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Trees, Ticks, and Spirochetes: The Story of the Lyme Disease Epidemic
“Lyme disease is an accident by all accounts.”--Alan G. Barbour, author of Lyme Disease: The
Cause, the Cure, the Controversy.
Lyme disease is very rarely fatal, but it can make people very sick and it’s sharply on the rise in
certain areas of the United States. It’s a good example of an outbreak that is environmentally
related because occurrence and location are so strongly linked. In nature, wild animal
populations are reservoirs for the bacteria which cause Lyme disease. Lyme disease is common
only where deer are abundant and where humans have encroached on their habitat, which is
exactly what’s happening in many suburban areas. Named after Old Lyme, Connecticut, Lyme
disease, a bacterial infection is transmitted by the bite of a tick--though it took many years to
figure that out. The infection itself is caused by a spirochete bacterium called Borrelia
burgdorferi after its discoverer Dr. Willy Burgdorfer. In the northeastern U.S. in the 1970’s, a
particular set of biological and environmental circumstances converged to create the perfect
conditions for a Lyme disease outbreak. Current estimates range as high as fifty to sixty
thousand cases of Lyme disease per year. “Physicians report thousands of cases each year, in a
steadily increasing pattern that promises to continue,” writes Andrew Spielman, Professor of
Tropical Public Health at the Harvard School of Public Health.
What’s a Vector?
In epidemiology, a vector is an organism that transmits the disease-causing agent between one
organism and another. (If an infectious agent can’t be transmitted by being swallowed, breathed
in, or through direct contact with an infected person, chances are the disease in question is
vector-borne--unlike, say, cancer or heart disease, which are not infectious.)
The Troublesome Tick
Ticks are parasites that feed by latching on to an animal host and sucking its blood. Their life
cycle has four stages: egg, larva, nymph, and adult. Larvae and nymphs each need a blood meal
in order to proceed to the next stage, and adult females need a blood meal before laying their
eggs. These meals may be months, or even years, apart. Because ticks feed on multiple
individuals in the course of their life cycles, they are ideal vectors for infectious microbes. Nine
diseases are known to be tick-borne in the U.S. alone, including Rocky Mountain spotted fever,
relapsing fever, and tularemia. The bacterium that causes Lyme disease isn’t a new species;
Borrelia burgdorferi is thought to have been in the U.S. for at least a hundred years. It may
have been brought over in ships from Europe by rats or livestock, or infected ticks may have
hitched rides on migrating birds. “What’s happened during the last twenty years is that the
range of tick that carries the bacteria that causes the disease has expanded,” explains Professor
Tom Daniels, of the Vector Ecology Lab at the Louis Calder Center of Fordham University.
In the United States, the disease is concentrated in three areas: the Northeast, the upper Midwest
(Minnesota and Wisconsin), and northern California and Oregon. People also get Lyme disease
in temperate zones in Europe, Russia, China, Japan, and Korea.
Figuring Out the Where and Why of the Disease
Understanding and preventing any infectious illness--not just Lyme disease--requires an
understanding of its ecology: the interactions between the disease agent or pathogen, any
vectors, its host(s) or reservoir(s), and human behavior. The Lyme disease pathogen is the
spirochete; the vector is Ixodes scapularis, commonly known as the “deer tick,” primarily in its
larval and nymphal stages; the major reservoir is the white-footed mouse (which is also the
preferred intermediate host for the nymphal and larval ticks); the preferred host for the adult
tick is the deer; and the relevant behaviors are deforestation, reforestation, and suburban
development of deer/tick habitat. Other smaller mammals, some birds, and humans, can also
serve as hosts for the tick.
More Woods + More Deer
The North American white-tailed deer population has been exploding since the mid-1900’s
because of human-induced environmental changes. Forest has returned to much of the land
cleared in the last century for farming, and suburban neighborhoods have developed. There’s
more forest in many places than any time in the previous hundred years. This secondary forest
occurs patchily around homes, creating “edge habitat,” which is ideal for deer, but not for the
predators that used to control their numbers. The deer’s only remaining significant predator is
man, and hunting is restricted in suburban areas. The white-tailed deer is an important host for
an adult tick because big hosts provide a better meal. A tick will stay on for a week or so,
turning into what Daniels cheerfully describes as “a Raisinet with legs. She converts that blood
meal into between 2500 and 3000 eggs, every one of which hatches into a larval tick.”
More Woods + More Deer + More Ticks + More People
The tick population has expanded along with that of the deer, and Americans first became
exposed to the related infections in the mid-1960s. Since ticks don’t travel far, “the only way
you can become infected is to go to where the tick is, and that’s what has happened in many
suburbs,” explains Daniels. “We’ve not only moved into the deer/tick habitat, but we have been
very busy making it even more attractive to the deer by doing ornamental planting, which, as
any gardener can tell you, the deer love to eat. So, we’ve gone where the bugs are and we’ve
done things that make it easier for the bugs to make a living.”
A Bum Rap for Deer?
