Tick Burdens in a Small-Mammal Community in Virginia
Leah R. Card, William J. McShea, Robert C. Fleischer, Jesús. E. Maldonado, Kristin Stewardson, Michael G. Campana, Patrick A. Jansen, and Justin M. Calabrese
Northeastern Naturalist, Volume 26, Issue 3 (2019): 641–655
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2019 NORTHEASTERN NATURALIST 26(3):641–655
Tick Burdens in a Small-Mammal Community in Virginia
Leah R. Card1,*, William J. McShea1, Robert C. Fleischer2, Jesús. E. Maldonado2,
Kristin Stewardson2, Michael G. Campana2, Patrick A. Jansen3,4, and
Justin M. Calabrese1
Abstract - Virginia has seen dramatic increases in reported cases of Lyme disease and
Rocky Mountain spotted fever, but basic knowledge on the community ecology of these
tick-borne diseases is poor. We examined the tick burdens of 5 small-mammal species in
northwest Virginia from October 2011 to December 2012. We live-trapped individuals,
quantified the tick burdens, assessed the burden structure, and tested a subset of the ticks for
tick-borne pathogens. We found the tick burdens to be composed predominantly of Ixodes
scapularis (Black-Legged Tick), and Ixodes sp. ticks, with Amblyomma americanum (Lone
Star Tick) and Dermacentor variabilis (American Dog Tick) also present at lower densities.
We detected Borrelia burgdorferi (prevalence 15%), Rickettsia spp. (4%), Anaplasma
phagocytophilum (4%), and Hepatozoon spp. (1%). Black-Legged Ticks, a species which
has shown range expansion in recent decades, tested positive for B. burgdorferi (17%) and
for multiple pathogens in individual ticks. For better predictions of tick-borne disease risk
across the Mid-Atlantic region, we recommend tracking changes in tick communities by
continuous monitoring of tick burdens, densities of questing ticks, and prevalence of tickborne
pathogens.
Introduction
Many of the tick-borne diseases affecting humans have increased in geographic
range and number of reported cases in the United States in recent years (Dantas-
Torres et al. 2012). This increase has been especially strong in Virginia, which has
seen dramatic increases in reported cases of Lyme disease and Rocky Mountain
spotted fever over the last decade (Brinkerhoff et al. 2014, CDC 2017, Nadolny et
al. 2014). Of the 10 states with the highest reports for Lyme disease from 2006 to
2016, Virginia has had the second highest rate of increase, following Pennsylvania
(CDC 2018). While no single factor seems primarily responsible, increases in
disease prevalence have been attributed to tick range expansion, increases in human–
tick interactions, and changes in biodiversity and habitat (Gubler et al. 2001,
Nadolny et al. 2014).
Small mammals play an important role in the prevalence of tick-borne pathogens,
as they are both hosts to feeding ticks and reservoir hosts for pathogens
1Conservation Ecology Center, Smithsonian Conservation Biology Institute, National
Zoological Park, 1500 Remount Road, Front Royal, VA 22630. 2Center for Conservation
Genomics, Smithsonian Conservation Biology Institute at the National Zoological Park,
3001 Connecticut Avenue NW, Washington, DC 20008. 3Center for Tropical Forest Science,
Smithsonian Tropical Research Institute, Roosevelt Avenue, Balboa, Ancón, Republic of
Panamá. 4Department of Environmental Sciences, Wageningen University, PO Box 47, 6700
AA Wageningen, Netherlands. *Corresponding author - leahrcard@gmail.com.
Manuscript Editor: Daniel Keppie
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(Dallas et al. 2012). While not adversely affected by the pathogens (Tilly et al.
2008), small-mammal species differ in quality as reservoir hosts (Ostfeld and Keesing
2012), with some being major carriers of pathogens (e.g., Peromyscus leucopus
(Rafinesque) [White-Footed Mouse] and the Lyme disease pathogen Borrelia burgdorferi
(Johnson)), and others as poor quality hosts, a “dead end” for the pathogen.
LoGiudice et al. (2003) found that high vertebrate biodiversity and community
composition can lead to lower prevalence of Lyme disease. Small-mammal hosts
can also impact overall tick populations as many immature ticks feed primarily on
such hosts (Smart and Caccamise 1988).
