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2012 SOUTHEASTERN NATURALIST 11(2):173–182
Temporal Aspects of Leprosy Infection in a Wild
Population of Nine-Banded Armadillos
Andrew J. Williams1 and William J. Loughry1,*
Abstract - Although Dasypus novemcinctus (Nine-banded Armadillo) is the only vertebrate
other than humans to exhibit naturally occurring infections with Mycobacterium
leprae, the causative agent in producing leprosy, little is known about patterns of infection
in wild populations. Here we provide data on some temporal aspects of infection, obtained
from sampling a population of armadillos in western Mississippi from 2005–2010. Annual
prevalence of infection varied between 4.5–15%. Incidence density estimates calculated
over progressively longer time intervals generated values ranging from 0.11–0.61 new
cases of infection/1000 animal days. Of 77 animals that tested seropositive over the course
of the study, 14 (18.2%) were seropositive in two consecutive years. Four of these animals
were seropositive in three consecutive years, but no armadillos tested positive in >3 straight
years. Finally, the proportion of seropositive animals increased with the number of years individuals
were enrolled in the study. Together, these data indicate a substantial potential for
transmission of infection within this population and confirm the view of leprosy as a slowacting
disease that is largely manifested in older individuals.
Aside from humans, Dasypus novemcinctus L. (Nine-banded Armadillo) is
the only free-ranging vertebrate known to exhibit naturally occurring infections
of Mycobacterium leprae, the causative agent in producing leprosy (Truman
2005, 2008). Although many aspects of the disease have been studied in captive
armadillos (review in Truman 2008), knowledge of infection dynamics in wild
populations remains sparse, with most studies limited to prevalence surveys (e.g.,
Loughry et al. 2009).
An important feature in understanding any disease concerns temporal aspects
of infection (Rohani and King 2010). For example, it is vital to know the probability
of an individual becoming infected. One standard way of assessing this is
by calculating an incidence density estimate (Page et al. 1995). Paige et al. (2002)
have provided the only such estimate for armadillos. Working with a population in
Louisiana with a high prevalence of M. leprae infection (≈18%), they calculated
an incidence density estimate of 3.5 new cases/1000 animal days. This rate was
considered quite high and likely to promote widespread transmission of infection
(Paige et al. 2002). Even so, estimates from other locales are useful to determine
the extent of variation in incidence density between sites. In addition, the estimate
of Paige et al. (2002) was derived from a relatively small number of animals (n =
23) recaptured over a single summer. Thus, it seems worthwhile to calculate an
estimate from a larger number of animals sampled over a longer time frame.
1Department of Biology, Valdosta State University, Valdosta, GA 31698-0015. *Corresponding
author - firstname.lastname@example.org.
174 Southeastern Naturalist Vol. 11, No. 2
Three other important temporal features of M. leprae infection concern (1) the
consistency of infection prevalence from year to year, (2) how infection status
changes with age, and (3) how long infected animals persist in a population.
Although many diseases exhibit cyclical fluctuations in prevalence (Rohani and
King 2010), the limited data available suggest this may not occur with M. leprae
infection in armadillos. In the only study of this issue to date, Truman et al.
(1991) found no differences in infection prevalence for a population of armadillos
in Louisiana that was sampled in 1960–1964 and again annually between
1984–1989. However, it is still necessary to determine the generality of this result
with data from other locales.
A standard finding in the epidemiology of any disease is that seroprevalence
increases with age. This is certainly the case with leprosy, where comparisons
between the broad age categories of juvenile, yearling, and adult have only found
infection in the adult cohort (Morgan and Loughry 2009, Truman et al. 1991). However,
it remains unknown how the likelihood of infection changes as adults continue
to age. The expectation is that the proportion of infected individuals should increase
as they become older, but no data have been available to test this hypothesis.
Regarding persistence, armadillos infected with M. leprae usually die as leprosy
spreads to many of the internal organs, with mortality often resulting from
secondary infections that occur due to compromised organ functioning (Truman
2008). However, because leprosy is a relatively slow-acting disease, it seems
likely infected individuals might survive in a population for extended periods
of time. At present, no data are available on how long infected armadillos might
persist in a population. Data on this issue are important because such animals
may represent an important source of infection transmission during the time they
remain active within a population while still relatively healthy.
