RESEARCH ARTICLE

Why are Svalbard Arctic foxes Brucella spp. seronegative?

Ingebjørg H. Nymo^1,2, Eva Fuglei³, Torill Mørk¹, Eva M. Breines², Karin Holmgren¹, Rebecca Davidson¹ & Morten Tryland^2,4

¹Animal Health, Welfare and Food Safety, Norwegian Veterinary Institute, Tromsø, Norway
²Department of Arctic and Marine Biology, UiT The Arctic University of Norway, Tromsø, Norway
³Norwegian Polar Institute, Tromsø, Norway
⁴Department of Forestry and Wildlife Management, Inland Norway University of Applied Sciences, Koppang, Norway

Abstract

Arctic foxes (Vulpes lagopus) are susceptible to smooth Brucella (s-Brucella) infection and may be exposed to such bacteria through the consumption of infected marine mammals, as implied by the finding of s-Brucella antibodies in polar bears (Ursus maritimus). Arctic foxes in Svalbard have not previously been investigated for s-Brucella antibodies, but such antibodies have been detected in Arctic foxes in Iceland, Alaska (USA) and Russia. We investigated blood from Svalbard Arctic foxes for s-Brucella antibodies using an indirect enzyme-linked immunosorbent assay (iELISA). The animals (0–13 years old) were either caught by fur trappers (1995–2003, n = 403) or found dead (1995 and 2003, n = 3). No seropositive animals were detected. Morbidity and mortality due to the infection cannot be ruled out. However, no known, large disease outbreaks of unknown aetiology have been reported. Furthermore, it is unlikely that the Svalbard Arctic fox is resistant to infection as Arctic foxes from other populations are susceptible, and there is circumpolar connectivity between populations. The discrepancy between the findings in Iceland and Svalbard is surprising as both populations are on islands with no known local sources of exposure to s-Brucella other than marine mammals. However, our negative findings suggest that marine mammals may not be a major source of infection for this species. Comparative investigations are needed in order to draw conclusions regarding the epizootiology of s-Brucella in Arctic foxes in Svalbard and Iceland.

Keywords
Infection; serology; smooth Brucella; s-Brucella; Vulpes lagopus; epizootiology

Abbreviations
s-Brucella: smooth Brucella
iELISA: indirect enzyme-linked immunosorbent assay
LPS: lipopolysaccharides
r-Brucella: rough Brucella
SD: standard deviation
%P: percent positivity

 

 

Introduction

The High-Arctic archipelago of Svalbard (78–81°N, 10–30°E) is located midway between the Norwegian mainland and the North Pole. The Arctic fox (Vulpes lagopus) is the only terrestrial mammalian predator in Svalbard (Fuglei et al. 2002). It is almost omnipresent, and the population is relatively stable, with no long-term population trends but with significant year-to-year variations (Nater et al. 2021). Arctic foxes in Svalbard belong to the coastal ecotype, being generalists in an ecosystem lacking cyclically fluctuating small mammal populations, feeding from both the marine and the terrestrial food webs (Fuglei & Ims 2008). Arctic foxes hunt ringed seal (Pusa hispida) pups and follow polar bears (Ursus maritimus) on the sea ice, scavenging remains of seals killed by bears (Frafjord 1993). Analysis of the stomach content of 898 Svalbard Arctic foxes revealed the remains of seal in 2.5% of the stomachs (Prestrud 1992), and stable isotope analysis revealed that marine resources, such as sea birds or marine mammals, are used by Arctic foxes in Svalbard (Ehrich et al. 2015). Svalbard reindeer (Rangifer tarandus platyrhynchus) and rock ptarmigan (Lagopus muta hyperborea) also constitute large parts of the Arctic fox diet all year round (Prestrud 1992; Fuglei et al. 2002; Eide et al. 2005; Eide et al. 2012). In spring and summer, additional food items are various birds and eggs (Frafjord 1993; Eide et al. 2005).

Brucella species occur in both smooth (s-Brucella) and rough (r-Brucella) forms, depending on the presence or absence of an O-polysaccharide on the cell surface, which influences virulence (Rittig et al. 2003). S-Brucella species include amongst others Brucella suis, Brucella ceti and Brucella pinnipedialis. S-Brucella may infect and cause disease in a range of marine mammal species and populations (Nymo et al. 2011). Antibodies against s-Brucella have been detected in numerous marine mammal species inhabiting the waters surrounding Svalbard (Table 1), signifying that the bacteria are circulating in these species. The seal-specific strain B. pinnipedialis has been isolated at a high prevalence from hooded seals (Cystophora cristata) caught in the so-called West Ice area, between Svalbard and Greenland (Tryland et al. 2005). S-Brucella antibodies have been detected in polar bears in Svalbard, with a seroprevalence of 4% in animals captured on land and 16% in polar bears captured on the sea ice east of Svalbard, suggesting ingestion of marine mammals as a source of infection (Tryland et al. 2001). Similarly, a study of polar bears from the Beaufort Sea population showed that animals that remained on the sea ice during summer had 2.5 times higher odds of being s-Brucella seropositive as compared to polar bears in more land-based habitats (Atwood et al. 2017).

