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Investigating the Role of White-footed Mice in the Transmission of Lyme
Disease on Fire Island, New York
James Fischer
Introduction
Lyme Disease is an infectious disease whose symptoms include
a bulls eye rash and flu-like symptoms, which include fatigue and
stiff joints. This disease has grown to epidemic levels in North America
since its first discovery. The agent of this disease is a spirochete bacterium
(Borrelia burgdorferi), which is passed between vertebrates (including
humans) by
Ixodes scapularis, technically known as the black-legged tick, but commonly
referred to as the deer tick. Although many mammals and birds have been
identified as competent reservoirs
of the disease, the primary vertebrate species that is associated with
this zoonosis is the white-footed
mouse (Peromyscus leucopus). The population demography
of white-footed mice has not been explored with respect to the etiology
of Lyme Disease. Since the autumn of 1995, the demography of white-footed
mice and deer tick populations on Fire Island, New York have been intensively
studied. This website presents some of the results of that study.
The Deer Tick
The Deer tick (Ixodes s
capularis)
is an arachnid that is found
mainly in the eastern North America. There are three instars
for this species, which are larva, nymph, and adult. Each instar requires
a blood meal and diapause to digest the meal
before molting into the next
stage. The larvae and nymphs feed on a wide variety of vertebrate hosts,
including white footed mice, while the adults feed primarily on white-tailed
deer (Odocoileus virginianus). The Deer tick quests for its hosts by climbing
onto vegetation, especially along well-used animal trails, to gain a better
vantage point to encounter its hosts. It does not have the ability to
fly or jump from any vantage point, therefore it requires that the host
come into contact with the vegetation and ticks. The spirochete is found
in the gut of the tick and is transmitted after the tick has been feeding
from 12 to 24 hours. The bacterium is primarily found in the guts of the
nymph and adult instars of 
this
species, but it can also be found in a few larvae. Therefore, the highest
probability of transmission
can occur when nymphs and adults are abundant and a tick bites you and
feeds for more than 12 hours. Lyme disease is one of the simplest diseases
to prevent with due diligence, however, by wearing light colored clothing
that is bound at the ankles, waist, and wrists and by removing any ticks
that are biting you as soon as you come in from the out of doors. The
favored habitat for this species is considered early
successional and typically contains shrubby
characteristics along field and forest edges.
The White-Footed Mouse
The white-footed mouse (Peromyscus leucopus) has uniformly brown fur on
the center of the back while blending to a reddish-orangish brown coat
color on the sides. The belly is an 
off-white
color and as the common name implies, it extends to the feet. It has long
whiskers and dark protruding eyes. The rodent feeds on seeds, fruit, invertebrates,
and fungus. This species is found over most of North America and lives
in habitats that are characterized as early successional, shrubby fields
and forest edges. These mice are mainly nocturnal and define territories
within their home ranges. The white-footed
mouse is commonly confused with the deer mouse (Peromyscus maniculatus)
because they have very similar pelage and habitat
associations, but the deer mouse has a slightly longer and noticeably
bicolored tail. Both species of Peromyscus are competent
reservoirs for the spirochete (Borrelia
burgdorferi) that causes Lyme disease.
Life History/ Population Models
We have produced two models
that incorporate the life history
and population fluctuations
of the white-footed mouse and deer tick populations that best represent
our observations. Our data was collected from the autumn of 1995 to the
summer of 1999. We incorporated computer
simulation models into our research in order to better understand
relationships within this complex biological system. Ecological models
are most often simplifications or abstractions of complicated relationships
that might never be fully understood. For example, a model that is built
by a hobbyist is a physical representation of a real object. While not
often exact, a model focuses on how the interaction among its components
affects the interpretation of the result. Therefore, results of a computer
simulation model should be interpreted conservatively because we have
not told the computer every detail of the complex biological relationship.
S
o then, why
would we want to use a computer model?
1) It helps us define and focus our questions
2) It helps us organize our ideas.
3) It helps us understand what we observe and
if any of our predictions correspond with our
observations.
4) It helps us communicate and test our understanding
of what we observed.
5) It helps us make conservative predictions as an intellectual tool.