Due to their importance, tick burdens (all of the ticks attached to an individual
host) have been the focus of many studies. Past research on tick burdens has often
been limited to examinations of a single tick species, mammal host species, or
tick-borne pathogen. Recent tick research in Virginia and the surrounding region
focused on pathogen prevalence in host-seeking, or “questing”, ticks (Henning et
al. 2014; Herrin et al. 2014; Nadolny et al. 2011, 2014), and have not assessed the
tick burdens of small mammals. All studies that have assessed tick burdens of small
mammals in Virginia and surrounding states are decades old (Levine et al. 1991,
Sonenshine and Haines 1985, Sonenshine and Stout 1968, Zimmerman et al. 1987).
Therefore, there is a strong need for tick-burden research in Virginia and the surrounding
region, to provide researchers and the medical community with important
updated information on ticks and tick-borne disease risk.
This study aims to address this deficiency. We surveyed ticks and tick-borne
pathogens within a small-mammal community in northwest Virginia. We examined
the tick burdens on small mammals for the following: tick abundance, tick species
composition, and tick-borne pathogen prevalence. In addition, we examined the
abundance, diversity, and pathogen prevalence of questing ticks.
Field-site Description
This study was conducted in forests and fields at the Smithsonian Conservation
Biology Institute (SCBI), near Front Royal, VA (Warren County; 38°53´15.6˝N,
78°9´54.6˝W). SCBI features long-term ecological monitoring projects including
a Smithsonian Institution Forest Global Earth Observatory (ForestGEO), a 25.6-ha
monitoring plot surveyed since 1990 (Bourg et al. 2013), and has been a National
Ecological Observatory Network core site since 2014. These features make SCBI
an ideal location for conducting a baseline tick survey, as the long-term data collected
at this site could provide ecological context for understanding any future
changes in the tick, pathogen, or host communities.
Methods
Small-mammal captures and tick collection
We trapped small mammals from October to November 2011 and April to October
2012 (5460 trap-nights) using 8 cm × 9 cm × 23 cm live traps (Sherman,
Tallahassee, FL). Each trapping session for the project consisted of 4 consecutive
nights of trapping. The trapping period during fall of 2011 and from spring to fall
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in 2012 coincided with tick phenology, thus capturing activity and abundance of
larval and nymphal ticks (Gatewood et al. 2009). For 2012, trapping in the forest
took place within the ForestGEO grid (680 m × 420 m), which was divided into 8
sections. The sections included 4 smaller sections (210 m x 160 m) with 88 trap
grid-points, and 4 larger sections (210 m x 180 m) with 99 trap grid-points. Trapping
took place at 2 of the 8 sections (including 1 of each size) with a total of 374
traps set at 187 trap locations. We set 2 traps facing opposite directions near each
trap grid-point site, with 20 m between locations. We randomly selected the 2 grids
(1 from each size) for trapping, which allowed for each section to be trapped for
multiple sessions across the trapping period, varying from 2 to 8 trapping sessions.
We also conducted trapping along 10 transects (100 m each), with 2 traps set
every 10 m, for 1 session each in 3 fields and 3 forests in 2011 and in 2 fields in
2012. Trapping along transects with a total of 22 traps per transect enabled us to
survey small mammals outside of the ForestGEO grid, in field and forest sections
across SCBI that varied in area and shape.
All captured animals were identified to species and sex, marked with an ear tag
if unmarked, weighed, searched thoroughly for ticks, and then released at the point
of capture. We did not use anesthesia during processing. We counted and collected
all observed, attached ticks, which we then placed into vials filled with ethanol
(Hersh et al. 2014, Schmidt et al. 1999). For all ticks, we also recorded the location
of the tick attachment upon the host using 3 location categories (ear, face [including
chin], and torso). All Peromyscus mice were identified as White-Footed Mice
based on tail characteristics (Reid 2006). However, we recognize that the study may
have included P. maniculatus (Wagner) (Deer Mice) as these Peromyscus species
are difficult to differentiate in the field (Rich et al. 1996). Procedures for all animal
captures were approved by the Smithsonian Institution’s Animal Use and Care
Committee (#11-30).