As part of a long-term project on the ecology of leprosy, we collected data
on the prevalence of infection in a wild population of armadillos located in
western Mississippi. Sampling occurred over multiple years, with many animals
captured in more than one year. Consequently, these data allowed us to examine
the temporal features of M. leprae infection described above. Specifically, we
had four goals in this study: (1) to describe changes in the annual prevalence of
infection in the population, (2) to calculate an incidence density estimate from a
large number of animals sampled over multiple years, (3) to determine how long
infected animals persisted in the population, and (4) to test whether the likelihood
of infection increased with age.
Sampling was conducted at the Yazoo National Wildlife Refuge (33º05'N,
90º59'W) near Hollandale, MS. Habitats there are diverse, including bottomland
hardwoods that normally flood on an annual basis, shallow cypress swamps, and
oxbow lakes that support Nyssa sylvatica (Tupelo Gum), Callitris intratropica
(Cypress), and Cephalanthus occidentalis (Buttonbush). There are also many
agricultural fields planted primarily with corn and soybeans.
2012 A.J. Williams and W.J. Loughry 175
Nine-banded Armadillos are medium-sized (adult body weight averages ≈4
kg) burrowing mammals (McBee and Baker 1982). They are primarily nocturnal
and feed on various soil invertebrates (McDonough and Loughry 2008). Dasypus
novemcinctus has the largest geographic distribution of any species of armadillo,
extending from northern Argentina to the southern United States (Abba and
Superina 2010). In the US, this range is continuing to expand northwards at an
estimated rate of 4 km/yr (Taulman and Robbins 1996). Molecular data indicate
armadillos initially acquired leprosy from humans subsequent to the colonization
of the Americas by Europeans and their African slaves (Monot et al. 2005). However,
the biogeographic distribution of infection remains unclear, with a mix of
infected and uninfected populations occurring over the vast range of the species
(Loughry and McDonough, in press; Loughry et al. 2009; Truman 2008).
Data were collected from 2005–2010. Sampling was limited in 2005 and
2006, consisting of 2–3 weeks of fieldwork in May of each year. In the remaining
four years, sampling was far more extensive, lasting from mid-May to late July
(50–55 days in the field/year).
In all years, we attempted to capture all armadillos observed during nightly
censuses that lasted from about 16:00–24:00 each day. Field procedures followed
those described previously by Loughry et al. (2009). Briefly, animals were captured
with dip nets, and marked with PIT tags for permanent identification and
with various shapes and colors of reflective tape glued to the carapace for longrange,
temporary identification. In addition to marking, captured armadillos were
measured and weighed, reproductive condition was noted, a piece of ear tissue
was taken for genetic analyses, and a blood sample was collected to screen for
M. leprae infection. Animals were then released at the point of capture. Although
many individuals were resighted and recaptured during a field season, blood
samples were only taken at the first capture of each year.
Data collection and analysis
Blood samples were collected onto Nobuto blood strips (Advantec, Dublin,
CA) and allowed to completely air dry. The samples were then screened for
the presence of immunoglobulin M antibodies to the M. leprae-specific PGL-I
antigen using an enzyme-linked immunosorbent assay (ELISA), following the
same protocols originally described by Truman et al. (1986; see also Loughry
et al. 2009). All sample analyses were performed at the National Hansen’s Disease
Center in Baton Rouge, LA. All samples were run at least twice to confirm
consistency. A minimum antibody titer of 0.70 optical density was required to
designate an animal as seropositive.
We calculated incidence density estimates from these data following the same
procedure as Paige et al. (2002). Beginning with 2006, we calculated a series
of estimates that progressively added one more year of sampling to the dataset.
176 Southeastern Naturalist Vol. 11, No. 2
Data consisted of the total number of days between date of first capture and the
date infected individuals first became seropositive, and the total number of days
between first capture and last recapture for those animals that never became infected.
