Lyme Disease
Transmission Ecology
In Virginia
Virginia Department of Health
David N. Gaines, Ph.D.
Public Health Entomologist
Division of Environmental Epidemiology
The Lyme Disease Agent
Lyme disease is an illness caused by the bites of ticks infected
with the bacteria species known as Borrelia burgdorferi.
B. burgdorferi is transmitted primarily between ticks, rodents
and other small mammals.
Ticks may also transmit B. burgdorferi to other mammals such
as dogs or people where it may cause disease.
The Lyme Disease Agent
Borrelia burgdorferi is one of several closely related (genetically
similar) species in the U.S.
In the U.S. mid-Atlantic Region, genetically related Borrelia
species include: B. bessettii, B. andersonii, B. americana,
and B. carolinensis.
Among the various closely related American Borrelia
species, only B. burgdorferi (sensu stricto) is known to
cause Lyme disease.
Tick Vectors of Disease in Virginia
Among the 16 tick species found in Virginia, only three species
commonly bite people.
Lone Star Tick
Blacklegged Tick
American Dog Tick
Only one species, the blacklegged tick transmits Lyme disease.
Blacklegged Tick Life-cycle Stages
egg
blood
meal
larva
blood
meal
blood
meal
Each tick has three blood meals during its the two–year life span.
Feeding Preferences of the Blacklegged Tick
Blacklegged nymphs have much broader feeding preferences
than the adult ticks.
Nymphs have been collected from over 100 different species
of animals as well as people.
Adult blacklegged ticks prefer to feed on deer and use deer as
a mating ground.
Adult blacklegged ticks may occasionally be found feeding on
other animals such as dogs, cats, foxes, or on people.
The main vectors of Lyme disease to people are nymph stage
blacklegged ticks - active from mid-spring to mid-summer.
Inch
(fractions)
Adult blacklegged ticks do not commonly bite people, but sometimes
do, and can transmit Lyme disease to people during the fall, winter
and early spring.
The
Blacklegged
Tick’s
Two-year
Life Cycle
Tick Feeding Time Required for B. burgdorferi
Transmission
Infected blacklegged larvae go through a 6-month dormant period
and then become nymphs - B. burgdorferi in the tick’s gut,
decline in number and become dormant.
When infected nymphs emerge in the spring, and feed on a warmblooded animal, or person, the B. burgdorferi in the tick’s gut
re-activate and begin to replicate.
As the B. burgdorferi replicate, they move through the tick’s gut
wall and migrate toward the tick’s salivary glands.
It takes about 24 hours before the tick’s salivary glands begin to
become infected. By 36 hours, some infected ticks will have
begun to transmit B. burgdorferi to the host in salivary fluids.
Feeding Time for B. burgdorferi Transmission
CUMULATIVE PROBABILITY OF B.
Cumulative
of B.FOR
burgdorferi
burgdorferiProbability
TRANSMISSION
EACH
Transmission
for ATACHED
Each Hour Attached
HOUR
1.0
PROBABILITY
0.8
T = (1 - exp[-λγ (t - G)γ ] )  k
0.6
WEIBULL MODEL
0.4
36 hours
0.2
0.0
0
24
48
HOURS ATTACHED
72
96
From DesVignes et al.
Reservoir Host Species for Lyme
White-footed mice are highly competent reservoirs of B.
burgdorferi.
Play a major role in maintaining
the infection cycle in the
blacklegged tick population.
Can be infected and remain
infectious with B. burgdorferi
for up to six months.
Competent and Non-Competent Host Species
Other animal species known to act as competent B. burgdorferi
reservoirs include:
Eastern chipmunks, meadow voles, short-tailed shrews,
masked shrews, robins (high to moderate competence)
Many other animal species have low competence as reservoirs,
or cannot be infected by B. burgdorferi, and play little or no
role in its transmission.
Low or non-competent host species include:
Eastern grey squirrels, raccoons, skunks, opossums, lizards,
white tailed deer, etc.
The Effect of Forest Environment on
Lyme Transmission
Studies have shown that animal species diversity in forest
environments influences the B. burgdorferi infection
rate in ticks.
If a mixture of competent and non-competent host species are
available for larval and nymph stage ticks to feed on, this can
dilute B. burgdorferi transmission.
Dilution Effect: Infected nymph stage ticks feed on noncompetent reservoir animals and their infection cannot be
passed on to other ticks that feed on these same animals.
The Effect of Forest Environment
Animal species diversity is greatest in undisturbed forests and
lowest in fragmented and disturbed forest environments.
Undisturbed, Intact Forest
There is a mixture of competent and non-competent host
species, and B. burgdorferi is diluted.
