Exposure to Borrelia burgdorferi and Other Tick-Borne
Pathogens in Gettysburg National Military Park,
South-Central Pennsylvania, 2009
George S. Han,1,2Ellen Y. Stromdahl,3David Wong,4and Andre C. Weltman2
Since 1998, Lyme disease cases have increased in south-central Pennsylvania, which includes Gettysburg
National Military Park (NMP). Limited information is available about tick populations or pathogens in this area,
and no data regarding frequency of tick bites or prevention measures among Gettysburg NMP employees are
available. To address these gaps, ticks were collected, classified, and replaced (to minimize disruptions to tick
populations) at two sites within Gettysburg NMP during April–September, 2009, among eight nonremoval
samplings. On two additional occasions during May and June, 2009, ticks were collected and removed from the
two original sites plus 10 additional sites and tested for tick-borne pathogens by using PCR. A self-administered
anonymous survey of Gettysburg NMP employees was conducted to determine knowledge, attitudes, and
practices regarding tick-borne diseases. Peak Ixodes scapularis nymph populations were observed during May–
July. Of 115 I. scapularis ticks tested, 21% were infected with Borrelia burgdorferi, including 18% of 74
nymphs and 27% of 41 adults; no other pathogen was identified. The entomologic risk index was calculated at
1.3 infected nymphs/hour. An adult and nymph Amblyomma americanum were also found, representing the first
confirmed field collection of this tick in Pennsylvania, but no pathogens were detected. The survey revealed that
most park employees believed Lyme disease was a problem at Gettysburg NMP and that they frequently found
ticks on their skin and clothing. However, use of personal preventive measures was inconsistent, and 6% of
respondents reported contracting Lyme disease while employed at Gettysburg NMP. These findings indicate a
need to improve surveillance for tick bites among employees and enhance prevention programs for park staff
substantial morbidity in the United States. In North
America, Lyme disease is caused by the spirochete Borrelia
burgdorferi. In the eastern United States, transmission to
humans is primarily by the bite of an Ixodes scapularis
nymph; 48–72h of tick attachment is likely required for
human infection (Hojgaard et al. 2008). During 2008, a total
of 28,921 confirmed cases of Lyme disease were reported to
ick-borne diseases, especially Lyme disease, cause
the Centers for Disease Control and Prevention (CDC 2010).
Lyme disease is concentrated in the northeastern United
States, and Pennsylvania consistently ranks among the top
three states with the most reported Lyme disease cases (CDC
of 3818 Lyme disease cases were reported to the Pennsyl-
vania Department of Health, with the vast majority having
illness onset during May–August (Pennsylvania Department
of Health, unpublished data, 2009); in 2012, the year with the
1Epidemic Intelligence Service, Centers for Disease Control and Prevention, Atlanta, Georgia
2Pennsylvania Department of Health, Harrisburg, Pennsylvania.
3U.S. Army Public Health Command (formerly U.S. Army Center for Health Promotion and Preventive Medicine), Aberdeen Proving
4National Park Service Office of Public Health, Albuquerque, New Mexico.
VECTOR-BORNE AND ZOONOTIC DISEASES
Volume 14, Number 4, 2014
ª Mary Ann Liebert, Inc.
most recent available data, 4146 cases were reported (Penn-
sylvania Department of Health, unpublished data, 2013). In
contrast, other tick-borne diseases are rarely reported in
Pennsylvania, including two (2008 and 2012) cases of ana-
plasmosis (caused by Anaplasma phagocytophilum), zero
(2008) and one (2012) case of Rocky Mountain spotted fe-
ver (caused by Rickettsia rickettsii), and zero (2008) and
three (2012) cases of ehrlichiosis (caused by Ehrlichia
chaffeensis and Ehrlichia ewingii). Although all of these
infections are reportable by state regulation, underdiagnosis
and underreporting are likely. The number of cases of babe-
rash illness (possibly associated with Borrelia lonestari) are
unknown; these conditions are not reportable in Pennsylvania.