Tick collection from the environment
To measure the density of questing ticks in the environment, we sampled ticks
opportunistically from October to November 2011 and from May to October 2012
at SCBI using standard drag cloths (90 cm × 185 cm) for dragging at ground level
(Brunner and Ostfeld 2008, Goddard 1993, Henning et al. 2014). We dragged the
area around each mammal trap for 40 m2 in each individual survey, with a total survey
area of 20,650 m2. These surveys provided a measure of questing tick density
and diversity in the area surrounding each trap site. We dragged at each trap site as
soon as logistically possible after mammal trapping, and sampling was completed
within 1 week following mammal captures.
Tick morphological identification
All collected ticks were observed under a 6.3:1 (5‒378x magnification) microscope
(Olympus CHA, Tokyo, Japan). Adult and nymphal ticks were identified
to species using characteristics (scutum, basis capituli, spurs, etc.) as shown in 2
pictorial keys (Keirans and Durden 1998, Keirans and Litwak 1989). We did not
morphologically identify the larval ticks to species as few keys exist for larvae, and
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identification is difficult and prone to error. Preliminary species identification was
checked by taxonomist R. Robbins (Air Forces Pest Management Board, US Army
Garrison-Forest Glen, Silver Spring, MD).
Pathogen screening and tick molecular identification
As we were unable to test all collected ticks due to time and budget constraints,
we selected a subsample (n = 250) of collected ticks, including both attached and
questing, for genetic testing. Attached ticks were selected randomly but stratified by
host and tick species across the study period. We randomly selected questing ticks
from each dragging location across the study period. This subsample was genetically
tested to verify tick species identification and screen for tick-borne pathogen
groups including Anaplasma phagocytophilum (Foggie) Dumler et al., Ehrlichia
spp., Theileria microti (França) (= Babesia microti), Borrelia burgdorferi, Coxiella
burnetii Derrick, Rickettsia spp., and Hepatozoon spp. We chose these pathogens
because they have been reported in the region (CDC 2017) and have had well-tested
protocols designed for their detection using PCR-based methods (Campana et al.
2016). We refer to genera that include pathogenic and non-pathogenic species, such
as Rickettsia spp., as pathogen groups with the understanding that the group may
include pathogenic and/or non-pathogenic species.
We homogenized whole ticks using 1.0-mm silica beads in a BeadBeater
(BioSpec Products, Inc., Bartlesville, OK). Each individual tick was processed
(Appendix 1). We identified morphologically unidentified ticks, mostly larvae, and
pathogens by previously published conventional PCR assays (primer pairs listed in
Appendix 1; detailed methodology listed in the appendix of Campana et al. 2016).
Rarefaction curves for community composition
We computed rarefaction-type curves, based on the Shannon–Weaver diversity
index (Dumas et al. 2011, Gauthier et al. 2010) to assess whether the sizes of our
subsamples of identified ticks were sufficient to characterize the tick communities
we sampled. We first divided our identified ticks into 4 groups: (1) attached adults,
(2) attached nymphs, (3) questing adults, and (4) questing nymphs. For each of these
groups with at least 2 species identified, we then randomly resampled (without replacement)
15,000 times, and computed the Shannon–Weaver index, as a function
of sample size, for each re-sampling. For each sample size (from 1 individual to
the total number of individuals in each group), we then averaged over the 15,000
re-samplings to obtain the rarefaction curve and its standard deviation. Finally,
we plotted the rarefaction curve for each group, together with ± 1 SD error bars,
against the full-data Shannon–Weaver diversity estimate for the focal group. This
latter value is simply the Shannon–Weaver estimate computed from all of the data
for a given group. We developed a small R package, shannonRarefy (https://github.
com/jmcalabrese/shannonRarefy), to automate these calculations. The package can
be installed using the devtools package via: devtools::install.github(“jmcalabrese/
shannonRarefy”).
These rarefaction curves allowed us to assess the extent to which species diversity
in each of the above-defined groups could be expected to change as sample sizes
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decreased. A small decrease in expected diversity produced by a large decrease in
sample size would suggest that the actual sample size of the focal group was sufficient
to characterize diversity. At the other extreme, a large decrease in expected
diversity produced by a small decrease in sample size would suggest a group-level
sample size that was likely too small to adequately characterize diversity.
