2007 SOUTHEASTERN NATURALIST 6(2):191–202
An Assessment of Leech Parasitism on Semi-aquatic
Turtles in the Western Piedmont of North Carolina
J. Clint McCoy1, Elisabeth L. Failey1, Steven J. Price1,
and Michael E. Dorcas1,*
Abstract - In 2005, we assessed the occurrence of leeches on semi-aquatic turtles in
nine ponds in the North Carolina Piedmont. Placobdella parasitica (smooth turtle
leech) was the only parasitic leech found on turtles and was present on turtles from
all ponds. Female Chrysemys picta (Painted Turtles) were more frequently parasitized
than males (females 54.7%, males 40.9%; p = 0.039), possibly because they are
larger and provide more surface for leech attachment. Chelydra serpentina (Snapping
Turtles) had the highest leech load of any species (mean = 32.3/turtle), which
we attributed to its large size and bottom-dwelling habits. Most leeches were found
attached to the underside of marginal scutes or between the plastron and inguinal
region. These sites likely offer protection from the environment when a turtle
emerges from the water.
Leeches are common ectoparasites of many freshwater vertebrates including
turtles (Ernst et al. 1994, Sawyer 1986), amphibians (Briggler et al.
2001), and fish (Pearse 1924). In North America, leeches of the genus
Placobdella are commonly found on turtles (Sawyer 1972, 1986), and
several studies have examined aspects of these host-parasite relationships
such as parasite loads (Brooks et al. 1990, Hulse and Routman 1982,
MacCulloch 1981), seasonal variation in parasite infestations (Ernst 1971,
Graham et al. 1997, Koffler et al. 1978), variation in attachment locations
(Brooks et al. 1990, Dodd 1988), recolonization rates (Dodd 1988, Ryan
and Lambert 2005), and leeches as vectors of hemogregarine blood parasites
(Paterson and Desser 1976; Siddall and Desser 1991, 2001).
Most studies of turtle-leech interactions have focused on only one or two
species of turtle. Bottom-dwelling species such as Chelydra serpentina
Linnaeus (Common Snapping Turtle) and the mud and musk turtles (Family:
Kinosternidae) generally have higher parasite loads than other semi-aquatic
turtles (Brooks et al. 1990, Ernst 1986). Aerially basking species (e.g.,
emydids) generally have reduced parasite loads, possibly because basking
forces leeches to detach to avoid desiccation (Ernst 1971, MacCulloch 1981,
Studies examining turtle-leech relationships have been primarily conducted
in relatively large bodies of water (e.g., lakes and rivers). Previous
studies have been conducted at the Qu’Appelle River in Saskatchewan
1Department of Biology, Davidson College, Davidson, NC, 28035. *Corresponding
author – firstname.lastname@example.org.
192 Southeastern Naturalist Vol. 6, No. 2
(MacCulloch 1981), the Upper Warrior Basin in Alabama (Dodd 1988), the
Central Canal in Indianapolis, IN (Ryan and Lambert 2005), and the Drysdale
and Isdell Rivers in Australia (Tucker et al. 2005).
In our study, we describe turtle-leech relationships within nine relatively
small, isolated golf course and farm ponds in the western Piedmont of North
Carolina. Specifically, we (1) describe gender and species-specific differences
in frequency of parasitism and leech load, (2) examine relationships
between turtle size and leech load, (3) examine relationships between parasitism
and turtle body condition, (4) describe variation in parasitism between
pond types, and (5) describe leech attachment sites on host turtles.
We assessed the occurrence of leeches on semi-aquatic turtles as part of a
mark-recapture study of turtles at nine man-made ponds in Mecklenburg,
Iredell, and Cabarrus counties, NC from 17 April to 28 July 2005. Four
ponds were located on golf courses, primarily surrounded by residential
neighborhoods and fairways. The other five were farm ponds, which contained
varying levels of mixed hardwood-coniferous forest and open field
within close proximity. Most of these ponds were historically used to provide
water for cattle, but none of our study ponds were part of operating
farms. The nine ponds ranged in size from 0.03 to 1.02 ha.
