American black bears (Ursus americanus) in Maryland, USA, live in forested areas in close proximity to humans and their domestic pets. From 1999 to 2011, we collected 84 serum samples from 63 black bears (18 males; 45 females) in five Maryland counties and tested them for exposure to infectious, including zoonotic, pathogens. A large portion of the bears had antibody to canine distemper virus and Toxoplasma gondii, many at high titers. Prevalences of antibodies to zoonotic agents such as rabies virus and to infectious agents of carnivores including canine adenovirus and canine parvovirus were lower. Bears also had antibodies to vector-borne pathogens common to bears and humans such as West Nile virus, Borrelia burgdorferi, Rickettsia rickettsii, and Anaplasma phagocytophilum. Antibodies were detected to Leptospira interrogans serovars Pomona, Icterohaemorrhagiae, Canicola, Grippotyphosa, and Bratislava. We did not detect antibodies to Brucella canis or Ehrlichia canis. Although this population of Maryland black bears demonstrated exposure to multiple pathogens of concern for humans and domesticated animals, the low levels of clinical disease in this and other free-ranging black bear populations indicate the black bear is likely a spillover host for the majority of pathogens studied. Nevertheless, bear populations living at the human–domestic-wildlife interface with increasing human and domestic animal exposure should continue to be monitored because this population likely serves as a useful sentinel of ecosystem health.

Maryland, USA, has a large population of black bears (Ursus americanus) in the four westernmost counties (Garrett, Allegany, Washington, and Frederick), with the highest bear population densities in Garrett and western Allegany counties (approximately 2,300 km2). Although evidence of a breeding population is confined to the western four counties, the Maryland Department of Natural Resources receives several bear sightings and reports of human/bear interactions in central and southern Maryland counties each year as subadult black bears disperse through these areas seeking territory. Maryland's black bear population is part of a larger, regional mid-Appalachian population shared with the neighboring states of Pennsylvania, Virginia, and West Virginia. Black bears within Maryland, and those within a large portion of their eastern range, live in proximity to humans and their domestic animals. Bears will at times approach human habitations and necessitate intervention by natural resources staff. A bear hunt was initiated in Maryland in 2004, causing further contact between humans and bears as hunters legally harvest black bears and consume the meat. Bears also come into contact with other free-ranging species that could transmit pathogens, including raccoon (Procyon lotor), red fox (Vulpes vulpes), gray fox (Urocyon cinereoargenteus), coyote (Canis latrans), bobcat (Felis rufus), and white-tailed deer (Odocoileus virginianus).

The knowledge of exposure to pathogens is important to assess the overall health of the population, as well as to determine possible infectious agents that may stem from human-related activities. Intensive health monitoring of this population began in the late 1990s and has expanded in the past decade. We examine diseases of greatest concern for this population, including those that are shared with other carnivores and zoonotic diseases with potential impact on humans. Our data will enable wildlife managers to better conserve and protect black bear populations and better understand public health risks involved with interactions with native wildlife.

From 1999 to 2011, 83 blood samples were collected from black bears in five Maryland counties (Garrett, Allegany, Washington, Frederick, and Cecil) (39°43′–39°13′N, 79°29′–75°47′W), with the majority of the samples collected in the two westernmost counties in the state, Garrett and Allegany, where bears are most prevalent. Elevations in this region are 128–1,024 m (Maryland Geological Survey 2008) (Fig. 1).

Figure 1.

State of Maryland, US: shaded labeled counties represent those from which black bears (Ursus americanus) were sampled (courtesy of the Maryland Geological Survey, Maryland Department of Natural Resources).

Figure 1.

State of Maryland, US: shaded labeled counties represent those from which black bears (Ursus americanus) were sampled (courtesy of the Maryland Geological Survey, Maryland Department of Natural Resources).