Regarding the latter, some animals tested negative in one year, were not
caught in one or more succeeding years, but then were recaptured and still tested
negative at some later date. Because infection status did not change over time
for these individuals, we included them in the data for those years they were not
caught, using the last day of sampling for that year as the recapture date. For example,
an animal might have tested negative in 2007, not been caught in 2008 or
2009, but tested negative upon recapture in 2010. If so, then this animal was classifi
ed as seronegative in 2008 and 2009, using the last date of sampling in each of
these years as the “recapture date”. However, data were not added from individuals
that, after an interval of one or more years, tested positive upon recapture,
because we could not know when their infection status had changed. Likewise,
because we only took a blood sample from each animal at the first capture each
year, we used that date, rather than any subsequent resighting in that same year,
for all animals that were not caught again in a subsequent year, in case their infection
status might have changed over the course of the summer. Over each time
interval, incidence density was calculated as the number of individuals that became
seropositive divided by the sum of the cumulative animal-days for animals
that never became seropositive plus one-half of the cumulative animal-days for
animals that did become leprous. For comparison with the value from Paige et al.
(2002), these values were then adjusted to an estimate per 1000 animal days.
To examine how the likelihood of infection changed with age, we performed a
Spearman rank-order correlation of the proportion of infected individuals versus
the number of years each individual was enrolled in the study. Because no juveniles
or yearlings were seropositive (see Morgan and Loughry 2009), we only
used data from adults. We chose years enrolled in the study for this analysis because
it is not possible to accurately age wild adult armadillos. The age categories
of juvenile, yearling, and adult are discernible by differences in body size and
weight (Loughry and McDonough 1996, McDonough et al. 1998), but at present
there is no way to know the precise age of an animal first caught as an adult. Even
so, our analysis is linked with age because, regardless of their true age (in years),
each individual armadillo was older with each successive year it was enrolled in
Finally, the persistence of infected individuals in the population was determined
as the number of intervening years between when an animal first tested
positive for infection with M. leprae and the last time it was seen or recaptured.
The prevalence of infected individuals varied in the Yazoo population from
a low of 4.50% in 2005 to a high of 14.92% in 2010 (Fig. 1). Mean prevalence
across all years was 9.29% (95% C.I. = 5.50–13.07%). Most differences in
prevalence between years were not significantly different from one another
(Fisher exact tests, all P > 0.10), except for 2005 versus 2010 (P = 0.004) and the
2012 A.J. Williams and W.J. Loughry 177
marginal cases of 2005 versus 2008 (P = 0.055), 2007 versus 2010 (P = 0.054),
and 2009 versus 2010 (P = 0.06).
Prevalence data were not entirely independent because some animals were seropositive
in more than one year (Table 1). However, although our data spanned a
6-year time frame, no seropositive individual persisted in the population for more
than 3 years (Table 1). Of 77 animals that tested seropositive over the course of our
study, 14 (18.2%) were positive in 2 consecutive years (Table 1). Four of these animals
tested positive in one additional year. There were no significant differences
between males and females in the proportion of infected individuals that persisted
for 2 or 3 consecutive years (Fisher exact tests: both P > 0.25).
Beginning with 2006, incidence density estimates were calculated that included
data from each succeeding year of the study (Table 2). These estimates varied
from year to year, but all were substantially lower than that reported by Paige
et al. (2002). In some years, a few of the seronegative data came from juveniles
Table 1. Persistence of adult male and female nine-banded armadillos that were seropositive for
infection with Mycobacterium leprae in more than one consecutive year at Yazoo National Wildlife
Refuge. Numbers in parentheses are the total number of individuals for which infection status was
known over each time frame.
2 years 3 years ≥4 years
Males 5 (61) 1 (21) 0 (9)
Females 9 (74) 3 (28) 0 (13)
Figure 1. Yearly prevalence of Mycobacterium leprae infection in a population of wild
Nine-banded Armadillos at Yazoo National Wildlife Refuge. Numbers in parentheses
under each point are the total number of animals sampled each year.
178 Southeastern Naturalist Vol. 11, No. 2
recaptured as yearlings. Inclusion of these individuals in the calculation of incidence
density estimates is debatable because no infection has ever been reported
in such young animals (see above). Consequently, we recalculated our estimates
excluding data from yearlings (Table 2). Doing so increased the values slightly,
Table 2. Annual incidence density estimates (new cases/1000 animal days) at Yazoo National Wildlife
Refuge. Each value was generated using data from all preceding years.
