Interview with Karyn D. Rode (U.S. Geological Survey, Alaska Science Center, Anchorage, Alaska, United States of America)

What first drew you to study the risk of pathogen exposure in polar bears?

Polar bears are listed as a threatened species under the United States Endangered Species Act. The U.S. polar bear management agency, the US Fish and Wildlife Service, developed a conservation management plan in response to the threats of sea ice loss to polar bears. Within that plan they specifically identify the lack of data on pathogen exposure and disease in polar bears, the potential for changes in transmission dynamics due to environmental change in the Arctic and the need for monitoring polar bear exposure to pathogens. This study is in direct response to that identified information need.

How did you choose to investigate this in the study, and why?

We identified that there was an opportunity to examine if exposure to pathogens may have changed in bears in the Chukchi Sea polar bear population, over a period when there were substantial declines in Arctic sea ice and associated other environmental changes, because there were blood serum samples available from the 1980s and 1990s and we had similarly collected serum and health data between 2008 and 2017. Collection of samples to improve surveillance for disease and pathogen exposure had also occurred in recent years, which gave us an opportunity to look at exposure to parasites as well. 

What are the key findings from your research?

The prevalence of serum antibodies to five pathogens increased among polar bears in the Chukchi Sea between 1987–1994 and 2008–2017, including: Toxoplasma gondii, Neospora caninum, Francisella tularensis, Brucella abortus/suis, and canine distemper virus. The proportion of bears testing for antibodies more than doubled for three of these pathogens and was higher than levels reported for other polar bear populations. Despite this change, however, the body condition, reproduction and cub survival of polar bears in the Chukchi Sea has been stable suggesting that this increased exposure has not impacted population dynamics.

What most surprised or interested you about your findings?

We were most surprised by the degree of increase in the proportion of the population testing positive for antibodies, particularly Neospora caninum which increased from 13.7% of the population testing positive for antibodies to 65.1%. We were also surprised that the proportion of bears testing positive for antibodies of land-based pathogens were not higher for bears that spent the summer on land compared to those that summered on the sea ice. However, information on summer land use was limited to female bears that could be fit with tracking collars, and most females den on land which could have resulted in similar exposure to land-based pathogens among most females.

Pathogen exposure is yet another of a number of challenges facing polar bears more generally. Could you tell us about these difficulties?

Polar bears, through much of their circumpolar range, are experiencing declines in their sea ice habitat. Concurrent to sea ice loss are other environmental changes in Arctic ecosystems. Where sea ice loss has occurred, increases in land use during the summer has been one of the most common responses. This change in habitat use alters feeding behavior and energetics, interactions and potential conflict with humans, and exposure to novel pathogens. Thus, polar bears face a combination of potential stressors associated with sea ice loss.

Do you think increased pathogen exposure further threatens the survival of polar bears?

The Chukchi Sea polar bear population has exhibited stable body condition and cub survival over the past several decades despite increased pathogen exposure. Thus, there is no evidence to conclude that pathogen exposure has negatively affected the health of Chukchi Sea polar bears. However, monitoring individual health and potential for symptomatic disease is very challenging. Although bears in our study tested positive for antibodies to pathogens, we don’t know if any of the bears ever had symptomatic disease in response to that exposure. It’s also difficult to monitor how sublethal effects, such as changes in physiology or body condition could interplay with other stressors to affect bear health or survival. Our study supports the need for increased surveillance of pathogen exposure and disease in polar bears to better understand if there could be health or survival impacts for polar bears.

What do you hope your findings might lead to, and what are the next steps for your research?

Our findings highlight that there are a myriad of changes occurring in Arctic ecosystems that have the potential to affect animal and human health. Our results that pathogen exposure has increased in a species that is at the top of the food chain, and that those changes were associated with diet, confirm that transmission dynamics are likely changing and warrant improved surveillance for disease in Arctic species.