We captured turtles using hoop-net traps (model MHNIA, 2.54-cm mesh,
Memphis Net and Twine, TN) baited with opened cans of sardines (replaced
every 4–5 days). Ten baited traps were used at each site, and were spaced
evenly around each pond. We checked traps every other day for a total of 20
days at each pond.
For each turtle we captured, we counted all leeches and recorded their
location on the host. We used 4 categories to assess leech attachment site:
(1) on the outside of the carapace, (2) on the outside of the plastron, (3) the
anterior body (soft tissues on the head, neck, and front limbs), and
(4) the posterior body (soft tissues on the hind limbs, inguinal region,
and tail). To prevent cross-contamination or loss of leeches, leeches were
counted in the field at the time of capture for all turtles except for Snapping
Turtles, which we transported individually back to the lab. We removed
turtles from the traps one at a time, and immediately examined each turtle for
the presence of leeches. We carefully counted leeches on each turtle, and
removed some leeches from small leech clusters in order to increase the
accuracy of our count. Each leech was classified as small (< 1 cm), medium
(between 1 cm and 2 cm), or large (> 2 cm). The same person was assigned
the task of counting and classifying leeches each time in order to maintain
consistency in leech counts and classifications. Leech data were only recorded
for the initial capture of each turtle; all recaptures were noted and
released without returning them to the laboratory. Representative leech
specimens were taken from turtles at each study pond, preserved in 95%
2007 J.C. McCoy, E.L. Failey, S.J. Price, and M.E. Dorcas 193
ethanol for identification, and deposited in the North Carolina Museum of
Natural Sciences. After collecting data in the field, all turtles were brought
to the laboratory, where we used digital calipers to measure carapace and
plastron length, maximal width, and maximal depth to the nearest 0.1 mm
for all turtles except large Trachemys scripta Schoepff (Yellowbelly Slider)
and Snapping Turtles which we measured to the nearest 1 mm. Large
Snapping Turtles, were placed in a canvas bag and measured using a spring
scale to the nearest 0.2 kg (the mass of the bag was subtracted from the total
mass), and we used a top-loading balance to measure the mass of all other
turtles to the nearest 0.1 g. All turtles were given a unique mark by filing
notches in their marginal scutes (Gibbons 1968, Sexton 1959). This ensured
that we would not double-count leeches on an individual turtle if recaptured.
Sex of adult turtles was determined by claw length, shell shape, and tail
length for Chrysemya picta Schneider (Eastern Painted Turtle), Pseudemys
concinna LeConte (Eastern River Cooter) and Yellowbelly Slider; shell
shape and tail length for Kinosternon subrubrum Lacépède (Eastern Mud
Turtle) and Sternotherus odoratus Latreille (Common Musk Turtle) (Ernst
et al. 1994); and by using the formula described by Mosimann and Bider
(1960) for Snapping Turtles. We attempted to age each turtle by counting the
rings on the plastron and/or carapacial scutes, and used a confidence scale
ranging from 0 to 3 to rate the accuracy of the count (Cagle 1946, Sexton
1959). We assumed for all turtles that one new growth ring was added each
year (but see Wilson et al. 2003). Many adult turtles could not be aged
accurately because the growth rings were not clearly visible; therefore, they
were categorized as “old.” We processed and returned all turtles to the
original capture site within 2–4 days of capture.