Number Number of
of new Total number seronegative Total number Incidence
Time interval seropositves of days recaptures of days density
2005–2006 3 1093 14 4993 0.54
2005–2007 1 742 9 6898 0.14
2005–2008 9 4788 50 24,861 0.31
2005–2009 4 2703 57 34,322 0.11
2005–2010 37 27,387 116 66,380 0.46
2005–2006 3 1093 12 4355 0.61
2005–2007 1 742 9 6898 0.14
2005–2008 9 4788 45 23,028 0.35
2005–2009 4 2703 53 32,845 0.12
2005–2010 37 27,387 101 61,150 0.49
Figure 2. The proportion of adult Nine-banded Armadillos at Yazoo National Wildlife
Refuge that tested positive for infection with Mycobacterium leprae as a function of the
number of years individuals were enrolled in the study. Data were pooled across males
and females (see Table 2). Numbers in parentheses under each point are the total number
of animals enrolled.
2012 A.J. Williams and W.J. Loughry 179
but all estimates still remained low, falling within a range from 0.12–0.61 new
cases per 1000 animal days (Table 2).
The proportion of infected individuals increased with the number of years
enrolled in the study (rs = 0.91, P = 0.04), with approximately half of all animals
enrolled for ≥4 years being seropositive (Fig. 2). A significantly higher proportion
of females enrolled for 2 years were seropositive than were males (Fisher
exact test: P = 0.038), but otherwise there were no significant differences in the
proportion of seropositive males and females enrolled for the same length of time
(Fisher exact tests: all P > 0.40; Table 3).
This study provides important details regarding several temporal aspects of
M. leprae infection in wild armadillos. As such, it contributes to a growing body
of work in which long-term ecological monitoring helps to illuminate epidemiological
patterns of wildlife diseases (review in Rohani and King 2010). Notably,
we found that (1) infection prevalence fluctuated significantly between some
years; (2) a higher proportion of older animals were infected; (3) some infected
animals were able to persist in the population for considerable lengths of time;
and (4) the likelihood of acquiring infection (i.e., the incidence density estimates)
was substantially lower than that reported from another population (Paige et al.
2002). We treat each of these findings in turn below.
Truman et al. (1991) sampled a population of armadillos in Louisiana sporadically
over a 20-year period and showed that infection prevalence remained
relatively stable. Within a year, they did find significant seasonal changes, with
prevalence higher in summer than in spring. This was attributed to the limited
availability of adult females for sampling in the spring because they were giving
birth and nursing young (Truman et al. 1991). Nonetheless, comparisons of prevalence
across years, but within the same season, revealed very little variation.
We cannot address seasonal changes in infection prevalence because all our data
were collected in the summer. However, in contrast to Truman et al. (1991), our
data indicated annual fluctuations can occur. For example, infection prevalence
in 2010 was over three times higher than that observed in 2005 (Fig. 1). Unfortunately,
at present there are no data available that might provide a plausible
explanation for such changes. In addition, a longer time series will be necessary
to determine if fluctuations in infection prevalence exhibit regular patterns, as has
been observed in some other pathogens (Rohani and King 2010).
Table 3. The number of adult male and female Nine-banded Armadillos that were seropositive for
infection with Mycobacterium leprae as a function of the number of years individuals were enrolled
in the study. The total number of individuals enrolled is given parenthetically.
Number of years enrolled
1 2 3 4 5 6
Male 20 (244) 7 (53) 5 (19) 7 (12) 1 (2) 0 (2)
Female 19 (225) 17 (56) 3 (21) 8 (19) 1 (2) 3 (4)
180 Southeastern Naturalist Vol. 11, No. 2
Collectively, our data support the conventional view of leprosy as a disease
that largely affects older individuals (Morgan and Loughry 2009; Truman 2005,
2008). To date, no study has found evidence of M. leprae infection in juvenile
or yearling armadillos (Morgan and Loughry 2009, Truman et al. 1991), which
suggests these individuals were either free of infection or the bacterium was
present but not abundant or active enough to trigger a detectable immunological
response. Our data extend these findings by showing that the proportion of seropositive
adult armadillos increased as enrolled animals became older. Indeed,
by the time animals had been enrolled for 4 years, the proportion of infected
individuals was nearly 50% (Fig. 2). We recognize that the small sample sizes in
the older enrollment classes must temper any conclusions about the relationship
between age and infection status. Nonetheless, it appears that adult armadillos
are at considerable risk of acquiring infection with M. leprae as they get older. It
would be interesting to compare survivorship, age structure, and other parameters
between a leprosy-free population of armadillos and an infected one in order
to explore the demographic consequences of this association between infection
status and age.