Results
Small-mammal captures
We captured 471 small mammals representing 5 species, including Blarina
brevicauda (Say) (Short-Tailed Shrew), White-Footed Mice, Zapus hudsonius
(Zimmermann) (Meadow Jumping Mouse), Microtus pennsylvanicus (Ord) (Meadow
Vole), and Tamias striatus (L.) (Eastern Chipmunk) (Table 1), all of which were
identified, measured, and examined for ticks. Most mammals (75%) were White-
Footed Mice. A total of 87 White-Footed Mice and 4 Eastern Chipmunks were
recaptured and reexamined for ticks.
Tick collection and identification
We collected 1114 ticks attached to the mammals, the majority of which were
larvae (Table 1). Most ticks were attached on ears (86%), with less on the face
region (9%) and the torso (5%). Ticks were not observed on 36% of all small mammals,
including the majority of Short-Tailed Shrews, a third of Eastern Chipmunks
and White-Footed Mice, and 1 Meadow Vole (Table 1). We identified a total of 164
ticks (115 genetically, 49 morphologically) to the following 3 species: Amblyomma
americanum (L.) (Lone Star Tick), Dermacentor variabilis (Say) (American Dog
Tick), and Ixodes scapularis (Say) (Blacklegged Tick). Tick abundance varied over
time for each life stage (Fig. 1), with peak abundance of adults in October (n = 12
ticks), nymphs in June (n = 66), and larvae in July (n = 234). Tick burdens were
lowest in November (91%) and peaked in July (16%). In addition, we collected
2389 questing ticks (Table 2). A subsample of 428 was identified (25 genetically,
403 morphologically) to the same 3 species. Most (57%) were Blacklegged Ticks.
Table 1. Small mammals captured in forests and fields in Virginia, and the ticks collected from them,
by species and life stage: Adult (A), Nymph (N), and Larvae (L). Average tick burdens (± 1 SD) are
given. Unidentified ticks are not listed except for those in Ixodes genus. A. a. = A. americanum (Lone
Star Tick), D. v. = D. variabilis (American Dog Tick), and I. s. = I. scapularis (Blacklegged Tick).
Average Unknown
Total tick A. a. D. v. I. s. Ixodes
Host species hosts burden N L A N L A N L Totals
Blarina brevicauda 55 0.51 (± 2.04) 0 0 0 0 6 0 0 8 26
Microtus pennsylvanicus 7 1.71 (± 1.60) 0 2 2 0 1 0 0 0 11
Peromyscus leucopus 353 3.04 (± 4.24) 6 11 16 34 54 1 55 527 930
Tamias striatus 55 2.85 (± 4.05) 0 0 6 11 12 0 14 95 144
Zapus hudsonius 1 3 0 0 0 0 3 0 0 0 3
Total ticks 6 13 24 45 76 1 69 630 1114
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The density of questing ticks peaked in June for adults (0.05 individuals/m2) and
for nymphs (0.14 individuals/m2), and in August for larvae (14.28 individuals/m2)
(Fig. 1).
We identified an additional 700 of the 1114 ticks collected from small mammals
(Table 1) and an additional 663 of the 2389 questing ticks to the Ixodes genus
(Table 2). Ten ticks were identified to the Ixodes genus using molecular techniques,
Figure 1.Tick burdens (number of ticks collected) and questing tick density (ticks/m2)
over time for: (A) larvae, (B) nymphs, and (C) adults. Sampling efforts across trapping
and dragging sessions were constant over time. No trapping took place between December
2011 and March 2012. No dragging for questing ticks took place between January and
March 2012.
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while the remaining 1353 ticks were identified morphologically. We were unable to
identify the remaining 1548 collected ticks (250 attached, 1298 questing) because
these specimens were either too damaged or non-Ixodes larvae, which are difficult
to identify due to their morphological traits being less conspicuous.
Taxonomist R. Robbins verified samples of adult and nymphal ticks that we had
identified morphologically. The ticks selected for genetic identification were those
that had not been morphologically identified to species, the majority of which were
larval ticks or too damaged to identify. We verified the morphological identification
of ticks to genus through genetic testing. All ticks that were genetically identified
as Blacklegged Ticks, or at least to the Ixodes genus, were morphologically identified
to the Ixodes genus. All larvae that were genetically identified as American Dog
Ticks and a single Lone Star Tick were morphologically identified as non-Ixodes.