Table 1 Isolation of Brucella spp. and the detection of antibodies against smooth Brucella spp. in seals, whales and polar bears in waters surrounding Svalbard and other areas.

Speciesa	Antibodies Svalbardb	Isolation other areasb, c	Antibodies other areas
Atlantic walrus (Odobenus rosmarus rosmarus) P	Scotter et al. 2018	No	Nielsen et al. 2001d
Bearded seal (Erignathus barbatus) P	Foster et al. 2018	Foster et al. 2018	Foster et al. 2018
Harbour seal (Phoca vitulina) P	No	Foster et al. 2002d	Nymo et al. 2018d
Harp seal (Pagophilus groenlandicus) S	Tryland et al. 1999d	Forbes et al. 2000d	Nielsen et al. 2001d
Hooded seal (Cystophora cristata) S	Tryland et al. 1999d	Foster et al. 1996	Nielsen et al. 2001
Ringed seal (Phoca hispida) P	Tryland et al. 1999	Forbes et al. 2000d	Nymo et al. 2018d
Atlantic white-sided dolphin (Lagenorhynchus acutus) S	No	Foster et al. 2002d	No
Beluga (Delphinapterus leucas) P	No	Whatmore et al. 2017	Nielsen et al. 2001d
Bowhead whale (Balaena mysticetus) P	No	No	No
Fin whale (Balaenoptera physalus) S	No	No	Tryland et al. 1999
Killer whale (Orcinus orca) S	No	Raverty et al. 2002	Jepson et al. 1997d
Minke whale (Balaenoptera acutorostrata) S	No	Tryland et al. 1999d	Tryland et al. 1999d
Narwhale (Monodon monoceros) P	No	No	Nielsen et al. 2001
White-beaked dolphin (Lagenorhynchus albirostris) S	No	Foster et al. 2002	No
Polar bear (Ursus maritimus) P	Tryland et al. 2001	No	Atwood et al. 2017d
aPermanently (P) or sporadically (S) present in waters surrounding Svalbard. bA citation indicates a positive finding. cThe only host species from which Brucella spp. has been isolated in the Svalbard area is the hooded seal (Cystophora cristata; Nymo, Tryland et al. 2013; Tryland et al. 2005), and the column indicating isolation from Svalbard has been omitted from the table to save space. dOther references exist in addition to this reference.

Arctic foxes are susceptible to infection with s-Brucella. Brucella suis biovar 4, which can cause brucellosis in reindeer/caribou (Rangifer; Josefsen et al. 2018), has been isolated from Arctic foxes in Russia (Zheludkov & Tsirelson 2010) and Alaska (Morton 1986). Arctic foxes harboured the bacterium after being fed reindeer meat with B. suis biovar 4 (Pinigin et al. 1970), and seropositive (50%, n = 4) wild Arctic foxes have been detected in areas with B. suis biovar 4 in Rangifer spp. (Morton 1986, 1989). In Iceland, s-Brucella has not been detected in any terrestrial species (European Centre for Disease Prevention and Control 2019), yet s-Brucella antibodies were detected in Icelandic Arctic foxes in a study by Czirják et al. (2016). The coastal foxes had a higher seroprevalence (63%) than the inland foxes (25%), and the authors suggested marine mammal meat as the source of infection.

Seropositive wild red foxes (Vulpes vulpes) have been described, and the source of infection was thought to be B. suis biovar 4 from infected Rangifer (Neiland 1975; Morton 1986, 1989; Zheludkov & Tsirelson 2010). Brucella suis biovar 4 has also been isolated from wild red foxes. Moreover, red foxes that were orally challenged with B. suis biovar 4 harboured antibodies for several months, and B. suis biovar 4 was isolated from tissues (Morton 1986). Brucella suis biovar 2 has also been isolated from red foxes, with the European brown hare (Lepus europaeus) the likely source of infection (Hofer et al. 2010). Other wild carnivore species are also susceptible to infection with s-Brucella and may seroconvert following exposure (Kosoy & Goodrich 2018).

