Documentation of the Rabies Virus in Free-ranging Fisher (Martes pennanti) in Pennsylvania
Jeffery L. Larkin, Jennifer Christine Wester, Walter O. Cottrell, and Melia T. DeVivo
Northeastern Naturalist, Volume 17, Issue 4 (2010): 523–530
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2010 NORTHEASTERN NATURALIST 17(4):523–530
Documentation of the Rabies Virus in Free-ranging Fisher
(Martes pennanti) in Pennsylvania
Jeffery L. Larkin1,*, Jennifer Christine Wester1, Walter O. Cottrell2,
and Melia T. DeVivo1
Abstract - Mammalian carnivores are the primary hosts for the rabies virus in terrestrial
disease cycles. While rabies prevalence in Vulpes spp. and Urocyon spp. (foxes),
Mephitis mephitis (Striped Skunk), and Procyon lotor (Raccoon) is well documented
in Pennsylvania, the reintroduction of Martes pennanti (Fisher) provides another
potential vector of this disease. We used a direct, rapid immunohistochemical test to
examine brain material from 46 free-ranging Fishers collected throughout Pennsylvania
from 2002–2008. Five Fishers had brain material unsuitable for rabies testing,
forty Fishers tested negative for the disease, and one individual tested positive. The
individual that tested positive was an adult male that was found to be positive for
the Eastern Raccoon strain of rabies. This individual was trapped and radio-collared
in July 2006 as part of a research project examining Fisher resource selection. Researchers
monitored this individual weekly starting July 2006 until October 2006,
when it was found dead beneath a brush pile. As Fisher populations continue to expand
throughout portions of the northeastern United States, their potential as a vector
of rabies should not be overlooked.
Introduction
Rabies is a disease caused by a lyssavirus that has a tropism toward
mammalian neural tissue and is one of the oldest recognized infectious
diseases (Rupprecht et al. 2001, Wunner 2002). The rabies virus attacks the
host’s nervous system and is almost always fatal (Williams-Whitmer and
Brittingham 1996). Rabies was primarily associated with Canis domesticus
L. (Domestic Dogs) when rabies surveillance began in the United States in
1938 (McLean 1970, Rupprecht et al. 2001). However, since 1960, wildlife
rabies cases have surpassed rabies cases in domestic animals (McLean 1970,
Wampler 2007). This transition was largely due to canine rabies control
efforts during the 1940s that included mandatory vaccination of Domestic
Dogs and removal of strays (Herman 2008, Rupprecht et al. 2001). Although
the number of documented wildlife rabies cases fluctuates yearly, a general
increase of rabies in wildlife has been observed in the United States (Blanton
et al. 2009). While bats are by far the most common species involved in
transmitting rabies to humans (Messenger et al. 2003), other wildlife species
such as Procyon lotor (L.) (Raccoon), skunks, and foxes are potential
risk factors for human exposure to this virus (Rupprecht et al. 2001). Such
species are known reservoirs of rabies virus and have adapted rather well
1Department of Biology, Indiana University of Pennsylvania, Indiana PA 15705.
2Pennsylvania Game Commission, Animal Diagnostic Laboratory, Orchard Road,
University Park, PA 16802. *Corresponding author - larkin@iup.edu.
524 Northeastern Naturalist Vol. 17, No. 4
to urban and agricultural landscapes (Adams et al. 2005). It is important to
recognize, however, that all unvaccinated wild or domestic mammals can
contract rabies and potentially infect other susceptible animals (Herman
2008).