We used chi-square tests to determine if there were gender- or speciesspecific
differences in the number of turtles parasitized. To better estimate
the potential impact of number and size of leeches, leech loads for each turtle
were calculated by multiplying the number of leeches by a size class assigned
to each leech. Small leeches were assigned a constant of 1,
medium-sized leeches were assigned a 2, and large leeches were given a
value of 3. For example, a turtle with one small leech and two medium
leeches would have a leech load of 5. This method assumes that larger
leeches have a greater impact on the animals than do smaller leeches and,
although this analysis only estimates the overall impact, it provides more
information than simply counting the number of leeches. We used a
Wilcoxon rank-sum test to determine any differences in leech load between
To determine if there were any differences in leech load between pond
types or species, we used a two-way analysis of variance (PROC GLM; SAS
Version 9.1, SAS Institute 1999, Cary, NC). The high number of zeros (i.e.,
turtles with no leeches) skewed the data so that tests could not be performed
194 Southeastern Naturalist Vol. 6, No. 2
due to violation in normality. Therefore, we used only data for turtles with at
least one leech in our analysis. Thus, many of our data points were omitted, so
we could only perform these tests on three species (Snapping Turtles, Eastern
Painted Turtles, and Eastern Mud Turtles). All leech loads were log-transformed
to normalize the dataset before analysis.
We also tested for differences in average leech load between pond types
for each species using the Wilcoxon rank-sum test. This test allowed us to
use all data points (including turtles with no leeches) to examine the relationship
between leech load and pond type for each species of turtle.
We conducted a linear regression analysis to examine the relationship
between plastron length and leech load for Eastern Painted Turtles and
Yellowbelly Sliders. We used carapace length for Snapping Turtles and Eastern
Mud Turtles to examine this relationship because their plastron lengths
are highly variable (Lindsay and Dorcas 2001). We also used linear regression
to determine if a relationship existed between turtle size and the size of
leeches parasitizing them. We performed three linear regressions for each
turtle species, one for each leech size class. We used the number of leeches
in each size class as the dependent variable and size of the turtle as the
To examine the relationship between parasitism and body condition, we
used a linear regression with leech load as the independent variable and body
condition as the dependent variable. Condition was calculated as the residuals
of a linear regression with mass as the dependent variable and plastron
length (Eastern Painted Turtles and Yellowbelly Sliders) and carapace
length (Snapping Turtles and Eastern Mud Turtles) as the independent
variable ( Budishak et al. 2006, Lindsay and Dorcas 2001). We used an alpha
of 0.05 for all analyses.
We observed leeches on all species captured, which included Eastern
Painted Turtles, Yellowbelly Sliders, Snapping Turtles, Eastern River
Cooters, Eastern Mud Turtles, and Musk Turtles. Placobdella parasitica Say
(smooth turtle leech) was the only parasitic leech found, and was present on
turtles from all ponds. During the study, we captured 221 Eastern Painted
Turtles, of which 47.5% were parasitized by at least one leech. We captured
34 Snapping Turtles, of which 67.6% were parasitized. Of the 37
Yellowbelly Sliders captured, 48.6% bore leeches, and of the 27 Eastern
Mud Turtles captured, 63% were parasitized (Table 1).
Female Eastern Painted Turtles were parasitized more frequently than
male Eastern Painted Turtles (2 = 4.24, p = 0.039) and had higher mean
leech loads than males (Wilcoxon rank-sum; p < 0.01). No significant sexspecific
differences in frequency of parasitism or leech loads were found
among the other three species: Snapping Turtles (2 = 0.13, p = 0.71),
Yellowbelly Sliders (2 = 0.24, p = 0.62), and Eastern Mud Turtles
(2 = 0.02, p = 0.88).
2007 J.C. McCoy, E.L. Failey, S.J. Price, and M.E. Dorcas 195
We found Snapping Turtles to have a greater mean leech load than any other
species of turtle captured in the study (Fig. 1; two-factor ANOVA: F = 3.9, df =
147, p = 0.02). Snapping Turtles were parasitized more frequently than Eastern
Painted Turtles (2 = 4.78, p = 0.02). Snapping Turtles and Eastern Mud Turtles
Table 1. Prevalence of leeches on four species of semi-aquatic turtles in nine ponds located in
Mecklenburg, Cabarrus, and Iredell counties, NC. Leech loads were calculated by multiplying
the number of leeches by the size class of each leech. Small leeches (< 1 cm) were assigned a
size class of 1, medium-sized leeches (between 1 cm and 2 cm) were assigned a 2, and large
leeches (> 2 cm) were given a value of 3.