As we know that both polar bears and Arctic foxes have a high degree of contact with, and consumption of, marine mammals (Prestrud 1992; Iversen et al. 2013), both species should be commonly exposed to s-Brucella. Moreover, a high seroprevalence has been found in Arctic foxes in Iceland (Czirják et al. 2016), and marine mammals were suggested as the source of infection. The aim of our study was to investigate whether Svalbard Arctic foxes are exposed to s-Brucella spp., and which geographical and physical traits affected their likelihood of seropositivity.

Materials and methods

Arctic foxes (n = 403) were caught in baited traps by fur trappers during winter (1996–2003). Three Arctic foxes found dead in 1995 and 2003 were also investigated. The Arctic foxes were frozen at −80 °C for minimum seven days to neutralize the eggs of the parasite Echinococcus multilocularis. The carcasses were then stored at −20 °C until necropsy. Sex (n = 405) and age (Grue & Jensen 1976) were determined and noted (n = 316). Blood was obtained (n = 406) from the heart or the thoracic or abdominal cavity during necropsy and centrifuged at 3500 rpm for 15 minutes, and the serum was stored at −20 °C until analysis.

Serum samples (n = 406) were analysed for s-Brucella antibodies with iELISA, a protein A/G iELISA (Nymo, Godfroid et al. 2013). A serum sample from a B. pinnipedialis bacteriology positive hooded seal (Tryland et al. 2005) was included on each plate as a positive control. The mean optical density of duplicate wells was expressed as a percentage of the reactivity of the positive control: ([optical density of the sample/optical density of the positive control] × 100) = %P. The cut-off was based on the mean value of the %P for hooded seal samples that were classified seronegative in the complement fixation test, the slow agglutination of Wright with ethylenediamine tetra-acetic acid and the Rose Bengal test plus 2.58 SDs. This provided a cut-off of 73.6 %P.

Descriptive statistics (i.e., range, mean and SD) were performed using the statistical analysis software JMP 14 (SAS Institute).

Results

The captured foxes (n = 403, 186 females, 216 males and one unknown) and the three foxes found dead (two males and one female) were obtained from various locations in Svalbard (Fig. 1); geographic origin was unknown for nine animals. Both inland (n = 45) and coastal (n = 307) resource areas were represented in all trap seasons (1996–97: nine coast/one inland, 1997–98: 42 coast/four inland, 1998–99: 94 coast/11 inland, 1999–2000: 53 coast/11 inland, 2000–01: 21 coast/one inland, 2001–02: 87 coast/16 inland, 2002–03: one coast/one inland).

Fig. 1 Geographic distribution of Svalbard Arctic foxes investigated for smooth Brucella spp. antibodies. (The figure was made using ©Mapbox, ©OpenStreetMap; www.mapbox.com; Washington, DC, and San Francisco, CA).

The captured foxes for which age could be determined (n = 313) were one year of age (n = 153), two (n = 84), three (n = 33), four (n = 10), five (n = 9), six (n = 5), seven (n = 6), eight (n = 8), 10 (n = 2), 11 (n = 2) and 13 (n = 1) years old. Carcass weights (n = 279; 1500–5100 g) and fat indexes (visible amount of subcutaneous and abdominal fat [Prestrud & Nilssen 1992]) were registered. The captured foxes were categorized as having fat index 0 (no fat, n = 16), 1 (low, n = 80), 2 (moderate, n = 130), 3 (considerable, n = 120), 4 (extensive, n = 54) and not registered (n = 3).

Of the three animals found dead, a male pup and a juvenile male were emaciated, so the cause of death was presumed to be starvation. The cause of death could not be determined for the third fox, an adult female in good condition.

All Arctic foxes were classified as seronegative. The iELISA results ranged 0.3–5.3 %P (mean 1.7 %P, SD = 0.9 %P).

Discussion

All Arctic foxes investigated in this study were seronegative. This finding was surprising as s-Brucella antibodies have been detected in marine mammal species in Svalbard waters. Many of these marine mammals were found to have Brucella antibodies in the same period as the Arctic foxes were sampled (Table 1). Ingestion of marine mammals has been suggested as the source of infection for seropositive polar bears in this region (Tryland et al. 2001) and in the Beaufort Sea (Atwood et al. 2017).