These results indicates that visual identification was accurate .
We computed Shannon–Weaver diversity-based rarefaction curves for attached
nymphs (n = 51), questing adults (n = 105), and questing nymphs (n = 320) (Fig. 2).
We were unable to compute a rarefaction curve for attached adults, as only 1 species
(Blacklegged Tick) was identified in this group. For the 3 groups of identified ticks
with ≥ 2 species per group, the rarefaction curves suggested that the estimated species
diversity of each group was not sensitive to changes in sample size. Specifically,
our analysis suggested that a 50% reduction in sample size of identified ticks would
lead to reductions in Shannon diversity of only 5.0% for attached nymphs (Fig. 2A),
1.4% for questing adults (Fig. 2B), and 0.3% for questing nymphs (Fig. 2C).
Pathogen detection
Using molecular methods, we tested 250 ticks (218 from small mammals and 32
questing) for 7 pathogens (Table 3). We detected varying prevalence of 4 pathogen
groups in tested ticks: A. phagocytophilum, B. burgdorferi, Hepatozoon spp., and
Rickettsia sp. (Table 3). Overall, 26% of the larvae (n = 33), 22% of the nymphs
(n = 19), and 26% of the adults (n = 9) tested positive for pathogens. Five Blacklegged
Ticks tested positive for multiple pathogens.
Discussion
Virginia has seen significant increases in reported cases of tick-borne diseases
over the past decade (CDC 2017), but basic knowledge on the community ecology
of these tick-borne diseases is poor. We examined the tick species and tick-borne
Table 2. Questing ticks captured in forests and field in Virginia, grouped by species and life stage.
Unidentified ticks are not listed except for those in Ixodes genus.
Amblyomma Dermacentor Ixodes Unknown
americanum variabilis scapularis Ixodes Total
Adult 45 20 40 1 106
Nymph 119 0 201 82 402
Larvae 1 0 2 580 583
Total 165 20 243 663 1091
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pathogens in questing ticks and within tick burdens of a small-mammal community
in forests and fields from October 2011 to November 2012 in northwest Virginia, in
order to generate valuable information regarding tick burdens and pathogen prevalence
that can be used as a baseline for future studies.
Among ticks collected from 5 small-mammal species, we found that Blacklegged
Tick was the most abundant tick species and Ixodes the most abundant
genus at each tick life stage. Lone Star Ticks and American Dog Ticks were
present in the burdens at much lower levels of abundance. It is likely that the
unidentified portion of attached larval ticks consisted of Lone Star Ticks and
American Dog Ticks, but even with these additional ticks, the 2 species remain at
low abundances when compared to Ixodes. These findings reflect that the range of
Figure 2. Rarefaction-type curves
representing Shannon-Weaver diversity
vs sample size (black curves)
for identified: (A) attached nymphs
(n = 51), (B) questing adults (n =
105), and (C) questing nymphs (n
= 320). The (vertical) error bars are
±1 SD, and the horizontal line is the
Shannon-Weaver diversity estimate
from the full dataset for the focal
group. Notice that the full sample
sizes noted above for each group are
reflected in the maximum sample
size value on the x-axis of each
panel. In all cases, cutting the full
sample size in half resulted in no
more than a 5% decrease in Shannon
diversity.
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Table 3. Pathogen prevalence in ticks collected from 3 small mammal species—Peromyscus leucopus (PELE; n = 166 ticks tested), Tamias striatus (TAST;
n = 38), Microtus pennsylvanicus (MIPE; n = 5)—and directly from the environment (q) in forests and fields in Virginia, grouped by species and life stage:
Larvae (L), Nymph (N), and Adult (A). Tested ticks from Blarina brevicauda (BLBR; n = 6) and Zapus hudsonius (ZAHU; n = 3) were negative for all
pathogens and are not listed below. Totals equal total number tested (number positive).
Tick life Anaplasma phagocytophilum Borrelia burgdorferi Hepatozoon spp. Rickettsia spp.