Close modal

During the study, annual sow health surveys were conducted in March. Radio-collared sows were anesthetized in the den with cubs, or rarely yearlings, with ketamine (KetaVed™, Vedco Inc., St. Louis, Missouri, USA) and xylazine (TranquiVed, Vedco), or a combination of tiletamine/zolazepam (Fort Dodge Animal Health, Fort Dodge, Iowa, USA), ketamine, and xylazine administered via dart gun or pole syringe. Supplementation with ketamine/xylazine or ketamine was administered via dart gun, pole syringe, or hand syringe when needed to enable safe approach and handling of the animals. Sows were recovered at the den site with the cubs. Samples were also collected from bears that approached humans and had to be removed or moved. These animals were anesthetized with ketamine/xylazine and were darted while free-ranging or in culvert traps. For some captured females, radio collars were placed, and the animals were followed and sampled in later years.

Blood was collected from the femoral or jugular vein and placed into serum separator tubes. Time from collection to centrifugation and separation of serum varied based on the site of collection from 1 to 4 hr. Blood was allowed to clot and kept cool on ice packs until centrifugation; serum was stored in a conventional freezer or in a −80 C ultralow freezer until testing.

Testing for antibodies to canine adenovirus-1 (CAV), canine distemper virus (CDV), West Nile virus (WNV), canine parvovirus-2 (CPV), and Toxoplasma gondii was performed at the Cornell University Animal Health Diagnostic Laboratory. Testing for antibodies to rabies virus was performed at the Kansas State Veterinary Diagnostic Laboratory. Testing for antibodies to Ehrlichia canis, Rickettsia rickettsii, Anaplasma phagocytophilum, and Borrelia burgdorferi was performed by ProtoTek Reference Laboratory and testing for antibodies to Brucella canis and to Leptospira interrogans serovars Pomona, Icterohaemorrhagiae, Canicola, Hardjo, Grippotyphosa, and Bratislava was performed by Iowa State University Veterinary Diagnostic Laboratory. Serum samples were sent in three batches to reference laboratories for analysis, and because of varying amounts of available serum, different subsets of testing were performed for each test. See Table 1 for testing modalities.

Table 1.

Testing method, positive titer cutoff values, number of samples tested, number of positive samples, range of titers, and prevalence of antibodies to infectious pathogens in free-ranging black bears (Ursus americanus) in Maryland, USA, 1999–2011.

Testing method, positive titer cutoff values, number of samples tested, number of positive samples, range of titers, and prevalence of antibodies to infectious pathogens in free-ranging black bears (Ursus americanus) in Maryland, USA, 1999–2011.
Testing method, positive titer cutoff values, number of samples tested, number of positive samples, range of titers, and prevalence of antibodies to infectious pathogens in free-ranging black bears (Ursus americanus) in Maryland, USA, 1999–2011.

Statistical analyses were performed using R, Version 3.0.1 (R Development Core Team 2013). Fisher's exact test was utilized to detect any differences in sex, county of sampling, or age group of individual antibody-positive bears at the time the bear initially seroconverted. Differences were calculated for the four diseases with the highest antibody prevalences (CDV, CPV, T. gondii, and B. burgdorferi) to provide sufficient numbers of samples. Results were considered statistically significant at P<0.05.

Of the 63 bears sampled, 18 were male and 45 were female. Forty-seven were captured and sampled once, nine were sampled twice, five were sampled three times, and two were sampled four times. Seventeen bears were subadults or cubs (≤2 yr) at first sampling, 46 were >2 yr and <15 yr, and none were considered geriatric at first sampling (≥15 yr), although one bear entered this age group upon subsequent sampling. The mean age of bears at first sampling was 5.0 yr. The mean age of the female bears at first sampling was 5.6 yr and the mean age for males was 3.6 yr.

Each disease agent tested, methods used, number of samples in each subset, positive titer cutoff values for each test, and test results are summarized in Table 1. Seven bears seroconverted to CDV during the study period. The prevalence of CDV antibodies was high in both of the main counties sampled (29% [19/65] in Garrett County and 42% [5/12] in Allegany). When comparing CDV-antibody–positive bears at first positive sampling within these two most sampled counties, Allegany bears were more likely to be antibody positive than Garrett County bears (P = 0.03; odds ratio [OR] 6.65; confidence interval [CI] 0.95–77.82), although the sample size for Allegany County was small. A similar number of males (five of 18 [28%]) and females (15 of 45 [33%]) were CDV antibody positive at one or more sampling points; no statistical significance was found between sexes (P = 0.77) or age groups (P = 0.11).