The fox carcasses in our study, and the blood samples obtained from them had undergone several freeze–thaw cycles. This may have reduced the amounts of antibodies in the samples (Cecchini et al. 1992; Pinsky et al. 2003); it also resulted in heavy haemolysis of the blood in the carcasses prior to sampling, which may hamper some serological tests. However, the iELISA used in the present study has previously shown to be robust when testing samples of similar quality from multiple species (e.g., whales, seals, polar bears and Rangifer; Nymo, Godfroid et al. 2013; Nymo, Tryland et al. 2013; Nymo et al. 2018), which is very convenient when working with wildlife. The iELISA is also a multi-species method validated for the detection of s-Brucella antibodies in seals, whales, polar bears and Rangifer (Nymo, Godfroid et al. 2013), but it is not validated for Arctic foxes. However, the plates were coated with Brucella abortus lipopolysaccharides, which is the immunodominant component of s-Brucella, shown to cross-react serologically with other s-Brucella (Cherwonogrodzky et al. 1990). As conjugate, we used the chimeric protein A/G (Harlow & Lane 1988), which has been used in an iELISA for Toxoplasma antibodies in Arctic foxes (Elmore et al. 2016). The most plausible source of infection for the Svalbard Arctic foxes is seals. The iELISA has been used to detect s-Brucella antibodies in many different seal species, with results that were supported by other serological methods and bacteriology (Nymo, Godfroid et al. 2013; Nymo et al. 2018). Taking these facts together, we assume that if the investigated Arctic foxes had seroconverted as a result of exposure to s-Brucella, the iELISA would have detected s-Brucella specific antibodies in spite of poor sample quality.

A potential explanation for the seronegative results could be that the Svalbard Arctic foxes are not susceptible to infection with s-Brucella. However, even though Svalbard is an archipelago, the Arctic fox population is not isolated. Satellite tracking of an Arctic fox revealed that it wandered from Svalbard to Ellesmere Island, Canada (Fuglei & Tarroux 2019). Analyses of population genetics have also shown that circumpolar connectivity is maintained amongst Arctic fox populations across most of the species’ distribution range, except Iceland (Dalén et al. 2004; Carmichael et al. 2007; Geffen et al. 2007; Noren et al. 2011). It seems unlikely that Arctic foxes in Svalbard would not be susceptible to infection with s-Brucella, whereas Arctic foxes in other locations are (Pinigin et al. 1970; Morton 1986, 1989).

The lack of seropositive animals amongst the 406 Arctic foxes tested could theoretically be due s-Brucella quickly killing infected foxes, so they were not captured. This also seems unlikely as the marine mammal brucellae are shown to have limited pathological potential in true seals (Phocidae; Nymo et al. 2011) and in cell models (Larsen et al. 2013) and mouse models (Nymo et al. 2016). Moreover, experimental inoculations of red foxes with the pathogenic B. suis biovar 4 yielded very limited pathology (Morton 1986, 1989). In addition, no larger disease outbreaks of unknown aetiology were reported in Svalbard Arctic foxes during the study period, although it is challenging to have a full overview of such events in wild populations.

When screening a large number of true seals, the highest seroprevalence against s-Brucella was found in one-year-olds and thereafter declined with age (Nymo et al. 2018). The Arctic foxes in our study were 0–13 years old, thus covering a wide age range and minimizing the odds of missing out a similar epidemiological trait in Arctic foxes. The Arctic foxes investigated also represented geographically distributed locations on the islands of Spitsbergen, Forlandet and Hopen, including numerous coastal locations associated with the marine ecosystem and food chain (Fig. 1).

The lack of s-Brucella seropositives amongst the 406 Svalbard Arctic foxes investigated herein is in sharp contrast to a recent screening of Arctic foxes in Iceland, reporting a prevalence of 63 and 25% in coastal and inland Arctic foxes, respectively, suggesting marine mammals as the source of infection (Czirják et al. 2016). The Arctic fox population in Iceland is completely isolated from other Arctic fox populations (Dalén et al. 2004), and s-Brucella has not been detected in any other Icelandic terrestrial species (European Centre for Disease Prevention and Control 2019), supporting the suspicion of marine mammals as the reservoir. However, our findings in Svalbard Arctic foxes suggest that marine mammals may not be a major source of infection for this population. Instead, our findings suggest the possibility of an unknown source of s-Brucella exposure in Iceland. Polymerase chain reaction amplicon sequences specific for Brucella spp. have been detected in common eider (Somateria mollissima), common loon (Gavia immer), great black-backed gull (Larus marinus), great cormorant (Phalacrocorax carbo), great shearwater (Puffinus gravis), herring gull (Larus argentatus) and northern gannet (Moras bassanus) from the Canadian–US coast between Kent Island and Virginia (Bogomolni et al. 2008). These birds are present in Iceland and all but the great cormorant and the great shearwater are present in Svalbard and may be part of the Arctic fox diet. Documentation of the ecological range of brucellae has also recently been extended to rodents (Tiller et al. 2010), frogs (Eisenberg et al. 2012), fish (Eisenberg et al. 2017) and soil (Scholz et al. 2008). Whether birds, rodents, fish or the environment is the source of exposure to s-Brucella for Arctic foxes in Iceland warrants further investigation.