Tick species stage PELE TAST q PELE TAST MIPE q PELE q PELE q
A. americanum L 0 (0) 0 (0) 1 (0) 0 (0) 0 (0) 0 (0) 1 (0) 0 (0) 1 (0) 0 (0) 1 (1)
N 1 (0) 0 (0) 3 (0) 1 (0) 0 (0) 0 (0) 3 (0) 1 (0) 3 (1) 1 (0) 3 (2)
A 0 (0) 0 (0) 1 (0) 0 (0) 0 (0) 0 (0) 1 (0) 0 (0) 1 (0) 0 (0) 1 (0)
D. variabilis L 11 (1) 0 (0) 0 (0) 11 (0) 0 (0) 2 (0) 0 (0) 11 (0) 0 (0) 11 (0) 0 (0)
A 0 (0) 0 (0) 2 (0) 0 (0) 0 (0) 0 (0) 2 (0) 0 (0) 2 (0) 0 (0) 2 (0)
I. scapularis L 54 (4) 12 (0) 2 (0) 54 (11) 12 (6) 1 (1) 2 (0) 54 (0) 2 (0) 54 (4) 2 (0)
N 34 (2) 11 (1) 13 (1) 34 (2) 11 (4) 0 (0) 13 (2) 34 (0) 13 (0) 34 (1) 13 (0)
A 16 (0) 6 (0) 6 (1) 16 (0) 6 (0) 2 (0) 6 (3) 16 (0) 6 (0) 16 (0) 6 (3)
Unknown Ixodes L 19 (0) 13 (0) 1 (0) 19 (4) 13 (0) 0 (0) 1 (0) 19 (1) 1 (0) 19 (0) 1 (0)
N 24 (0) 1 (0) 0 (0) 24 (1) 1 (1) 0 (0) 0 (0) 24 (1) 0 (0) 24 (0) 0 (0)
A 0 (0) 0 (0) 0 (0) 0 (0) 1 (1) 1 (1) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)
Totals 250 (10) 250 (37) 250 (3) 250 (11)
% of ticks testing positive 4.0 14.8 1.2 4.4
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the Blacklegged Tick has been expanding geographically over the past 20 years,
and is moving from the East Coast inland to the west (Brinkerhoff et al. 2014,
Brownstein et al. 2003). Our findings confirm Brownstein et al. (2003), who modeled
future distributions of Blacklegged Ticks, and predicted prominent increases
in Virginia. We found that the Blacklegged Tick has become the dominant tick
species in burdens in northwest Virginia.
Blacklegged Ticks tested positive for Borrelia burgdorferi, Anaplasma phagocytophilum,
and Rickettsia spp., and some specimens even tested positive for multiple
pathogens. Given the major roles that Blacklegged Ticks and White-footed Mice
have in the transmission and maintenance of Lyme disease (B. burgdorferi; Brunner
and Ostfeld 2008, Schmidt et al. 1999), the dominance of Blacklegged Ticks in
tick burdens can help explain the drastic increases in Lyme disease and other tickborne
diseases in Virginia (Brinkerhoff et al. 2014). Monitoring of the expansion
of Blacklegged Ticks across North America would be needed to track and predict
increases in associated tick-borne pathogen prevalence and resulting disease risks
to human health.
Our survey of questing ticks showed high abundances of Blacklegged Ticks and
Lone Star Ticks; however, many of the larvae could not be identified. In surveys
in southeastern Virginia, Lone Star Ticks represented 95% of the questing ticks
collected (Nadolny et al. 2014). As Lone Star Ticks do not use small mammals as
primary hosts (Kollars et al. 2000), studies should include larger-sized hosts to better
understand the interactions of this species. Our rarefaction analyses suggested
that our sampling efforts were sufficient to characterize the tick assemblages we
studied. For the specific samples we collected, these analyses demonstrated that
halving the sample size would be expected to cause a negligible change in estimated
species diversity for attached nymphs, questing adults, and questing nymphs. However,
we caution that there is no guarantee employing such reduced sample sizes in
future studies would sufficiently characterize diversity in these groups. We also caution
that no method can “see” what was not in a sample. However, based on the data
we do have, the rarefaction results lend some confidence that our results would not
have changed much even if we had collected substantially smaller samples. We detected
4 tick-borne pathogens: Rickettsia spp., B. burgdorferi, A. phagocytophilum,
and Hepatozoon spp., the agents of Rickettsia diseases (i.e., Rocky Mountain Spotted
Fever), Lyme disease, human granulocytic anaplasmosis, and hepatozoonosis,
respectively (CDC 2017). While pathogen presence in ticks collected from mammalian
hosts does not necessarily imply that the mammal is infected, it does provide
information regarding the infection risk to the host species. With at least a quarter of
the tested ticks having one or more pathogen, these data demonstrate the high risk
for tick-borne disease in the study area.