Only one bear seroconverted to CPV during the study period. All CPV-antibody–positive bears but one were sampled in Garrett County (14% of Garrett County samples were positive), and all but one were female. No statistically significant difference was seen between sexes (P = 0.66), counties (P = 0.51), or age groups (P = 0.17). All four bears with detectable antibody to rabies virus were from Garrett County with similarly low prevalences in male (6% [1/18]) and female (5% [3/65]) bears.

Toxoplasma-antibody–positive bears were found in each of the sampled counties, with no statistically significant difference in prevalence between Garrett and Allegany counties (P = 1.00). There were more Toxoplasma-antibody–positive males (78%) than females (39%), although this was not statistically significant (P = 0.72). More females than males had high titers (defined as ≥1∶1,024) to T. gondii (28% of males and 66% of female samples) with marginal statistical significance (P = 0.05; OR 3.22; CI 0.88–13.67). Each of the 13 antibody-positive females that were sampled more than once during the study had antibody titers to T. gondii that rose or remained at the same high levels over multiple years.

Two samples were positive for antibody to L. interrogans serovar Pomona, three for Icterohaemorrhagiae, one for Canicola, four for Grippotyphosa, and four for Bratislava. Two samples had antibody to three serovars, and one to two serovars. Of the rickettsial agents tested, one sample had antibody to both A. phagocytophilum and B. burgdorferi, and another to R. rickettsii and B. burgdorferi at the same time point. There was no statistically significant correlation between sex and age for B. burgdorferi, but significantly more bears had antibody to B. burgdorferi in Allegany County than in Garrett County (P = 0.01; OR 12.75; CI 1.46–140.96). One bear in the study seroconverted to B. burgdorferi during the study.

We observed no clinical signs suggestive of disease caused by these agents in any bear. Alopecia and mild to moderate traumatic wounds were found in a few bears. Many bears had fractured canine teeth, molar wear, and generalized tartar accumulation. One bear was euthanized because of severe sarcoptic mange. One bear died a week after sampling; no abnormalities were found at the time of the examination.

Black bear populations are stable or increasing in many regions of North America and are found in areas of high human densities, especially in Appalachian regions of the eastern US. Bears frequently come in direct contact with humans and their domestic animals, providing opportunities for pathogens to be encountered as well as passed to humans and their pets and livestock. Because of this proximity, black bears in most eastern states are carefully managed by state wildlife departments, making population surveys an important tool in making management decisions to protect the bears, the ecosystem, and the people sharing that ecosystem. Knowledge of zoonotic diseases and diseases passed between black bears and domestic carnivores are an essential component to such studies.

The black bear population in Garrett and western Allegany counties increased during this study from an estimate of 227 adults and subadults in 2000 to 701 adults and subadults in 2011. The human population in the area is also increasing. Bear sightings have increased in Maryland counties to the east of occupied bear range because of the increasing number of dispersing subadult black bears associated with the increasing bear population (Maryland Department of Natural Resources, Wildlife and Heritage Service 2013).

Domestic dogs (Canis lupus familiaris) and bears are susceptible to several infectious diseases, such as CAV, CDV, rabies, T. gondii, and WNV (Pursell et al. 1983; Schönbauer et al. 1984; Kiupel et al. 1987; Taylor et al. 1991; Dutton et al. 2009). The bears we surveyed had been exposed to several infectious diseases that could have been caused by contact with domestic or wild canids, including CAV, CDV, and CPV, and could have implications for domestic carnivores in this region. Alternately, these diseases may be endemic now in the bear population. A few bears in this study had antibody to CAV-1. All but one of the positive results was considered a low positive titer (<1∶16), and three of the seven low-positive samples were from the same female captured three times from 2006 to 2010. This virus causes infectious canine hepatitis in domestic dogs and other canids, and rarely is reported to cause clinical disease in ursids, including two reports in captive black bears (Pursell et al. 1983; Collins et al. 1984). In most serosurveys in wild bear species, similarly low prevalences for CAV-1 antibody have been noted as in this study (Zarnke and Evans 1989; Dunbar et al. 1998; Philippa et al. 2004). Canine adenovirus is unlikely to pose a threat to this population.