In the study performed in Iceland by Czirják et al. (2016), the SVANOVIR® Brucella-Ab I-ELISA and the serum agglutination tests were utilized, whereas we used a Protein A/G iELISA (Nymo, Godfroid et al. 2013). As the two Arctic fox populations were investigated with different serological methods, the results are not directly comparable. Nonetheless, the large difference in results is surprising. To further explore the contrasting findings from Svalbard and Iceland, additional investigations of Arctic foxes from both places, including a larger serum sample size and organ samples, and using the same serological tests and in addition bacteriological and molecular methods, should be performed. An interesting future avenue would also be to use molecular methods for determining dietary composition, and thus help with identifying potential sources of exposure. Taken together, this could provide interesting information on the epizootiology of Brucella spp. in Arctic foxes. If this difference in exposure remains, further investigations are warranted in Iceland in order to identify the source of s-Brucella infection in the Arctic foxes there.

Acknowledgements

The authors thank the local fur trappers in Svalbard for handing in the Arctic fox carcasses to the Norwegian Polar Institute. The authors are grateful to Stian Hanssen, Chief Technology Officer at Helseapps AS, Tromsø, Norway, for graphics assistance.

References

Atwood T.C., Duncan C., Patyk K.A., Nol P., Rhyan J., McCollum M., McKinney M.A., Ramey A.M., Cerqueira-Cezar C.K., Kwok O.C.H., Dubey J.P. & Hennager S. 2017. Environmental and behavioral changes may influence the exposure of an Arctic apex predator to pathogens and contaminants. Scientific Reports 7, article no. 13193, doi: 10.1038/s41598-017-13496-9.

Bogomolni A.L., Gast R.J., Ellis J.C., Dennett M., Pugliares K.R., Lentell B.J. & Moore M.J. 2008. Victims or vectors: a survey of marine vertebrate zoonoses from coastal waters of the northwest Atlantic. Diseases of Aquatic Organisms 81, 13–38, doi: 10.3354/dao01936.

Carmichael L.E., Krizan J., Nagy J.A., Fuglei E., Dumond M., Johnson D., Veitch A., Berteaux D. & Strobeck C. 2007. Historical and ecological determinants of genetic structure in Arctic canids. Molecular Ecology 16, 3466–3483, doi: 10.1111/j.1365-294X.2007.03381.x.

Cecchini G., Bekele T. & Kasali O.B. 1992. The effect of repeated freezing and thawing of serum on the activity of antibodies. Veterinary Research Communications 16, 425–428, doi: 10.1007/BF01839019.

Cherwonogrodzky J.W., Dubray G., Moreno E. & Mayer H. 1990. Antigens of Brucella. In K. Nielsen & R. Ducan (eds.): Animal brucellosis. Pp. 19–64. Boca Raton, FL: CRC Press.

Czirják G.-Á., Bela D., Miklos G., Unnsteinsdottir E.R., Pall H., Muhldorfer K. & Greenwood A.D. 2016. You are where you eat: high Brucella seroprevalence in coastal but not inland Arctic foxes. Paper presented at the 12th Conference of the European Wildlife Disease Association, 26–31 August, Berlin.

Dalén L., Fuglei E., Hersteinsson P., Kapel C.M., Rot J.D., Samelius G., Tannerfeldt M. & Angerbjørn A. 2004. Population history and genetic structure of a circumpolar species: the Arctic fox. Biological Journal of the Linnean Society 84, 79–89, doi: 10.1111/j.1095-8312.2005.00415.x.

Ehrich D., Ims R.A., Yoccoz N.G., Lecomte N., Killengreen S.T., Fuglei E., Rodnikova A.Y., Ebbinge B.S., Menyushina I.E., Nolet B.A., Pokrovsky I.G., Popov I.Y., Schmidt N.M., Sokolov A.A., Sokolova N.A. & Sokolov V.A. 2015. What can stable isotope analysis of top predator tissues contribute to monitoring of tundra ecosystems? Ecosystems 18, 404–416, doi: 10.1007/s10021-014-9834-9.

Eide N.E., Eid P.M., Prestrud P. & Swenson J.E. 2005. Dietary responses of Arctic foxes Alopex lagopus to changing prey availability across an Arctic landscape. Wildlife Biology 11, 109–121, doi: 10.2981/0909-6396(2005)11[109:DROAFA]2.0.CO;2.