During 1998–2008, Lyme disease cases in Pennsylvania
were increasingly reported beyond the endemic southeastern
region, extending to the central and western regions of the
state (Pennsylvania Department of Health, unpublished data,
Gettysburg National Military Park (NMP); however, no
studies regarding tick populations or Lyme disease in Adams
County or Gettysburg NMP have been published. (Gettys-
burg National Military Park and the adjoining Eisenhower
National Historic Site are administered jointly by the Na-
Gettysburg NMP encompass approximately 2720 hectares
and receive >1.6 million visitors annually. Composed of
mixed hardwood forest, fields, trails, and brushy areas,
Gettysburg NMP surrounds the town of Gettysburg and also
serves as a local park for area residents. Substantial expo-
sure to tick habitat can occur within the park, especially
during peak visitation months in the late spring and summer.
Gettysburg NMP’s limited size and emphasis on historical
events might cause visitors to perceive the park as an outdoor
museum rather than a natural environment and to therefore
not use precautions against tick-borne diseases. In addition,
park employees who work outdoors are potentially at in-
creased risk for Lyme disease (Smith et al. 1988). In general,
however, the risk for exposure to tick-borne pathogens in
national parks is not well described (Eisen et al. 2013). Our
objectives were to determine the burden and seasonality of
I. scapularis and other ticks at Gettysburg NMP, estimate the
prevalence of B. burgdorferi and other tick-borne pathogens,
and characterize the extent of tick exposures and prevention
practices of park employees.
Materials and Methods
We adapted a tick-sampling method described by Nicholson
and Mather (1996). White flannel flags measuring 0.5 by 1.0
30-s drag, flags were examined for ticks, which were counted
done 50 times/site, without repeating areas previously sampled
that day, yielding a total of 25 sampling min/site.
We developed one tick-sampling protocol to estimate tick
population size and a second protocol to estimate pathogen
prevalence. For the first protocol, we chose two sites within
Gettysburg NMP (Fig. 1, sites 1 and 8) that were recognized
by park employees as prime tick habitats (hardwood forest).
From these two sites, ticks were sampled and identified ap-
proximately every 2–3 weeks for a total of eight times during
April–September, 2009, months that reflect the anticipated
seasonality of the nymph population (Falco et al. 1999). In
this protocol, ticks were returned to their habitat after spe-
cies identification to minimize disruptions to the local tick
For the second protocol, ticks were not only counted and
identified, but also removed and placed in vials for pathogen
testing at the U.S. Army Public Health Command’s Tick-
Borne Disease Laboratory. Tick-removal sampling was per-
formed at 12 sites (Fig. 1, sites 1–12) on May 13 and June 15,
during the anticipated peak period for nymph populations.
These sites included the two original sites and 10 additional
sites chosen on the basis of park employee knowledge of tick
habitat, visitor points of interest, and employee work loca-
tions. Because Gettysburg NMP is relatively limited in size,
of higher or lower risk but rather to understand the tick
population across the whole park environment.
We calculated the entomologic risk index (ERI) for
I. scapularis nymphs at Gettysburg NMP. ERIs measure the
number of infected ticks to which a person might be exposed
in tick habitat. Because the nymphal life stage is responsible
for most transmission of B. burgdorferi to humans, ERIs are
calculated for nymphs only. We estimated the risk for en-
countering an infected nymph per hour of exposure time
spent in tick habitat byusing the method described byMather
et al. (1996).
Polymerase chain reaction
Genomic tick DNA was extracted by using the Zymo
Genomic DNA II Kit? (Zyme Research Corporation, Or-
ange, CA) according to the manufacturer’s instructions
(Stromdahl et al. 2011). I. scapularis ticks were tested for the
presence of B. burgdorferi (Straubinger 2000), A. phagocy-
tophilum (Bell and Patel 2005),and B. microti (Tonnetti et al.