Martes pennanti (Erxleben) (Fisher) is one such species that historically
has had a low documented incidence of rabies. The low number of
documented rabies cases in Fisher most likely reflects limited investigation
of Fisher as a species potentially affected by this disease. However, the
generalist diet of this forest-dwelling carnivore includes species that are
known reservoirs for the rabies virus, including Raccoon, Mephitis mephitis
(Schreber) (Striped Skunk), Vulpes vulpes L. (Red Fox), and vespertilionids
(bats) (Dzialak et al. 2005, Wester 2009). For example, Raccoon occurred
in the diets of Maryland, West Virginia, and Pennsylvania Fishers (Dzialak
et al. 2005, Wester 2009), and Red Fox, bats, and Striped Skunk were also
observed in the diets of Pennsylvania Fishers (Wester 2009). Fishers likely
both kill and scavenge carcasses of potentially rabid animals when available
(Dzialak et al. 2005). In Pennsylvania, Raccoons are frequently struck by
vehicles, and thus may be available to Fisher as carrion. During the time of
our study (2002–2008), 1798 positive cases of Raccoon rabies in Pennsylvania
were reported to the Centers for Disease Control (Blanton et al. 2006,
2007, 2008, 2009; Krebs et al. 2003a, 2004, 2005). Additionally, 9542 Raccoons
tested from 2000–2008 in the northeastern United States were positive
for rabies (Blanton et al. 2006, 2007, 2008, 2009; Krebs et al. 2001, 2002,
2003a, 2004, 2005).
The reintroduction of Fisher to Pennsylvania and other northeastern
states combined with the species popularity among trappers further warrants
concern regarding the species’ potential as a source of rabies virus
transmission. The Pennsylvania Veterinary Laboratory tested two Fishers
for rabies previously, and both were found to be negative (K. Herman,
Pennsylvania Veterinary Laboratory, Harrisburg, PA, pers. comm.). Nationally,
10 Fishers have been documented with rabies since widespread
testing began in 1960. All 10 cases were restricted to the northeastern states
including New York (n = 3; 1965, 1994, and 2007), Maine (n = 2; 1973 and
1996), Massachusetts (n = 2; 1995 and 2002), New Hampshire (n = 2; 1994
and 1995), and Vermont (n = 1; 1996) (Blanton et al. 2008; Krebs et al.
1995, 1996, 1997, 2003a, 2003b). The most recent documented case of a
Fisher with rabies occurred in the suburbs of Albany, NY (Grondahl 2007).
This infected individual was shot by law enforcement after it attacked and
bit a woman outside of her residence (Grondahl 2007).
Although reports of rabies among Fisher are rare throughout the range
of the species, Fisher may be of regional importance as a potential rabies
virus transmitter. To determine whether rabies among Fisher is of concern
in Pennsylvania, we tested Fisher carcasses collected from 2002–2008. Our
objective was to document occurrences of rabies in Pennsylvania Fisher and
to discuss its implications for Fisher management protocols.
2010 J.L. Larkin, J.C. Wester, W.O. Cottrell, and M.T. DeVivo 525
Methods
From the Pennsylvania Game Commission (PGC), we obtained roadkilled
and accidental trapper-killed Fisher carcasses that had been collected
and stored from February 2002 to March 2008. One additional carcass was
obtained from a radio-telemetry study that was conducted in southwestern
Pennsylvania. Fisher carcasses were stored in freezers at PGC regional
offices, transferred to Indiana University of Pennsylvania - Biology Department,
and stored frozen (-20 °C) until necropsied. With few exceptions, each
carcass had an accompanying kill report which included date and location of
kill, cause of death, the name of the collector, and any other details regarding
the carcass and cause of death.
Rabies virus testing was performed by the United States Department of
Agriculture Animal and Plant Health Inspection Service (USDA-APHIS)
in New Castle, PA using direct, rapid immunohistochemical test (dRIT) as
described by Lembo et al. (2006). Freezing specimens did not compromise
the dRIT sensitivity to the rabies virus (Lembo et al. 2006). All previously
detected genotype 1 variants of the rabies virus and representative lyssaviruses
are recognized by this test (Lembo et al. 2006). The sensitivity and
specificity of the dRIT were 100% (95% CI = 93.9%–100.0%) and 100%
(95% CI = 96.3%–100.0%), respectively. These values are comparable to
direct fluorescent-antibody assay (DFA), the traditional standard in rabies
diagnosis (Lembo et al. 2006). The identification of the rabies virus variant
(i.e., raccoon vs. skunk) associated with positive cases was determined
via immunofluorescence using the Light DiagnosticsTM, Rabies Monoclonal
Antibody Typing Set (Millpore Inc., Billerica, MA).