Number Percent Mean Range
Species n parasitized parasitized leech load (leech load)
Eastern Painted Turtles 221 105 47.5 3.84 ± 0.59
Male 115 47 40.9 2.97 ± 0.77 0–68
Female 106 58 54.7 4.78 ± 0.91 0–71
Snapping Turtles 34 23 67.6 32.26 ± 14.47
Male 17 12 70.6 21.18 ± 8.40 0–129
Female 17 11 64.7 43.35 ± 27.88A 0–468
Yellowbelly Sliders 37 18 48.6 7.11 ± 3.06
Male 19 10 52.6 6.05 ± 2.79 0–50
Female 18 8 44.4 8.22 ± 5.65 0–102
Eastern Mud Turtles 27 17 63.0 6.33 ± 2.52
Male 14 9 64.3 9.64 ± 4.64 0–66
Female 13 8 61.5 2.77 ± 1.21 0–16
ALeech load was 16.81 ± 9.1 when we excluded an outlier value of 468.
Figure 1. Mean leech loads for C. serpentina (Snapping Turtles), C. picta (Eastern
Painted Turtles), T. scripta (Yellowbelly Sliders), and K. subrubrum (Eastern Mud
Turtles). Leech loads were calculated by multiplying the number of leeches by the
size class of each leech. Error bars represent ± one standard error.
196 Southeastern Naturalist Vol. 6, No. 2
were parasitized at similar frequencies, meaning both species had a similar
percentage of individuals with at least one leech (2 = 0.15, p = 0.70).
Leech size varied widely from less than 0.5 cm to greater than 4 cm. We
found no relationship between leech size and the size of the host turtle (p >
0.05 for all analyses). However, our data suggest that larger leeches were
more prevalent on larger species of turtles, Snapping Turtles and
Yellowbelly Sliders (Table 2). We found no relationship between the turtles’
body condition and the leech load (all p-values > 0.19). We found a positive
relationship between leech load and plastron length for Eastern Painted
Turtles (R2 = 0.033, p = 0.004), although the low R2 value explains little of
the variation, possibly rendering this finding biologically meaningless.
Table 2. Number and prevalence of leeches, grouped by size class, on four species of turtles
from nine ponds located in Mecklenburg, Cabarrus, and Iredell counties, NC. Small leeches
(< 1 cm) were assigned a size class of 1, medium sized leeches (between 1 cm and 2 cm) were
assigned a 2, and large leeches (> 2 cm) were given a value of 3.
Size class 1 Size class 2 Size class 3
Species n #/turtle n #/turtle n #/turtle
Snapping Turtles (n = 34) 204 6.00 216 6.35 17 0.50
Eastern Painted Turtles (n = 221) 369 1.67 137 0.62 69 0.31
Yellowbelly Sliders (n = 37) 97 2.62 56 1.51 18 0.48
Eastern Mud Turtles (n = 27) 156 5.78 3 0.11 3 0.11
Figure 2. Mean leech loads for C. picta (Eastern Painted Turtles),C. serpentina
(Snapping Turtles), T. scripta (Yellowbelly Sliders), and K. subrubrum (Eastern
Mud Turtles) in golf course ponds and farm ponds. Numbers represent sample size
(number of turtles). Leech loads were calculated by multiplying the number of
leeches by the size class of each leech. Error bars represent ± one standard error.
2007 J.C. McCoy, E.L. Failey, S.J. Price, and M.E. Dorcas 197
There were no overall trends in mean leech load between pond types
(two-factor ANOVA: df = 147, F = 0.30, p = 0.58). However, mean leech
loads for Snapping Turtles were higher in farm ponds than in golf course
ponds (Fig. 2; Wilcoxon rank-sum: p < 0.001). Eastern Painted Turtles had
higher mean leech loads in golf course ponds than in farm ponds (Fig. 2;
Wilcoxon rank-sum: p = 0.02).