2009). Samples positive for B. burgdorferi in the initial PCR
1999). Dermacentor variabilis ticks were tested for spotted
fever group Rickettsia species (Jiang et al. 2005). Amby-
lomma americanum ticks were tested for B. lonestari (Bacon
et al. 2004) and for E. chaffeensis and E. ewingii (Bell and
Park employee survey
To determine self-reported exposure to ticks and tick
habitat and to measure knowledge, attitudes, and practices
regarding Lyme disease, we implemented an anonymous
survey that was adapted from two prior survey instruments
(one was adapted from two prior survey instruments: one
that was used in U.S. Forest Service employees in Cali-
fornia, and the other was used among Connecticut
homeowners). (The entire 33-question survey is available
by e-mail from the corresponding author.) Because the
survey did not collect personally identifiable information,
it was determined to be exempt from Institutional Review
Board assessment by both the Pennsylvania Department of
Health and the Centers for Disease Control and Prevention
2 HAN ET AL.
(CDC). Paper copies were distributed to park employ-
ees during September and December, 2009, at two staff
meetings. Additionally, Gettysburg NMP administration
e-mailed employees an Internet link for an online version of
the survey created using SurveyMonkey?(SurveyMonkey,
LLC, Palo Alto, California) three times during September–
Tick sampling and identification
During April–September, 2009, a total of 141 ticks (110 I.
scapularis nymphs, 15 I. scapularis adults, and 16 D. var-
iabilis adults) were counted at sites 1 and 8, among eight
nonremoval and two removal samplings (Fig. 2). The
center surrounded by national park, overlaid with the locations of the 12 tick-sampling sites. Sites 1 and 8 were the locations
where nonremoval sampling occurred eight times from April to September, 2009. All 12 sites, including sites 1 and 8, were
sampled for ticks on May 13 and June 15, 2009, for tick removal and pathogen testing. (Source: National Park Service.)
National Park Service visitor’s map of Gettysburg National Military Park indicating the town of Gettysburg in the
EXPOSURE TO Borrelia burgdorferi IN GETTYSBURG, PA3
I. scapularis nymph population peaked during May–July; the
adult population was low throughout the sampling period. A
limited number of D. variabilis adults and no D. variabilis
nymphs were found. One A. americanum adult and one
nymph were found on different dates (May 13 and June 15,
respectively) at the same site (Fig. 1, site 2).
On May 13 and June 15, a total of 115 I. scapularis, 44 D.
sites where we performed removal sampling for pathogen
testing; the only tick-borne pathogen identified was
B. burgdorferi. Of 115 I. scapularis ticks, 13 (18%) of 74
nymphs and 11 (27%) of 41 adults were infected. Neither
scapularis ticks. R. rickettsii was not identified among any D.
were identified among the two A. americanum ticks.
To calculate the ERI for Gettysburg NMP, we first calcu-
lated the total time spent in tick habitat on the basis of the 25
sample-minutes of flagging conducted at each of the 12 sites,
multiplied by two collection visits, which equals 600 sample-
minutes (10 sample-hours). Of the 115 I. scapularis ticks
collected, 74 were nymphs, yielding an average of 7.4
nymphs collected/h. This nymph encounter rate multiplied
by 0.18 (proportion of nymphs infected) results in an ERI of
1.3 infected nymphs/h for Gettysburg NMP during the 2009
peak nymph season.
Park employee survey responses
Of 110 employees, 79 (72%) completed the survey.
Table 1 displays employee response rates by occupational
division, where typical activities include fence repair, mon-
ument preservation, and trail maintenance; the next highest
response rate came from park rangers, which include inter-
pretive guides and law enforcement officers, followed by
resource managers, who manage and conduct research on
natural vegetation and wildlife; the lowest response rate was
from the administration division. Characteristics of survey
who reported working outdoors ranged from 11 during winter
months to 60 during April–September, when park visitation
peaks. Approximately half (49%) of all employees reported
working outdoors >20 hours during an average workweek.
checking for ticks on their clothes and body were the most
common prevention measures used.
Lyme disease was thought to be a somewhat or very serious
problem at Gettysburg NMP by 82% of employees; 62%
thought they were somewhat or very likely to acquire Lyme
disease while working at Gettysburg NMP. At least one unat-
tached tick was found by 84% of employees during the past
year, and 51% found one or more attached ticks during the past
year. Nine (11%) employees reported ever having received a
diagnosis of Lyme disease by a physician, and five (6%) had
received their diagnosis while employed at Gettysburg NMP.
This study of the distribution of tick species in Gettysburg
NMP revealed I. scapularis to be the predominant species;
americanum adult was collected on May 13 and an A. americanum nymph was collected on June 15; both were found at site
2 (see Fig. 1).