Over half of the CDV-antibody–positive bears had high antibody titers, indicating either recent exposure or active infection. Three adult sows that were sampled repeatedly had persistently high levels of CDV antibodies, which may suggest repeated exposure to the virus. Domestic dogs are the natural host of CDV, although the prevalence of nonvaccinated, infected dogs in this area is likely low. Ursids have rarely been documented with clinical CDV infection, although there is a report of fatal perinatal CDV infections in polar bears (Ursus maritimus) and a spectacled bear (Tremarctos ornatus) (Schönbauer et al. 1984). Other native carnivores, including raccoons, red and gray foxes, and striped skunks (Mephitis mephitis), can also become infected and transmit CDV in nasal and oral secretions and may be a possible source for the bears in this study because these species can pose a risk to other nondomestic carnivores and dogs (Deem et al. 2000). It is also possible that CDV is endemic in the black bear population. The CDV antibody titers in this population were higher than those reported in other black bear populations (Dunbar et al. 1998; Philippa et al. 2004).

Overall there was a low prevalence of antibody to CPV, although most titers were high and all but one antibody-positive bear was in the county with the highest bear population, Garrett County. Dogs are the main host of CPV, and puppies, especially, can become sick and shed this hardy virus into the environment in feces until vaccine protection is complete at 4–5 mo of age. Raccoons harbor a similar parvovirus that may cross-react on serology, and wild canids could also contribute to exposure in black bears. Clinical signs related to parvovirus infection or isolation of the virus has not been reported in any bear species, so that the impact of CPV on black bears remains unknown (Barker and Parrish 2001). The evidence of exposure to CDV and CPV in the bears of this study underlines the importance of maintaining vaccinations in domestic dogs in the vicinity and avoiding contact between dogs and bears, as well as the need to monitor other wildlife for infectious agents and disease outbreaks to understand the impact on the black bear population.

The diseases caused by several of the agents examined in this study are zoonoses, most notably rabies virus, toxoplasmosis, leptospirosis, and canine brucellosis. Four bears in the study had detectable antibodies to rabies virus, two of which had high titers. Until recently, exposure to rabies virus was considered inevitably fatal, but evidence is mounting in both animals and humans that survival after exposure to rabies is possible (Deem et al. 2004; Jackson et al. 2008; Jorge et al. 2010; Gilbert et al. 2012), and some animals that are exposed mount an immune response but do not develop signs of rabies. A black bear with neurologic signs and abnormal behavior was diagnosed with rabies in 2007 in Garrett County, but cases of rabies are considered rare in free-ranging bears, with only three other reported cases in bears between 1992 and 2011 in the US, all in black bears in the eastern US (Krebs et al. 2000, 2004, 2005; Blanton et al. 2008). All of these other cases presented with neurologic signs and were found to be infected with the raccoon variant of the rabies virus (New York State Department of Health 2011), which is the variant that is endemic in Maryland. Based on the results of this survey, rabies does not seem to be a concern for this population, and black bears in Maryland are likely spillover hosts and do not appear to pose a significant risk to the public because of the low prevalence we observed.

