourself, your pets, and your kids each night for ticks. If you get a bite, or come down with a flu-like condition, or find a red circle around a bite mark, get treated immediately and hope for the best. There's also a new Lyme disease vaccine which should be available shortly, and which has shown pretty good efficacy.

Disclaimer: I am a microbiologist. I am not a medical doctor, and don't play one on TV, so take or ignore this advice at your own risk. It's all just my personal opinion. Even I might disagree with it at some point in the future.

Alan Dove, Ph.D.
N3IMU
a.dove@natureny.com
a.dove@erols.com
http://128.59.173.136/Poliolab/Alan/Dove.html

Reference:
Chain reactions linking acorns to gypsy moth outbreaks and Lyme disease risk.
      Science Feb 13, 1998 v279 n5353 p1023(4)

Last updated: 01/18/01