Unlike many typical host-parasite relationships, which have evolved over time to be quite
specific, I. Scapularis parasitize different animals through various stages of its life. “In fact,”
write Daniels and entomologist Richard C. Falco in Natural History magazine, “the immature
deer tick’s catholic taste in potential hosts insures that virtually all warm-blooded wildlife in the
wooded areas of Westchester County will eventually be infected.” Indeed, a Harvard
University study demonstrated that the deer population would have to be virtually eliminated in
order to have any effect on tick numbers. Though deer are an important host, the name “deer
tick” is something of a misnomer, and the role played by deer has been exaggerated.
The Plot Still Thickens: Meet the White-Footed Mouse
When they hatch, 99% of the larval ticks are born uninfected and harmless. That’s where mice
come in. “The larval ticks need to feed on a competent reservoir,” explains Daniels, “and the
best one is the white-footed mouse, which is very good at maintaining the spirochete in its
body.” An individual white-footed mouse is capable of infecting nearly 100% of the larval ticks
that feed on it, though typically the infection rate is around 40%. In the southern United States,
by comparison, immature Ixodes scapularis ticks feed mainly on lizards, which can’t be
infected by the spirochete. A similar tick that carries the disease on the Pacific coast, Ixodes
pacificus, also feeds on species that are not as highly infected.
Enter the Acorn
Another factor in the ecology of Lyme disease is acorns: both deer and mice love them. Every
three to five years--called “mast years”--oak trees produce bumper crops. And an abundance of
acorns in the fall has been statistically correlated to peak population densities of white-footed
mice, larval ticks, and nymphs the following summer--the tick stage most likely to infect
humans. In 1994, scientists documented a mast year in Duchess County, NY. In 1996 the
Center for Disease Control in Atlanta recorded the highest number of Lyme disease cases on
record, 30 to 40% higher than much of the preceding decade. This correlation doesn’t prove the
theory, of course. The point, as Richard S. Ostfeld of the Institute of Ecosystem Studies puts it,
“is that understanding the diversity of interactions in ecological systems may be beneficial for
predicting, and therefore avoiding, the times and places at which we are at highest disease risk.”
Then There’s Climate--Another Reason the Northeast Is Hit So Hard
Because of northeastern winters when it’s too cold for ticks to be active, I. scapularis has a twoyear life cycle. “That’s critically important to maintaining this disease in nature,” explains
Daniels, “because the seasonal activity pattern allows an overlap of generations.”
Nymphs, the next-to-last life stage of a tick, become active in the spring and infect all the
wildlife “just a month or so ahead of the next generation of larvae, hatching out in the
summertime,” explains the scientist. “The larvae will feed on all kinds of wildlife, but all of it
has just been reinfected by nymphs. This system allows the spirochete to be very efficiently
transmitted form one generation to another.” In the milder climate of the Southwest, by
comparison, the life cycle of the tick has been compressed to one year, so the likelihood of
spirochete transmission from one generation to the next is greatly reduced. Nymphal infection
rates are around 5%, compared to 25-30% in the northeast. Nymphs, harder to spot than adults,
are responsible for nearly 90% of Lyme disease cases caused by deer ticks. The range of ticks
also seems to be expanding. “Ticks that carry the infection have also been found in some areas,
such as northern Sweden, that used to be too cold for ticks to survive in, a finding that suggests
a regional, if not a global, change in climate,” notes Dr. Barbour.
And Last But Not Least, Human Behavior
“Superimpose these variables with the edge forest that deer like, put yourself in harm’s way . . .
” Tom Daniels’ sentence ends in a shrug. What appears to be an ideal residential environment-free-standing houses on large, wooded lots--can be extremely dangerous, an example of the law
of unintended consequences. The risk of contracting Lyme disease is highest in densely
populated suburban areas like New York’s Westchester County or the suburbs of Vienna,
Austria--not on a hike in the wilderness. But people like to live near wildlife. Scientists are at
work on models that predict a higher incidence of Lyme disease in certain areas at certain times,
so that people can be warned to take extra preventive measures.
The Bacterium Hasn’t Changed, But the Environment Has
Nobody’s actually looked at whether the pattern of suburbanization and reforestation is in place
everywhere that Lyme disease has increased, “but I would suspect it is,” says Daniels.
“There are more deer across the country (an estimated 25-30 million, a historic high as far as we
can tell), more forest (of the fragmented variety, which is more deer-friendly and more peoplefriendly), and more development,” which puts humans into contact with the disease vectors.
The irony, as Daniels points out, “is that normally we think of these changes in the
environment--re-planting forests and so forth--as good things.” It’s deforestation that’s
implicated in the spread of many infectious diseases.
“In general, reforestation results in better health, not an increase in disease,” corroborates Dr.
Durland Fish of the Yale University School of Public Health. “Nobody really understands the
patterns of environmental change that cause disease.”
BioBulletin by the American Museum of National History
http://www.amnh.org/sciencebulletins/biobulletin/biobulletin/story986.html
1. Lyme disease is caused by what organism?
2. Cases of the disease have increased due to humans doing what?
3. What is a vector?
4. Why are ticks ideal vectors for infectious microbes?
5. Adult ticks like to feed on what animal?
6. The larval ticks like to feed on what animal?
7. What is a reservoir?
8. How does the presence of acorns affect the number of cases of Lyme disease?
9. As climate changes, how have the ticks been able to adapt?
10.What are 3 factors that have increased human contact with the disease vector?
11.It’s _____ that’s implicated in the spread of many infectious diseases.