Leeches attached more frequently on the posterior region of Eastern
Painted Turtles and Eastern Mud Turtles, while they attached more frequently
on the anterior region of Snapping Turtles. They were found in
relatively equal proportions on the anterior and posterior regions of
Yellowbelly Sliders. Leeches were found on the carapace of Snapping
Turtles more frequently than any other species (Fig. 3).
The smooth turtle leech is the most common species of leech found on
turtles in the northern United States and Canada (Klemm 1995, Sawyer
1972). In our study, the smooth turtle leech was the only leech found
parasitizing turtles, confirming that the species is also well established in the
western Piedmont of North Carolina. All turtle species captured were parasitized,
although the frequency of parasitism and mean leech load varied
considerably among and between species, gender, and pond type.
Figure 3. Leech attachment sites for C. picta (Eastern Painted Turtles),C. serpentina
(Snapping Turtles), T. scripta (Yellowbelly Sliders), and K. subrubrum (Eastern
Mud Turtles). Each bar represents the number of leech observations at each region as
a proportion of the total number of leeches found on each species.
198 Southeastern Naturalist Vol. 6, No. 2
In our study, the only species to show gender-specific differences in
parasitism was the Eastern Painted Turtle, in which females were more
frequently parasitized and had higher leech loads than males. Sexual dimorphism
is prominent in Eastern Painted Turtles, with females generally larger
than males (Ernst et al. 1994). We also found a significant, though weak,
relationship between increased plastron length and leech load for Eastern
Painted Turtles; therefore, the higher prevalence of leeches on female Eastern
Painted Turtles may be due to their larger size. Although there were no
significant differences between sexes of other species, male Snapping
Turtles had a higher rate of parasitism than female Snapping Turtles. Sexual
dimorphism is also prominent in Snapping Turtles, where males are typically
larger than females (Ernst et al. 1994). Brooks et al. (1990) reported
more leech clusters on male Snapping Turtles than females and attributed it
to size differences. Thus, size could be a determining factor in both frequency
of leech parasitism and leech loads for Eastern Painted Turtles and
Snapping Turtles. Future studies could experimentally examine rates of
parasitism between sexes and sizes by removing leeches and examining
Snapping Turtles were parasitized more frequently and had higher
mean leech loads than any other species in our study. Brooks et al. (1990)
found extremely high rates of leech parasitism on Snapping Turtles in
Algonquin Park, ON. Due to their large size and bottom-dwelling habits
(Ernst et al. 1994), Snapping Turtles may be more likely than other species
to come in contact with smooth turtle leeches, which are notably poor
swimmers, spending much of their lives attached to a host or crawling
along the pond bottom (Sawyer 1986). Activity level may be a determining
factor in turtle-leech relationships. Both Snapping Turtles and Eastern Mud
Turtles (bottom-dwellers) had higher frequencies of parasitism than the
emydids, Eastern Painted Turtles and Yellowbelly Sliders (active swimmers/
The frequency of parasitism may also be explained by the “desiccating
leech” hypothesis (Ernst 1971, MacCulloch 1981, McAuliffe 1977), which
proposes that turtles' basking may force leeches to detach from their hosts
to avoid desiccation. Eastern Painted Turtles and Yellowbelly Sliders regularly
leave the water to bask on pond edges, logs, and other debris, and in
our study they had a lower frequency of parasitism than Snapping Turtles
and Eastern Mud Turtles. Our findings correlate with the “desiccating
leech” hypothesis, although experimental evidence casts doubt on this
hypothesis. Ryan and Lambert (2005) showed that a bottom-dwelling species,
the Musk Turtle, acquired more leeches than an aerially basking
species, Graptemys geographica Lesueur (Common Map Turtle), even
when turtles were not allowed to bask. This implies that basking alone does
2007 J.C. McCoy, E.L. Failey, S.J. Price, and M.E. Dorcas 199
not explain the differences in leech infestations of basking and non-basking
species. Combining these findings with evidence that leeches are able
to survive after losing up to 92% of the water in their body (Hall 1922), the
“desiccating leech” hypothesis may lack some support, but leeches may
still choose hosts that are less likely to bask in order to reduce their
chances of desiccation (Ryan and Lambert 2005).