The incidence density estimates we calculated varied from year to year
(Table 2). Even so, all the estimates were substantially lower than that obtained
by Paige et al. (2002) for a population of armadillos in Louisiana. Presumably,
our final estimate (2005–2010) is reasonably precise because of the long time
interval and large number of animals included. However, it remains difficult to
evaluate this value against those generated over shorter time spans or at other
locations. Thus, at present, we can offer no complete explanation for annual
or between-sites variability. Undoubtedly, differences in infection prevalence
between years and between sites are at least partially responsible. However,
the extent to which other factors, such as sampling effort, population density,
environmental conditions, and so on, may have contributed to this variation is
still unknown. Nonetheless, we would point out that, even though lower, the
incidence density estimates for the Yazoo population still indicate a substantial
potential for transmission of infection, particularly given the large, dense population
of armadillos that is present there (Loughry et al. 2009).
The persistence of infected armadillos within a population may represent an
important source of infection for other individuals. Most likely this would occur
as infected animals shed M. leprae from the respiratory tract, thus potentially
contaminating soil at foraging sites and within burrows (see Lavania et al. 2008).
Although considered solitary (McBee and Baker 1982), armadillos do exhibit
considerable overlap in space use, including using some of the same burrows
(albeit not normally at the same time; McDonough and Loughry 2008). Thus, it
seems likely an infected armadillo, particularly one that remained active in the
population for an extended period of time, could play a significant role in transmitting
M. leprae to non-infected individuals. However, arguing against this
possibility is the failure to find any evidence of the spatial clustering of infection
(Morgan and Loughry 2009, Paige et al. 2002), which suggests infected animals
do not represent “hotspots” of disease transmission. Perhaps social network
2012 A.J. Williams and W.J. Loughry 181
models (e.g., Fenner et al. 2011, Perkins et al. 2009) may prove a useful tool in
further analyzing the consequences of infected armadillos persisting in populations
over multiple years.
Given the foregoing, one might expect relatively rapid and widespread transmission
of infection within populations of armadillos (see also Truman et al.
2011). This, coupled with the high vagility of the animals (Taulman and Robbins
1996) would seem to lead to the prediction that M. leprae infection should
be sweeping across the broad distribution of the species in the southern United
States (and, possibly, elsewhere). However, prevalence surveys do not support
this hypothesis, with most seropositive populations occurring in a narrow band
along the west side of the Mississippi River and the Gulf of Mexico coast from
Louisiana to southern Texas (Loughry et al. 2009; Truman 2005, 2008). Truman
(1996) proposed that certain environmental conditions (e.g., wet, humid soils),
coupled with high densities of armadillos, may be necessary to permit longterm
maintenance of M. leprae infection. Thus, it seems plausible that leprosy
has been widely disseminated throughout wild populations of armadillos in the
US, but rapidly disappeared from those areas unable to sustain the bacterium.
Targeted prevalence surveys, based on predictive models of the occurrence of
leprosy (Truman et al. 2011), will be necessary to further our understanding
of the biogeographical distribution of this disease.
We conclude by noting that our results must be considered preliminary. Armadillos
may reach ages of 8–12 years old in the wild (Loughry and McDonough,
in press) and our time series only extended over 6 years. Thus, continued surveillance
of this population (and, ideally, others) will be necessary to confirm and
extend the findings reported here.
We thank the staff of the Yazoo National Wildlife Refuge for all their help and support
of our work. Thanks also to K. Ancona, M. Ard, M. Bernhardt, W. Burnett, C. Gibson,
R. Hooks, R. Morgan, and B. Spychalski for assistance collecting blood samples. We are
extremely grateful to R.W. Truman and his lab for screening all of those samples. This
work was financially supported in part by grants from the National Geographic Society,
and Faculty Research Awards from Valdosta State University. Thanks to J.M. Lockhart,
C.M. McDonough, D. Scholl, M. Superina, and R.W. Truman for comments on earlier
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