Results
Forty-six Fisher heads were collected from 21 Pennsylvania counties
(Bedford, Blair, Bradford, Cambria, Centre, Clarion, Columbia, Elk,
Fayette, Forest, Jefferson, Luzerne, Lycoming, Mercer, Mifflin, Somerset,
Tioga, Union, Venango, Warren, and Westmorland; Fig. 1), and submitted to
USDA-APHIS for rabies testing. Thirty-eight carcasses were vehicular mortalities,
four were accidentally killed by trappers, and the cause of death for
four individuals was unknown. Of the 46 Fisher heads submitted for rabies
testing, 41 had brain material suitable for testing. Of the 41 Fishers tested,
one (2.4%) tested positive for the Eastern Raccoon rabies virus variant. The
individual that tested positive was a radio-collared adult male found dead
under a brush pile in October 2006. The location of the mortality was such
that vehicular strike may have actually been the cause of death. The animal
had blood from multiple body orifices, but no other visible clues suggesting a
reason for his death. A necropsy revealed extensive lung bruising and blood
in the urine, but little else. The lung discoloration may have been an artifact
from death or, even more likely, freezing. However, because the animal
tested positive for rabies, no further examination or testing of the tissues
526 Northeastern Naturalist Vol. 17, No. 4
taken at necropsy was performed. Thus, the actual cause of death could not
be determined.
Discussion
Pennsylvania has long been recognized as a state with high endemic indices
of rabies (USDA-APHIS 2005). In addition to the one Fisher that tested
positive for rabies during our study, 283 Raccoons, 62 Striped Skunks, 41
bats, 32 foxes, 6 Marmota monax L. (Groundhog), 4 Odeocoleus virginianus
Zimmermann (White-tailed Deer), 2 Canis latrans Say (Coyote), and 1 Felis
rufus Schreber (Bobcat) also tested positive for the disease in Pennsylvania
during 2006 (Blanton et al. 2007). Among the counties from which Fisher
carcasses in our study were obtained, the reported incidences of rabies from
January 2002 to March 2008 included 512 Raccoons, 109 Striped Skunks, 69
bats, 49 Felis silvestris L. (Domestic Cat) , 47 foxes, 9 Bos taurus L. (Domestic
Cow), 6 White-tailed Deer, 4 Bobcat, 4 Groundhog, 1 Domestic Dog, 1
Ovis aries L. (Domestic Sheep), 1 Capra hircus L. (Domestic Goat), 1 Ursus
americanus Pallas (Black Bear), and 1 Fisher (Pennsylvania Animal Diagnostic
Laboratory, unpubl. data, http://www.padls.org/notes/rabies.html)
Figure 1. Distribution of confirmed cases of domestic and wild mammals infected
with rabies in Pennsylvania from 2002–2008. Counties with stars had at least one
Fisher carcass tested for rabies during this study. The circled star indicates the county
from which a Fisher tested positive for rabies. (Data provided by A. Snow, Pennsylvania
Department of Agriculture, Bureau of Animal Health and Diagnostic Services,
Meadville, PA).
2010 J.L. Larkin, J.C. Wester, W.O. Cottrell, and M.T. DeVivo 527
(Fig. 1). The single Fisher that tested positive for the rabies virus in our study
was only the eleventh (n = 11) Fisher to have ever tested positive for this
virus (Blanton et al. 2007, 2008; Krebs et al. 2003a, b). All 11 cases were
from the northeastern United States (Blanton et al. 2007, 2008; Krebs et al.
2003a, b). However, Fishers in western and central portions of the species
range have not been thoroughly investigated for rabies infections.
Results from a non-invasive hair-snare sampling effort in southwestern
Pennsylvania from 2007–2008 indicated that the location where the rabiesinfected
Fisher was found may support one of the highest density Fisher
populations reported for the entire range of the species (Larkin et al. 2009).