Starfield, A. M. and Bleloch, A. L. 1986. Building models for conservation
and wildlife management. Macmillan Publishing Company, New York. pp 253
Mouse Model
We wanted to better understand how an uncertain demography of mice would
effect the transmission of the bacterium from mice to ticks. The mouse
population statistics that we focused our attention on were abundance,
the probability of survival
for adult mice, the probability of survival for juvenile mice, and the
potential birth
rate. A thorough analysis of these parameters would help us understand
why a mouse population increases or decreases throughout and between seasons.
We cre
ated
a computer simulation model
using a sophisticated software program called STELLA. It is an icon-based
simulation environment that allows relationships among interacting variables
to be constructed and explored through iterative calculations. The effect
of each model component can be appreciated using sensitivity
analysis. The abundance of white footed mice increases throughout
the season and declines during the autumn. Although new juvenile mice
appear throughout the season, two distinctive birth pulses were observed
during the season -- one in
June and another in late August, but with some variability from one season
to the next.
Deer Tick Model
We wanted to describe the tick population so that we could describe the
mice's tick burden throughout the season. We characterized the deer tick
population by accounting for the phenology
of the active instars. The
larval stage is the first instar
of the deer tick life cycle, yet they emerge in the month of August. The
nymphs, which is the next instar, are active in late May and early June.
The abundance of the adults
was observed to be lower on Fire Island than at other study sites. The
abundance of larvae is consistently greater than the number of nymphs,
which is greater than the number of adults. This suggests that a large
proportion of ticks die before maturing into the next instar, yet if an
individual does survive to the adult instar it is likely that it will
reproduce.
Interaction
We wanted to understand how population fluctuations in both tick and mice
affect the transmission of the Lyme Disease bacterium from mice to the
ticks and vice versa. A very low proportion of larvae are infected with
the bacterium, while a much larger proportion of 
the
nymphs are infected. At the taking of each blood meal, a tick increases
the probability that it will receive the bacterium from an infected host.
The white-footed mouse is a competent reservoir for the bacterium and
only feeds the larvae and nymph instar of the tick. The deer tick receives
the bacterium by feeding on an infected host The majority of juvenile
mice emerge at two periods during the summer. One juvenile group emerges
during the peak activity of the nymph instar and the other group emerges
dur
ing
peak larvae activity. The first group of mice is at a greater risk of
being exposed to an infected tick than the latter group. Because each
season exhibits variation around the times of emergence of juvenile mice
and tick instars, we predict great variation between years in the epizootic
potential, thus differing risks to humans in areas where Lyme Disease
is endemic. By modeling the variation in mouse and tick abundance as time
series, we expect to make predictions about the relative degree of risk
for contracting Lyme Disease from one year to the next. Our approach uses
the phenology of emergence (and its variation) of each life stage of both
ticks and mice, which is fairly predictable, rather than the factors affecting
survival and reproduction which are not.
Contact Information
Please feel free to contact us if you have any questions or comments. Your feedback is greatly appreciated. James Fischer
email: jpfische@mailbox.syr.edu
Dr. H. Brian Underwood
email: hbunderw@mailbox.syr.edu
Address:
350 Illick Hall
State University of New York College
of Environmental Science & Forestry
Syracuse, NY 13210
About The Author: James Fischer
B.S. 1996
State University of New
York, College of Environmental Science and Forestry. My M.S. thesis is
understanding the role of white-footed mice in the transmission of the
Lyme disease bacterium to the deer tick. My research was conducted on
Fire Island, which is located along the southern coast of Long Island,
New York. I am interested in many things, but primarily those that allow
me to work outdoors in conditions that most people would find unbearable.
My plans for the future are to continue doing research on topics where
I can learn more about natural history and ecology.
Links
Centers
for Disease Control, Introduction to Lyme Disease
Medline Plus Health Information on Lyme Disease
American Lyme Disease Foundation, Inc.
American College of Physicians-American Society of Internal Medicine
Bacteriology at University of Wisconsin-Madison
Iowa State Entomology Index: Medical Entomology
Morbidity and Mortality Weekly Report
Emerging Infectious Diseases Journal
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