Indeed, the progressively declining Arctic Sea ice thickness represents a leading cause of the profound ecological, behavioural, feeding and dietary patterns modifications reported with an increased frequency in polar bears (1).
Within such an alarming context, the animals' chronic stress deriving from prolonged starvation is a further concern issue. As a matter of fact, besides acting as a powerful and efficient machinery allowing us and all the other living organisms to cope with a huge number of environmental stressors, chronic stress responses are invariably characterized by immunosuppression, originating from enhanced cortisol production (2). Therefore, while increased cortisol levels should be reasonably expected to occur in the blood of polar bears experiencing chronic starvation (1), it should be additionally emphasized that these top predators may also become, at the same time, much more susceptible to a wide range of microbial pathogens impacting their already threatened health and conservation.
This could be especially true for Toxoplasma gondii, a cosmopolitan and zoonotic protozoan parasite infecting a large number of aquatic mammal species, including polar bears. In this respect, anti-T. gondii antibodies have been previously reported in almost half of the polar bears from Svalbard, Norway, with males showing higher seroprevalence values than females and with the infection's frequency turning out to have doubled in comparison to an earlier investigation carried out in the same area (3).
Furthermore, the well-known apex predator position occupied by polar bears within the marine food chain makes these animals fully prone to bioaccumulate and biomagnify a huge number of persistent environmental pollutants within their body tissues, including immunotoxic heavy metals like methyl mercury (methyl Hg) (4).
Based upon the above, the immunosuppression induced by the chronic starvation-derived, adaptive stress response, coupled with that due to the high body tissue concentrations of immunotoxic environmental xenobiotics, could render polar bears much more susceptible toward the development of infectious disease processes impacting the already threatened health and conservation status of this iconic and highly endangered animal species.
As a concluding remark, a multidisciplinary effort and a One Health-based approach are strongly recommended in order to get proper insight into the alarming risk of ending with another noble "piece of biodiversity" irreversibly lost!
References
1) Pagano, A.M., Rode, K.D., Lunn, N.J., et al. Polar bear energetic and behavioral strategies on land with implications for surviving the ice-free period. Nat. Commun. 15, 947 (2024). https://doi.org/10.1038/
2) O'Leary, A. Stress, emotion, and human immune function. Psychol. Bull. 108, 363-382 (1990). doi: 10.1037/0033-2909.108.3.363.
3) Jensen, S.K., Aars, J., Lydersen, C., et al. The prevalence of Toxoplasma gondii in polar bears and their marine mammal prey: evidence for a marine transmission pathway? Polar. Biol. 33, 599-606 (2010). https://doi.org/10.1007/
4) St Louis, V.L., Derocher, A.E., Stirling, I., et al. Differences in mercury bioaccumulation between polar bears (Ursus maritimus) from the Canadian high- and sub-Arctic. Environ. Sci. Technol. 45, 922-928 (2011). doi: 10.1021/es2000672.
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