Number of ticks by species collected at sites 1 and 8 from April 17 to September 15, 2009. An Amblyomma
Table 1. Survey Response Rates by Division:
Gettysburg National Military Park, 2009
aIncludes Interpretation and Visitor & Resource Protection
4 HAN ET AL.
D. variabilis and A. americanum were present, but less
common. The I. scapularis peak nymph population (May–
July) coincides with the highest levels of visitor and em-
abundance and distribution data derived from ticks submitted
to the Pennsylvania State University Insect Identification
Laboratory for 1900–2000, which revealed a statewide dis-
tribution of D. variabilis and I. scapularis, and limited
numbers of A. americanum (http://ento.psu.edu/extension/
factsheets/pdf/ticks.pdf). Additionally, data regarding tick
abundance, distribution, and prevalence of tick-borne path-
Kit (HTTK) Program (Stromdahl et al. 2001) revealed pop-
ulations of I. scapularis and D. variabilis, and substantially
fewer numbers of A. americanum (E.Y. Stromdahl, unpub-
lished data, 2012).
Our study is the first to detect A. americanum from envi-
ronmental sampling in Pennsylvania. Transportation by birds
might account for an occasional finding of A. americanum,
but the finding of an adult and a nymph during 2 different
months indicates an emerging A. americanum population at
Gettysburg NMP. Presence of an established population of A.
americanum ticks in Pennsylvania is of concern, not only
because they bite aggressively and can be infected with hu-
man pathogens (Childs and Paddock 2003), but also because
A. americanum is a hardier species that is less restricted by
habitat than I. scapularis and can rapidly spawn large pop-
ulations (Ginsberg et al. 1991). Prior to 2011, the CDC map
of A. americanum distribution did not show the range of the
tick extending beyond the southern border of Pennsylvania.
In 2011, an updated map increased the range across the state
to reflect new information on distribution of the tick (www
B. burgdorferi was the only tick-borne pathogen identified
at Gettysburg NMP. Although this might be a result of the
limited sample size of collected I. scapularis (n=161), this
finding is consistent with the substantial number of reported
Lyme disease cases but infrequent cases of other tick-borne
diseases in Pennsylvania. The B. burgdorferi infection rates
of 18% among nymphs and 27% among adults are also
consistent with the nymphal (22%) and adult (28%) infection
rates among I. scapularis (n=209) submitted to HTTK dur-
ing 2008–2010 from two military installations near Gettys-
burg, Fort Indiantown Gap and Carlisle Barracks (E.Y.
surveys from the northeastern United States (Ullmann et al.
2005, Schulze et al. 2006, Diuk-Wasser et al. 2012).
The ERI for Gettysburg NMP of 1.3 infected nymphs/h
is similar to ERIs for the mid-Atlantic region and is the only
one that has been calculated in south-central Pennsylvania.
ERIs of 0–22.7 (mean, 4.4) were calculated across 31 sites at
Aberdeen Proving Ground, Maryland (E.Y. Stromdahl, un-
published data, 2009), and ERIs of 0–13.2 were calculated
from six Rhode Island towns (Mather et al. 1996). ERIs only
estimate the encounter rate of infected nymphs, not neces-
sarily the attachment rate of nymphs in tick habitat, and they
only apply to persons who enter tick habitats.
Table 2. Characteristics of Survey
Age (years) No. (%)
Employment status No. (%)
Length of employment at Gettysburg
National Military Park (years) No. (%)
Hours worked outdoors during
an average work weekNo. (%)
aNot all respondents answered all questions.
Military Park employee exposures and prevention practices
(n=79). (A) Percentage of employees with exposure to a
given outdoor habitat or activity one or more times/week.
(B) Percentage of employees who used selected tick pre-
Survey responses regarding Gettysburg National
EXPOSURE TO Borrelia burgdorferi IN GETTYSBURG, PA5
number of visitors increases during the summer months.
Casual visitors who restrict themselves to paved paths are
likely at low risk for tick-borne diseases, but visitors with
venture beyond paved paths and into tick habitats, thereby
increasing their exposure to ticks. Data on off-trail use or the
incidence of Lyme disease among visitors to Gettysburg
NMP are not routinely collected.
