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Infectious diseases in polar bears and their impact on population health

Written by Anna Fagre, 2014-2015 Sustainabiltiy Leadership Fellow and Ph.D. Student in the Professional Veterinary Medicine Program.

As a poster child of the Arctic, polar bears have been receiving an increasing amount of attention in the media due to documented loss of the sea ice habitat. What does this mean for polar bears? Loss of sea ice limits the ability of the bears to access marine mammal prey and range over long distances. Other implications for sea ice loss include increased frequency of long-distance swims (Durner et al. 2012), reduced body condition, lower survival rates, and declines in abundance (Regehr et al. 2007; Rode et al. 2010; Bromaghin et al. 2015) which may lead to immunosuppression.  Another potential threat that is less easily examined but being more frequently studied as of late is the effect of environmental contaminants on the polar bears’ immune systems. These factors have led to projections by some scientists that 2/3 of the world’s polar bears may be gone by the year 2100 (Amstrup et al. 2008).

Over the course of the past year, I have had the opportunity to collaborate with a multidisciplinary and trans-institutional team whose main goal is examining the health of polar bear subpopulations and factors influencing it. My primary role in this process has been conducting a systematic literature review examining all available studies on the presence of infectious disease in polar bear subpopulations worldwide. This information was synthesized to determine whether or not infectious diseases are a true threat to polar bear subpopulations given their changing physical environment, increased interactions with novel species, and factors potentially contributing to immunosuppression.

In a nutshell, our literature review determined that most reports of infectious agents causing pathologic changes or mortality in polar bears occur in captivity, largely in geographic regions where polar bears do not historically thrive (e.g. equatorial zoos). The majority of information on infectious agents in free-ranging polar bears documents evidence of exposure to numerous viruses, parasites, and bacteria but very few studies correlate signs of illness or health impacts with these findings. Despite the paucity of information on infectious agents adversely affecting the health of polar bears, an even greater lack of knowledge exists surrounding the synergistic effects of infectious agents and environmental contaminants. Lie et al. (2004) have determined the presence of organochlorines in this species, and as such, the possibility of comorbidity effects is a very real one.

There are many challenges associated with collecting information on polar bears in the wild. First and foremost, they often travel alone and may range across areas of sea ice as large as 125,100 km2 (Ferguson et al. 1999). Procuring fresh and systematic samples requires intensive manpower and financial resources, as it is nearly uniformly performed aerially.  Otherwise, samples are often collected opportunistically from hunter-harvested carcasses rather than live animals and may be unrepresentative of the population as a whole. Infectious agents discovered associated with these carcasses are primarily incidental findings.

Approximately 57% of articles that fit our inclusion criteria were on parasitic findings in polar bears, many of these being enteric and nonpathologic. A more well-known event among researchers focusing on Arctic megafauna was that of a rabid polar bear shot by hunters when it was seen dragging its hindlegs (Taylor et al. 1991). However, this is relatively isolated information, in that not many other reports describe infectious agents causing pathology in free-ranging bears. Worth noting is that many of the infectious agents reviewed are zoonotic, meaning they possess the capability to be passed between man and animal. The significance of these agents to polar bear health was unclear, but it may put those at risk that ingest polar bear meat, such as indigenous natives. Already familiar to many of you ecologists and conservationists, funding in these fields is increasingly more difficult to obtain, though often more readily available when diseases of interest have impacts on human health or may pose an economic burden. The work of this team is solely focused on polar bear conservation, rather than taking into account zoonotic infections contracted from the consumption of polar bears.

In a broader scope, the individuals that I have been collaborating with on this project have taught me a lot and I feel incredibly lucky to have learned from them. I have gotten to work with wildlife biologists, epidemiologists, pathologists, and veterinarians from federal agencies and universities both here in Colorado and in Alaska. In Alaska, they have been routinely sampling the polar bear population since 1985, looking at specific health parameters in bloodwork and for evidence of exposure to infectious diseases.