Our study provides a snapshot of the tick burdens and tick-borne disease risk
in Virginia, and can provide a baseline for future research. Given the range expansion
of various tick species across the Mid-Atlantic region and the possible
consequences for human health, we recommend further monitoring of the changes
in both the tick community and variation in prevalence and disease risk over time.
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Acknowledgments
We thank Justin Lock for initial testing of ticks at the Center for Conservation Genomics
lab; Caitlin Kupferman and Giulia Manno for assisting with the trapping and tick collection;
Richard Robbins of the Armed Forces Pest Management Board and Hillary Young for assistance
with tick identification; Rick Ostfeld and Kelly Oggenfuss for advice regarding
tick collection; Holly Gaff and Jory Brinkerhoff for advice regarding the use of PCR for
testing ticks; and Lorenza Beati of the US National Tick Collection for advice regarding tick
identification. This study was funded by a Smithsonian Institution Grand Challenges Award
to W.J. McShea and P.A. Jansen. The collection of mammals was done under the approval
of the Smithsonian Institutional Animal Care and Use Committee (#11-30). No competing
financial interests exist.
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Appendix 1. We genetically identified ticks to species using the cytochrome c oxidase
subunit 1 (cox1) barcode region and the 16S rRNA gene (Folmer et al. 1994, Hebert
et al. 2004, Ivanova and Grainger 2006, Nadolny et al. 2011). The Apicomplexan
primer pair BT-1F/BT-1R (Criado-Fornelio et al. 2003) sometimes amplified the tick
18S rRNA gene, permitting identification of ticks to genus. We identified pathogens
by previously published diagnostic regions (Criado-Fornelio et al. 2003, Ghafar
and Eltablawy 2011, Levin and Fish 2000, Mediannikov et al. 2010, Shih and Chao
2002, Ujvari et al. 2004). All PCR set-ups included an extraction negative control
and a PCR negative control (containing water rather than DNA). Positive controls
were obtained for the 8 pathogens from known infected animals. Therefore, all non-
Apicomplexan PCR set-ups included positive controls. PCRs were conducted in 25-
μl volumes containing 1× AmpliTaq Gold PCR buffer (Life Technologies, Carlsbad,
CA), 2 mM of MgCl2, 1 mM of dNTPs, 0.4 μM of each primer, 20 μg of BSA, 1U of
AmpliTaq Gold (Life Technologies) and 2‒3 μl of DNA.
Thermocycling for the ectoparasite cytochrome c oxidase subunit I (cox1) reactions
included the following: an initial 5-min denaturation step at 95 °C; 5 cycles of
30 s at 95 °C, 40 s at 45 °C, and 1 min at 72 °C; 35 cycles of 30 s at 95 °C, 40 s at 51
°C, and 1 min at 72 °C; and a final 10-min extension step at 72 °C. Thermocycling
for B. burgdorferi, C. burnetii, Rickettsia sp., and ectoparasite 16S rRNA assay programs
consisted of an initial 5-min denaturation of 95 °C; 40 (B. burgdorferi, Rickettsia
sp.) or 35 (C. burnetii, ectoparasite) cycles of 1 min at 94–95 °C, 1 min at
annealing temperature (60 °C for B. burgdorferi and C. burnetii, 55 °C for Rickettsia
sp., 50 °C for ectoparasites), and 1 min at 72 °C; and a final 5-min extension
step of 72 °C. For the A. phagocytophilum and Ehrlichia sp. assays, thermocycling
consisted of an initial 5-min denaturation of 95 °C; 35 cycles of 30 s at 94 °C, 30 s
at annealing temperature (55 °C for Ehrlichia sp., 58 °C for A. phagocytophilum),
and 30 s at 72 °C; and a final 5-min extension step of 72 °C. Thermocycling for the
apicomplexan assays consisted of an initial 5-min denaturation of 95 °C; 40 (BTH-
1F/BTH-1R primer pair) or 35 (HepF300/HepR900 primer pair) cycles of 30 s at
94 °C, 30 s at 60 °C, and 60 s (BTH-1F/BTH-1R) or 45 s (HepF300/Hep4900) at
72 °C; and a final 5-min extension step of 72 °C. Sequencing was done on an ABI
3130 (Life Technologies) for representative subsamples of positive PCR products
following standard protocols.