The employee survey demonstrated that Gettysburg NMP
employees are concerned about ticks and Lyme disease, and
that they frequently work outdoors during months with high
nymph populations. Our finding that 6% of employees re-
ported contracting Lyme disease while employed at Gettys-
burg NMP is consistent with a serosurvey of outdoor workers
in New York that reported that 6.5% of 414 employees had
serologic evidence of past B. burgdorferi infection (Smith
et al. 1988). Using tick protective measures was irregular
among park employees. Apart from clothing-related mea-
sures, employees frequently performed tick checks on
themselves but often did not apply insect repellent.
Gettysburg NMP, the entire NPS, and all U.S. Department
of the Interior bureaus use the Safety Management Infor-
mation System (SMIS) for employees to report work-related
injuries and illnesses. Tick bites can also be recorded;
however, only 15 tick bites were reported among Gettysburg
NMP employees during 2006–2009 (NPS, unpublished data,
2010). Because a primary function of SMIS is to manage and
process workers’ compensation claims, park employees in-
dicated that SMIS likely underestimates the true number of
tick bites, particularly in cases where employees remain
asymptomatic. Data from our anonymous survey validate
underreporting of tick bites in SMIS. Pilot efforts are un-
derway to encourage reporting of all tick bites among NPS
employees on separate tick logs via SMIS.
One limitation of this study is that we chose our tick
sampling sites on the basis of likely tick habitats and the
locations of visitor and employee activities, rather than ran-
domly selecting locations around the park. Therefore, our
study was not designed to determine whether certain areas of
the park are riskier than others. Additionally, not all park
employees responded to the survey. Employees who did not
Lyme disease, which can affect their perceptions of risk and
their use of personal preventive measures. Finally, because
park visitors and local residents were not surveyed, our study
cannot address their tick exposures and prevention practices.
Our findings demonstrate that I. scapularis ticks are
present at Gettysburg NMP and are infected with B. burg-
dorferi, posing a potential risk for Lyme disease. Ad-
ditionally, A. americanum, an aggressive biter, might be an
emerging problem at the park. To address these issues, Get-
tysburg NMP should continue to support an evidence-based
employee tick and Lyme disease prevention program that
includes education on symptoms and treatment, providing
and encouraging use of insect repellents, consistently con-
ducting tick checks after outdoor activity, and showering
within 2h of coming indoors (Mead 2011). Because NPS
policy is to leave natural ecosystems unimpaired, interven-
tions to suppress tick populations are not viable options in
national parks, except under rare situations. Factory-treated,
permethrin-impregnated uniforms for outdoor use are an-
other potential prevention measure. These uniforms are used
by the U.S. Army and have been shown to be effective in a
study involving outdoor workers in North Carolina (Vaughn
and Meshnick 2011).Finally, although notdirectly addressed
by our data, NPS should consider enhancing tick and Lyme
disease education for visitors through ranger-led programs
(Wong and Higgins 2010), park Internet sites, and visitor
center displays. By developing a comprehensive tick pre-
vention program, Gettysburg NMP can help decrease the
risk of Lyme disease and other tick-borne diseases for both
employees and the visiting public.
The authors thank the numerous persons from the Pennsyl-
vania Department of Health, the U.S. Army Public Health
Command, and the U.S. Army 1stand 9thArea Medical La-
boratories who assisted in tick sampling efforts. We also thank
Mary Vince and the Tick-Borne Disease Laboratory for their
contribution to the laboratory identification of tick pathogens
and the National Park Service Office of Public Health for pro-
and the administrators and employees of Gettysburg National
Military Park and Eisenhower National Historic Site for their
participation in the survey and assistance with selecting tick-
sampling sites, as well as Gettysburg College professor Steven
James for local background information. Finally, we acknowl-
edge the contributions of Rebecca Eisen, Heidi Brown, Paul
Mead, Kevin Griffith, and Joseph Piesman of the Division of
Vector-Borne Diseases, National Center for Emerging and
Zoonotic Infectious Diseases, CDC for providing invaluable
feedback regarding the study design, tick sampling protocols,
and the employee survey.
Author Disclosure Statement
No competing financial interests exist. The findings and
conclusions in this report are those of the authors and do not
necessarily represent the official position of the Centers for
DiseaseControland Prevention, U.S.Army, or National Park
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Address correspondence to:
Ellen Y. Stromdahl
U.S. Army Public Health Command
5158 Blackhawk Road
Aberdeen Proving Ground, MD 21084
EXPOSURE TO Borrelia burgdorferi IN GETTYSBURG, PA7