Another study published by this team at the same time our literature review was submitted was on the concept of defining wildlife health (Patyk et al. 2015). Utilizing the Delphi approach, multiple expert opinions were taken into consideration in determining the most important threats to the species and also in defining specific metrics for determining the health of polar bear subpopulations. In turn, changes in these parameters may be more easily monitored. Not surprisingly, the largest threat to polar bears as determined by this Delphi model is climate change.

Efforts to quantify health metrics and specific threats on which to focus on as we move forward will hopefully allow for more streamlined collaboration and study design. Many different groups study polar bears, as nineteen subpopulations exist in five countries: US (Alaska), Canada, Russia, Greenland, and Norway. Recent research identified a pattern of recent directional gene flow north towards the Canadian archipelago, which is likely to be one of the last regions in the Arctic to be affected by global warming and thus function as a long-term refugium for polar bears (Peacock et al., 2015). Clearly, the future of this species heavily relies on interdisciplinary and international collaboration as well as increased funding for their research efforts.

While the circumpolar population projection for these animals may appear grim, the climate-induced loss of sea ice habitat can be stopped, and possibly reversed, by mitigating greenhouse gas emissions. Knowledge and awareness surrounding climate change and species losses gives us the power to educate others on its very real and devastating effects. More information on polar bear conservation may be found below:

 

References

Amstrup, S. C., Marcot, B.G., and Douglas, D.C. 2008. A Bayesian network modeling approach to forecasting the 21st century worldwide status of polar bears. Pages 213-268 in E. T. DeWeaver, C. M. Bitz, and L.-B. Tremblay, editors. Arctic sea ice decline: Observations, projections, mechanisms, and Implications. Geophysical Monograph Series 180. American Geophysical Union, Washington, D. C.

Bromaghin, J., McDonald, T., Stirling, I., Derocher, A., Richardson, E., Regehr, E., Douglas, D.,  Durner, G., Atwood, T., and Amstrup, S. 2015. Polar bears in the Beaufort Sea: population decline and stabilization in the 2000’s. Ecological Applications 25:634-651.

Durner, G., J. Whiteman, H. Harlow, Amstrup, S.P., Regehr, E., and Ben-David, M. 2011. Consequences of long-distance swimming and travel over deep-water pack ice for a female polar bear during a year of extreme sea ice retreat. Polar Biology 34:975-984.

Ferguson, S.H., Taylor, M.K., Born, E.W., Rosing-Asvid, A., and Messier, F. 1999. Determinants of home range size for polar bears (Ursus maritimus). Ecology Letters 2:311-318.

Lie, E., Larsen, H.J., Larsen, S., Johansen, G.M., Derocher, A.E., Lunn, N.J., et al. 2004. Does high organochlorine (OC) exposure impair the resistance to infection in polar bears (Ursus maritimus)? Part I: Effect of OCs on the humoral immunity. Journal of Toxicology and Environmental Health 67:555-582.  

Patyk, K.A., Duncan, C., Nol, P., Sonne, C., Laidre, K., Obbard, M., et al. 2015. Establishing a definition of polar bear (Ursus maritimus) health: a guide to research and management activities. Science of the Total Environment 514:371-378.

Peacock, E., Sonsthagen, S.A., Obbard, M.E., Boltunov, A., Regehr, E.V., Ovsyanikov, N., et al. 2015. Implications of the circumpolar genetic structure of polar bears for their conservation in a rapidly warming climate. PloS one 10:e112021.

Regehr, E. V., Lunn, N. J., Amstrup, S.C., and Stirling, I. 2007. Effects of earlier sea ice breakup on survival and population size of polar bears in western Hudson Bay. Journal of Wildlife Management 71:2673-2683.

Rode, K. D., Amstrup, S.C., and Regehr, E.V. 2010. Reduced body size and cub recruitment in polar bears associated with sea ice decline. Ecological Applications 20:768-782.

Taylor, M., Elkin, B., Maier, N., and Bradley, M. 1991. Observation of a polar bear with rabies. Journal of Wildlife Diseases 27: 337-339.

Photo credit: United State Geological Survey

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