Southern Pennsylvania also consistently has the highest number of documented
cases of Raccoons infected with the rabies virus annually (Pennsylvania
Animal Diagnostic Laboratory, unpubl. data, http://www.padls.org/notes/
rabies.html). This observation is important because a high Fisher density in
a region where Raccoon rabies is enzootic creates conditions where contact
between Fishers and rabid animals may become increasingly probable.
Rabies transmission primarily occurs between conspecific individuals
(Blanton et al. 2009). Infection from rabies reservoirs associated with a
specific virus variant to other species, known as spillover, occurs but rarely
initiates subsequent infections (Blanton et al. 2009, Krebs et al. 2003b).
However, rabies spillover and maintenance has occurred in foxes, skunks,
bats, Coyote, and Raccoon within the United States (Guerra et al. 2003). Due
to rabies spillover, rabies variants may adapt to secondary hosts and become
enzootic within that new species (Guerra et al. 2003). The rabies-infected
Fisher in our study was a result of spillover because it tested positive for the
rabies virus variant associated with Raccoons. Fortunately, rabies spillover
rarely leads to maintenance in carnivores that are not the primary hosts for a
specific rabies virus variant (Krebs et al. 2003b). Additionally, the likelihood
of rabies spillover and cross-species adaption in Fisher is unlikely due to
limited intraspecific contact resulting from the species’ territorial behavior
(Powell 1993). However, rabies-infected animals tend to behave abnormally
due to the disease’s effect on the central nervous system (Williams-Whitmer
and Brittingham 1996); therefore, intraspecific infection of rabies among
Fisher is possible, particularly in areas with high Fisher population densities.
Additionally, Fishers in Pennsylvania are known to consume Raccoon,
Red Fox, Striped Skunk, and bats, which are all common reservoirs for
rabies (Wester 2009). While transmission of rabies through consumption of
infected animals is rare (West 1972), Pennsylvania Fisher may be more susceptible
due to their frequent contact with many prey species that are known
reservoirs of the rabies virus.
The single rabies-infected Fisher documented in our study warrants
concern. As Fisher populations in the northeastern United States
expand, human encounters with rabies-infected animals may become
more frequent. Presence of rabies among Fisher is perceived as uncommon;
however, Fishers are reclusive animals and a rabid Fisher may go
528 Northeastern Naturalist Vol. 17, No. 4
unnoticed. In fact, if the individual that tested positive for rabies in our
study had not been radio-collared, we would not have detected the infection.
Although documented rabies cases among Fisher are rare, they can be
a source of rabies exposure for humans and domestic animals (Krebs et al.
2003b). Moreover, rabies in wildlife is of great concern because it can lead
to “silent epizootics,” where wildlife acts as reservoirs for the virus to grow
and propagate, often reviving the virus in areas that were once considered
rabies-free (West 1972). Occurrence of rabies among Fisher may become
of regional importance, especially if managers intend to trap and transfer
Fisher from areas of high population density to areas with low or variable
Fisher population density (Lovallo 2008). Translocation of potentially
infected animals could have detrimental effects not only on Fisher populations,
but also on other wildlife, domestic animals, and people (Chipman
et al. 2008). The Ontario Ministry of Natural Resources imposed a ban on
the translocation of rabies vector species within areas of the province endemic
for Raccoon rabies (Rosatte et al. 2009). An intrastate translocation
protocol that prohibits moving Fisher from areas of Pennsylvania with high
incidences of rabies to areas with low incidences should be developed and
employed. Additionally, trapper education regarding the safe handling of
carcasses should be considered in response to the recent decision to initiate
a Fisher-harvest season in 2010. As Fisher populations continue to expand
throughout portions of the northeastern United States, their potential as a
vector of rabies should not be overlooked.
Acknowledgments
We are grateful to Molly Giles, Christopher Kirkhoff, Jim Kauffman, Andrea Evans,
and Eric Ludwig for their contributions in the field. We are also thankful to Dr.
Jan Humphreys and Mourad Gabriel who reviewed earlier drafts of this manuscript.
This work was conducted under the approval of Indiana University of Pennsylvania-
IACUC protocol # 02-0506 and Pennsylvania Game Commission general use permit
# 75-2006.
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