PCR products were visualized on a 1.5% agarose gel stained with GelRed
(Biotium Inc., Freemont, CA). We purified and sequenced representative samples
of positive PCR products on an ABI 3130 sequencer (Life Technologies) following
standard protocols (sequences available in FASTA format upon request from the
authors). Sequences were edited using Sequencher® 5 (Gene Codes Corporation,
Ann Arbor, MI) and then aligned against the GenBank non-redundant nucleotide
database using Megablast to determine tick and pathogen identities (Zhang et al.
2000). We identified pathogen and tick species by their best Megablast matches.
In the case of multiple best (or near-best) matches, we conservatively identified
sequences to genus. All pathogens identified to species were at least 97% identical
with publicly available reference sequences.
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The following table provides the primer pairs used to identify tick and pathogen
species by polymerase chain reaction. Published primer names are given with the
marker in parentheses.
Organism Marker Forward (5′→3′) Reverse (5′→3′) Reference
Tick cox1 TAA CTT CAG GGT CAA CAA Folmer et al. 1994
(HC02198/LC01490) GGT GAC CAA ATC ATA AAG
AAA TCA ATA TTG G
Tick cox1 ATT CAA CCA TAA ACT TCT Hebert et al. 2004
(LEPF1/LEPR1) ATC ATA AAG GGA TGT CCA
ATA TTG G AAA ATC A
Tick cox1 AYT CAA CYA CCW GTY CCA Ivanova and Grainger
(dgLEPF1/dgMLEPR1) ATC AYA AAG GCW CCA KWT 2006
AYM TTG G TC
Tick 16S rRNA CTG CTC AAT GTC TGA ACT Nadolny et al. 2011
(16s + 1/16s - 1) GAT TTT TTA CAG ATC AAG
AAT TGC TGT T
Anaplasma 16S rRNA GGC ATG TAG CCC CCA CAT Ghafar and Eltablawy
(E1/E2) GCG GTT CGG TCA GCA CTC 2011
TAA GTT ATC GTT TA
Apicomplexa 18S rRNA GTT TCT GAC CAA ATC AAG Ujvari et al. 2004
(HepF300/HepR900) CTA TCA GCT AAT TTC ACC
TTC GAC G TCT GAC
Apicomplexa 18S rRNA GGT TGA TCC GCC TGC TGC Criado-Fornelio et al.
(BT-1F/BT-1R) TGCC AGT CTT CCT TA 2003
AGT
Borrelia ospA AAT AGG TCT CTA GTG TTT Shih and Chao 2002
(SL_F/SL_R) AAT AAT AGC TGC CAT CTT
CTT AAT AGC CTT TGA AAA
Borrelia Flagellin CGG CAC ATA CCT GTT GAA Levin and Fish 2000
(FLA297/FLA652) TTC AGA TGC CAC CCT CTT
AGA CAG GAA CC
Coxiella IS1111 CAA GAA ACG CAC AGA GCC Mediannikov et al.
(CbISF/CbISR) TAT CGC TGT ACC GTA TGA 2010
GGC ATC
Ehrlichia 16S rRNA CAA TTG CTT TAT AGG TAC Ghafar and Eltablawy
(HE1F/HE3R) ATA ACC TTT CGT CAT TAT 2011
TGG TTA TAA CTT CCC TAT
AT
Rickettsia ompB GGC AAT TAA GCA TCT GCA Eremeeva et al. 1994
(BG1-21/BG2-20) TAT CGC TGA CTA GCA CTT
CGG TC