Eide N.E., Stien A., Prestrud P., Yoccoz N.G. & Fuglei E. 2012. Reproductive responses to spatial and temporal prey availability in a coastal Arctic fox population. The Journal of Animal Ecology 81, 640–648, doi: 10.1111/j.1365-2656.2011.01936.x.

Eisenberg T., Hamann H.P., Kaim U., Schlez K., Seeger H., Schauerte N., Melzer F., Tomaso H., Scholz H.C., Koylass M.S., Whatmore A.M. & Zschock M. 2012. Isolation of potentially novel Brucella spp. from frogs. Applied Environmental Microbiology 78, 3753–3755, doi: 10.1128/AEM.07509-11.

Eisenberg T., Risse K., Schauerte N., Geiger C., Blom J. & Scholz H.C. 2017. Isolation of a novel ‘atypical’ Brucella strain from a bluespotted ribbontail ray (Taeniura lymma). Antonie van Leeuwenhoek 110, 221–234, doi: 10.1007/s10482-016-0792-4.

Elmore S.A., Samelius G., Al-Adhami B., Huyvaert K.P., Bailey L.L., Alisauskas R.T., Gajadhar A.A. & Jenkins E.J. 2016. Estimating Toxoplasma gondii exposure in Arctic foxes (Vulpes lagopus) while navigating the imperfect world of wildlife serology. Journal of Wildlife Diseases 52, 47–56, doi: 10.7589/2015-03-075.

European Centre for Disease Prevention and Control 2019. Brucellosis. Annual epidemiological report for 2017. Annual epidemiological report for on communicable diseases in Europe. Stockholm: European Centre for Disease Prevention and Control.

Forbes L.B., Nielsen O., Measures L. & Ewalt D.R. 2000. Brucellosis in ringed seals and harp seals from Canada. Journal of Wildlife Diseases 36, 595–598, doi: 10.7589/0090-3558-36.3.595.

Foster G., Jahans K.L., Reid R.J. & Ross H.M. 1996. Isolation of Brucella species from cetaceans, seals and an otter. Veterinary Research 138, 583–586, doi: 10.1136/vr.138.24.583.

Foster G., MacMillan A.P., Godfroid J., Howie F., Ross H.M., Cloeckaert A., Reid R.J., Brew S. & Patterson I.A. 2002. A review of Brucella sp. infection of sea mammals with particular emphasis on isolates from Scotland. Veterinary Microbiology 90, 563–580, doi: 10.1016/s0378-1135(02)00236-5.

Foster G., Nymo I.H., Kovacs K.M., Beckmen K.B., Brownlow A.C., Baily J.L., Dagleish M.P., Muchowski J., Perrett L.L., Tryland M., Lydersen C., Godfroid J., McGovern B. & Whatmore A.M. 2018. First isolation of Brucella pinnipedialis and detection of Brucella antibodies from bearded seals Erignathus barbatus. Diseases of Aquatic Organisms 128, 13–20, doi: 10.3354/dao03211.

Frafjord K. 1993. Food-habits of Arctic foxes (Alopex-lagopus) on the western coast of Svalbard. Arctic 46, 49–54.

Fuglei E. & Ims R.A. 2008. Global warming and effects on the Arctic fox. Science Progress 91, 175–191, doi: 10.3184/003685008X327468.

Fuglei E., Øritsland N.A. & Prestrud P. 2002. Local variation in Arctic fox abundance on Svalbard, Norway. Polar Biology 26, 93–98, doi: 10.1007/s00300-002-0458-8.

Fuglei E. & Tarroux A. 2019. Arctic fox dispersal from Svalbard to Canada: one female’s long run across sea ice. Polar Research 38, article no. 3512, doi: 10.33265/polar.v38.3512.

Geffen E., Waidyaratne S., Dalen L., Angerbjorn A., Vila C., Hersteinsson P., Fuglei E., White P.A., Goltsman M., Kapel C.M. & Wayne R.K. 2007. Sea ice occurrence predicts genetic isolation in the Arctic fox. Molecular Ecology 16, 4241–4255, doi: 10.1111/j.1365-294X.2007.03507.x.

Grue H. & Jensen B. 1976. Annual cementum structures in canine teeth in Arctic foxes (Alopex lagopus (L.)) from Greenland and Denmark. Danish Review of Game Biology 10(3), 3–12.