= competent reservoir species
The Effect of Forest Environment
In fragmented and disturbed forest environments, whitefooted mice often become the predominant forest species.
Fragmented, Disturbed Forest
Competent reservoirs predominate and help amplify B.
burgdorferi in the local tick population.
= competent reservoir species
Effect of Suburbanization on Whitefooted Mouse Habitat
Suburbanization often results in fragmented and disturbed
forest environments that favor white-footed mouse
populations.
Man-made structures around the edges of suburban forests
(e.g., sheds, wood piles, etc.) may provide added nesting
sites for the white-footed mouse population.
The Role of White-tailed Deer
White-tailed deer are important
because:
They serve as an important
mating ground for adult
blacklegged ticks.
Serve as the primary source
of blood for egg production
by female blacklegged
ticks.
The Role of White-tailed Deer
Deer ears covered by ticks
An engorged female tick that drops off of a deer may have
the potential to lay up to 3,000 eggs.
The Role of White-tailed Deer
Studies have shown that total elimination of the local whitetailed deer population results in a 99% reduction of the
local blacklegged tick population (It will also eliminate the
lone star tick population).
Blacklegged tick population density is generally positively
correlated with deer population density.
Higher tick population densities can increase the rate of B.
burgdorferi transmission by ticks.
White-tailed Deer Habitats
White-tailed deer prefer forest edge environments.
Feed on vegetation that grows in sun-lit areas along the
forest edge and use the forest for refuge.
A square mile of solid forest will support a much smaller
population of deer than a square mile of fragmented forest.
Fragmented forests have more of the edge habitat and
forage that deer require.
Effect of Suburbanization on
Deer Habitat
Suburbanization can result in a pattern of fragmented forest
and increase white-tailed deer habitat and populations.
Suburbanized areas restrict deer hunting activities to small
geographic areas.
Deer are smart - properties and neighborhoods that are offlimits to hunters become a refuge for deer during the
hunting season.
Effect of Suburbanization
Deer have adapted well to suburban environments and can
survive in relatively densely developed areas.
Ornamental vegetation and gardens around suburban
homes can provide good forage for deer.
Lyme Disease in Your Own Backyard!
Mated, fed, female ticks drop off deer as they travel through
backyards and suburban forest blocks.
Fed larval ticks drop off of white-footed mice as they move
through backyards and forests.
Engorged ticks that land in suitable habitats will survive until
the next season.
Surviving females produce and lay eggs that become larvae.
Surviving larvae become nymphs, and if infected, may bite
and infect people in their own back yards.
Blacklegged tick infection
rates detected by recent
surveillance and testing
efforts in Virginia
46%
20%
15%
20%
6%
18%
0%
4%
5%
3%
8%
Tick Drags 2012 - Fairfax County Department of Health / Johns Hopkins University, School of Public Health
Tick Drags 2013 - Loudoun County Lyme Commission / Johns Hopkins University, School of Public Health
Tick Drags 2011 - University of Richmond, Dept. of Biology
Deer Hunt Collections 2011 - Virginia Dept. of Health / University of Richmond, Dept. of Biology
Tick Drags 2011 - Old Dominion University, Dept. of Biology/ U. of Richmond
Incidence of Lyme disease by county in
Virginia in 2012
Summary
Lyme disease is a growing problem in Virginia and endemic zones
are expanding southwestward and appear to be linked to
suburbanization and development.
The transmission ecology of Lyme disease involves the complex
interactions of blacklegged ticks, B. burgdorferi, competent
animal hosts, deer, and forest environments.
More tick surveys and testing will be needed to help predict local
levels of Lyme disease risk in Virginia, and understand the local
factors influencing transmission ecology.
Acknowledgements
Nelson Lafon – Virginia Dept. Game and Inland Fisheries
Julia Foutz & Jim Broyhill – VDH, Division of Environmental Epidemiology
Rexford Dwamena , PhD – VDH, Office of Minority Health and Health Equity
Local Epidemiologists – VDH, Division of Surveillance and Investigation
Azizul Islam – VDH, Division of Surveillance and Investigation
Jorge Arias, PhD & Joshua Smith – Fairfax County Dept. of Health
Jory Brinkerhoff, PhD – University of Richmond, Department of Biology
Dr. David Goodfriend – Loudoun County Dept Of Health
Andy Lima – Clarke Mosquito Control,
Holly Gaff, PhD, & Chelsea Wright – Old Dominion University, Dept. of Biology
For further information on Lyme disease and other tick-borne
diseases in Virginia, visit:
www.vdh.virginia.gov/TickBrochure