The high prevalence of exposure to T. gondii in this population is noteworthy but not surprising and is similar to results obtained in black bears in New Jersey (Briscoe et al. 1993) and other neighboring states with a similar ecosystem. Although there was no significant difference in antibody prevalence between males and females for T. gondii, there was a marginal statistical significance for females to have higher titers than males, which may be because of their feeding strategies, greater longevity (females in this study were on average 2 yr older than males at first sampling) and therefore greater chance of exposure over time, or sampling bias. Black bears are infected by oocysts, either by ingesting food or water contaminated with cat feces or by ingestion of small mammals with tissue cysts. Because of the high titers and high prevalence, Maryland bear hunters are at risk of contracting toxoplasmosis, and should wear gloves when dressing the bears and wash hands well with soap and water afterward. Bear meat should be cooked thoroughly to at least 67 C for 3 min (Dubey et al. 1990), which is sufficient to kill both T. gondii and Trichinella ssp., the latter of which has been found in screened hunter-killed Maryland bears (Dubey et al. 2013). Low levels of exposure were found to L. interrogans serovars, and bears likely represent an accidental host via ingestion of small rodents or contaminated water or food, although any mammal that recovers from infection can shed leptospires in the urine for many months postinfection (Sykes et al. 2011). Pilfering at garbage cans that are often also frequented by rats or mice is also a plausible method of transmission to bears in a rural population (Slavica et al. 2010), but this appears to not be a significant factor for transmission of this disease in this region because of the overall low antibody prevalence.

Several vector-borne diseases were surveyed in this study and can infect both black bears and humans or their pets, such as WNV and the rickettsial agents. The role that native bears play in the pathogenesis of these diseases is not fully elucidated. West Nile virus is a mosquito-transmitted flavivirus that has been present in Maryland since 1999. Few of the bears in this survey had evidence of exposure to WNV, and five of the seven positive samples were from the same two bears over several years, one of which was alive 1999–2002, when the majority of WNV cases were diagnosed in birds, equids, and humans in Maryland, so antibodies likely were persistent from previous exposure during this period. Low prevalence of antibodies to WNV has been noted before in black bears in New Jersey in 2002 (Farajollahi et al. 2003). Low overall prevalence of antibody to this virus likely indicates that black bears are a spillover host for this virus. The virus occasionally affects carnivores, but has only once before been reported to cause clinical disease in an ursid, a captive polar bear (Ursus maritimus) (Dutton et al. 2009).

Of the rickettsial agents tested, low or no evidence of exposure was found except to B. burgdorferi, the causative agent of Lyme disease. Antibody-positive bears were found in three of the five counties surveyed, and bears in Allegany County were more likely exposed than those in Garrett County, although overall numbers and statistical power were low. Maryland had one of the higher incidences of human Lyme disease in the US during the study period (Centers for Disease Control and Prevention 2012). Black bears are not known to be a major reservoir of the disease, but these results do indicate that the bears were exposed and produced a response. The prevalence seen in this black bear population was much lower than that seen in a recent study in Pennsylvania, in which 21% of black bears had antibody to A. phagocytophilum/Ehrlichia equi and 33% of samples from the same study area had antibody to B. burgdorferi (Schultz et al. 2002).

Although the mere examination of serologic titers to diseases in a population has limitations and gives information regarding only exposure to pathogens, when it is used broadly for a population, information can be gleaned about trends and concerns for the population, as well as diseases that may be applicable for hunters and those living in proximity to bears in this and other densely populated regions in the eastern US. Black bears in this region had evidence of significant exposure to CDV and T. gondii, and low but notable exposure requiring further study to rabies virus and B. burgdorferi. Although all four of these agents either are zoonotic or could impact human health and the health of domestic pets, black bears are unlikely to be a source and are likely spillover hosts given that clinical disease is rarely noted in wild bear populations. Ongoing studies should monitor for further trends in these infections, and management strategies should be designed to lessen the impact of these pathogens on black bears and the humans living among them.

We sincerely thank the many dedicated Department of Natural Resources field biologists and Maryland Zoo veterinarians that have contributed to this ongoing project: Rob Harvey, Nick Stonesifer, Mike Fazenbaker, Georgia Johnson, Rande Brown, Clarissa Harris, Mike Cranfield, Mary Denver, Vikki Milne, Carol Bradford, Allison Wack, Jennifer Kilburn, Richard Sim, and Jennifer Hausmann. Special thanks also to Jennifer Sohl for sample preparation and analysis and Martin Zak for helpful assistance with statistical analysis. We also thank Cyndi Holland at ProtaTek International, Inc., and Amy Glaser at the Cornell Animal Health Diagnostic Center for collaboration and helpful consultation on testing methods.

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