Harlow E. & Lane D. 1988. Reagents. In E. Harlow & D. Lane (eds.): Antibodies. A laboratory manual. Pp. 613–634. New York: Cold Spring Harbor Laboratory Press.

Hofer E., Reisp K., Revilla-Fernandez S., Plicka H., Romanek G., Bago Z., Winter P. & Kofer J. 2010. Isolation of Francisella tularensis and Brucella suis from red foxes (Vulpes vulpes). Tieraerztliche Umschau 65, 229–232.

Iversen M., Aars J., Haug T., Alsos I.G., Lydersen C., Bachmann L. & Kovacs K.M. 2013. The diet of polar bears (Ursus maritimus) from Svalbard, Norway, inferred from scat analysis. Polar Biology 36, 561–571, doi: 10.1007/s00300-012-1284-2.

Jepson P.D., Brew S., MacMillan A.P., Baker J.R., Barnett J., Kirkwood J.K., Kuiken T., Robinson I.R. & Simpson V.R. 1997. Antibodies to Brucella in marine mammals around the coast of England and Wales. Veterinary Recearch 141, 513–515, doi: 10.1136/vr.141.20.513.

Josefsen T.D., Mørk T. & Nymo I.H. 2018. Bacterial infections and diseases. In M. Tryland & S. Kutz (eds.): Reindeer and caribou: health and disease. Pp. 237–272. Boca Raton, FL: CRC Press.

Kosoy M. & Goodrich I. 2018. Comparative ecology of Bartonella and Brucella infections in wild carnivores. Frontiers in Veterinary Science 5, article no. 322, doi: 10.3389/fvets.2018.00322.

Larsen A.K., Nymo I.H., Briquemont B., Sorensen K.K. & Godfroid J. 2013. Entrance and survival of Brucella pinnipedialis hooded seal strain in human macrophages and epithelial cells. PLoS One 8, e84861, doi: 10.1371/journal.pone.0084861.

Morton J.K. 1986. Role of predators in reindeer brucellosis in Alaska. Rangifer 6, Special Issue 1, 368, doi: 10.7557/2.6.2.682.

Morton J.K. 1989. Brucella suis type 4 in foxes and their role as reservoirs/vectors among reindeer. PhD thesis, University of Alaska Fairbanks.

Nater C.R., Eide N.E., Pedersen Å.Ø., Yoccoz N.G. & Fuglei E. 2021. Contributions from terrestrial and marine resources stabilize predator populations in a rapidly changing climate. Ecosphere 12, e03546, doi: 10.1002/ecs2.3546.

Neiland K.A. 1975. Further serologic observations on the occurence of rangiferine brucellosis in Alaskan carnivores. Journal of Wildlife Diseases 11, 45–53, doi: 10.7589/0090-3558-11.1.45.

Nielsen O., Stewart R.E.A., Nielsen K., Measures L. & Duignan P. 2001. Serologic survey of Brucella spp. antibodies in some marine mammals of North America. Journal of wildlife Diseases 37, 89–100, doi: 10.7589/0090-3558-37.1.89.

Noren K., Carmichael L., Fuglei E., Eide N.E., Hersteinsson P. & Angerbjorn A. 2011. Pulses of movement across the sea ice: population connectivity and temporal genetic structure in the Arctic fox. Oecologia 166, 973–984, doi: 10.1007/s00442-011-1939-7.

Nymo I.H., Arias M.A., Pardo J., Alvarez M.P., Alcaraz A., Godfroid J. & Jimenez de Bagues M.P. 2016. Marine mammal Brucella reference strains are attenuated in a BALB/c Mouse Model. PLoS One 11, e0150432, doi: 10.1371/journal.pone.0150432.

Nymo I.H., Godfroid J., Asbakk K., Larsen A.K., das Neves C.G., Rodven R. & Tryland M. 2013. A protein A/G indirect enzyme-linked immunosorbent assay for the detection of anti-Brucella antibodies in Arctic wildlife. Journal of Veterinary Diagnostic Investestigation 25, 369–375, doi: 10.1177/1040638713485073.

Nymo I.H., Rødven R., Beckmen K., Larsen A.K., Tryland M., Quakenbush L. & Godfroid J. 2018. Brucella antibodies in alaskan true seals and eared seals-two different stories. Frontiers in Veterinary Science 5, article no. 8, doi: 10.3389/fvets.2018.00008.

Nymo I.H., Tryland M., Frie A.K., Haug T., Foster G., Rodven R. & Godfroid J. 2013. Age-dependent prevalence of anti-Brucella antibodies in hooded seals Cystophora cristata. Diseases of Aquatic Organisms 106, 187–196, doi: 10.3354/dao02659.