Combining all species together, we found no overall trend in leech
parasitism between farm and golf course ponds. However, there were differences
in rates of parasitism between pond types when we examined species
individually. Eastern Painted Turtles had higher leech loads in golf courses
than in farm ponds. Golf courses generally do not have logs and other debris
in their ponds, which would force aerially basking turtles such as Eastern
Painted Turtles to bask on the pond edges. Increased presence of humans on
and around golf courses may discourage these turtles from basking on the
pond edges. Thus, turtles may be forced to spend more time in water,
increasing their likelihood of being parasitized.
Leech attachment sites
The attachment locations of smooth turtle leeches on turtles in our study
were similar to those found in other studies (Brooks et al. 1990, Dodd 1988,
Ernst 1971, Hulse and Routman 1982, Koffler et al. 1978, MacCulloch 1981).
The most preferred site of attachment was the posterior region. The majority
of leeches in this region were found attached to the underside of the marginals,
where they could access the adjacent soft tissue, or between the inguinal
region and the plastron. These sites would most likely provide the most
protection from desiccation and abrasion when the turtle left the water, as well
as protection from predators such as grackles (Vogt 1979), other turtles, and
the hosts themselves (Hendricks et al. 1971). Many leeches were found
attached in limb sockets where they could also find protection from the
environment. Leeches found on the anterior region of the turtles were commonly
found on the underside of the marginals, above the head, as well as on
the plastron below the head. Similarly, these sites would offer protection from
the environment when the turtle was basking. Snapping Turtles had more
leeches attached to the carapace and plastron than any other species. Although
it is not known if smooth turtle leeches can feed on bony tissues, Placobdella
ornata (Verrill) (predacious leech) has been observed feeding on bony tissues
of turtles (Siddall and Gaffney 2004). The relatively high frequency of
occurrence of smooth turtle leeches on the carapace and plastron of Snapping
Turtles suggests that smooth turtle leeches can obtain a blood meal from these
Several turtle species serve as hosts to leeches. Turtles can be captured
easily, and the presence of ectoparasites on each turtle can be readily
observed. Studies of leech parasitism on semi-aquatic turtles is an
200 Southeastern Naturalist Vol. 6, No. 2
underutilized method of investigating host-parasite relationships, and these
turtle-leech relationships can serve as a model system for studying other
host-parasite interactions (Dodd 1988, Ernst 1971).
We would like to thank Eddie Campbell, Sam Linker, Bo Miller, and Darrin
Spierings for permission to access the ponds on their golf courses. We thank Dennis
Testerman of Cabarrus County Soil and Water Conservation District for his help in
finding and granting permission to access study ponds. We also thank J.D. Willson for
his help with statistical analysis, Donald Klemm for help identifying leeches, and
Travis Ryan, Judy Greene, Thomas P. Wilson and two anonymous reviewers for
providing comments that improved the manuscript. Leech specimens are catalogued as
numbers 45049–45054 at the North Carolina Museum of Natural Sciences. All turtles
were collected under scientific collecting permit # 0902 issued by the North Carolina
Wildlife Resources Commission to M.E. Dorcas. All research was approved by
Davidson College’s IACUC protocol # 03-05-11. Manuscript preparation was aided
by the Environmental Remediation Sciences Division of the Office of Biological and
Environmental Research, US Department of Energy through Financial Assistance
Award no DE-FC09-96SR18546 to the University of Georgia Research Foundation.
This research was supported by Duke Power and National Science Foundation grants
(DEB - 0347326 and DBI-1039153) to M.E. Dorcas.
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