Nymo I.H., Tryland M. & Godfroid J. 2011. A review of Brucella infection in marine mammals, with special emphasis on Brucella pinnipedialis in the hooded seal (Cystophora cristata). Veterinary Research 42, article no. 93, doi: 10.1186/1297-9716-42-93.

Pinigin A.F., Zabrodin V.A. & Nikulina V.I. 1970. Brucellosis in Arctic foxes, Alopex lagopus. Krolikovodstvo i Zverovodstvo 5, 39–40.

Pinsky N.A., Huddleston J.M., Jacobson R.M., Wollan P.C. & Poland G.A. 2003. Effect of multiple freeze–thaw cycles on detection of measles, mumps, and rubella virus antibodies. Clinical and Diagnostic Laboratory Immunology 10, 19–21, doi: 10.1128/CDLI.10.1.19-21.2003.

Prestrud P. 1992. Food-habits and observations of the hunting behavior of Arctic foxes, Alopex lagopus, in Svalbard. Canadian Field-Naturalist 106, 225–236.

Prestrud P. & Nilssen K. 1992. Fat deposition and seasonal variation in body composition of Arctic foxes in Svalbard. The Journal of Wildlife Management 56, 221–233, doi: 10.2307/3808816.

Raverty S.A., Gaydos J.K., Nielsen K., Nielsen O., Ross P.S., Lambourn D.M. & Jeffries S.J. 2005. An overview of marine mammal diagnoses in the Pacific Northwest from 1999 to 2004. Paper presented at the International Association for Aquatic Animal Medicine Conference, 14–19 May, Seward, AK.

Rittig M.G., Kaufmann A., Robins A., Shaw B., Sprenger H., Gemsa D., Foulongne V., Rouot B. & Dornand J. 2003. Smooth and rough lipopolysaccharide phenotypes of Brucella induce different intracellular trafficking and cytokine/chemokine release in human monocytes. Journal of Leukocyte Biology 74, 1045-1055, doi: 10.1189/jlb.0103015.

Scholz H.C., Hubalek Z., Nesvadbova J., Tomaso H., Vergnaud G., Le Fleche P., Whatmore A.M., Al Dahouk S., Kruger M., Lodri C. & Pfeffer M. 2008. Isolation of Brucella microti from soil. Emerging Infectious Diseases 14, 1316–1317, doi: 10.3201/eid1408.080286.

Scotter S.E., Tryland M., Nymo I.H., Hanssen L., Harju M., Lydersen C., Kovacs K., Klein J., Fisk A.T. & Routti H. 2018. Pollutants in Atlantic walruses part 1: relationships between exposure, diet and pathogen prevalence. Environmental Pollution 244, 9–18, doi: 10.1016/j.envpol.2018.10.001.

Tiller R.V., Gee J.E., Frace M.A., Taylor T.K., Setubal J.C., Hoffmaster A.R. & De B.K. 2010. Characterization of novel Brucella strains originating from wild native rodent species in north Queensland, Australia. Applied Environmental Microbiology 76, 5837–5845, doi: 10.1128/AEM.00620-10.

Tryland M., Derocher A.E., Wiig Y. & Godfroid J. 2001. Brucella sp. antibodies in polar bears from Svalbard and the Barents Sea. Journal of Wildlife Diseases 37, 523–531, doi: 10.7589/0090-3558-37.3.523.

Tryland M., Kleivane L., Alfredsson A., Kjeld M., Arnason A., Stuen S. & Godfroid J. 1999. Evidence of Brucella infection in marine mammals in the North Atlantic Ocean. Veterinary Recesarch 144, 588–592, doi: 10.1136/vr.144.21.588.

Tryland M., Sørensen K.K. & Godfroid J. 2005. Prevalence of Brucella pinnipediae in healthy hooded seals (Cystophora cristata) from the North Atlantic Ocean and ringed seals (Phoca hispida) from Svalbard. Veterinary Microbiology 105, 103–111, doi: 10.1016/j.vetmic.2004.11.001.

Whatmore A.M., Dawson C., Muchowski J., Perrett L.L., Stubberfield E., Koylass M., Foster G., Davison N.J., Quance C., Sidor I.F., Field C.L. & St Leger J. 2017. Characterisation of North American Brucella isolates from marine mammals. PLoS One 12, e0184758, doi: 10.1371/journal.pone.0184758.

Zheludkov M.M. & Tsirelson L.E. 2010. Reservoirs of Brucella infection in nature. Bulletin of the Russian Academy of Sciences 37, 709–715, doi: 10.1134/S106235901007006X.