Polar Bears Are Exposed to More Parasites, Viruses and Bacteria as the Arctic Heats Up

As the planet heats up, polar bears are being exposed to more parasites, bacteria and viruses, likely through their food, according to a new study published Wednesday in the journal PLOS ONE. Since these massive marine mammals are at the top of the food chain, the findings suggest that other Arctic animals are also at greater risk of contracting pathogens as the climate warms.

It’s not clear whether or how the increased exposure to pathogens is affecting the bears’ health. But researchers say the findings are just one more example of the ways the Arctic ecosystem is shifting amid human-caused climate change.

The Arctic Ocean is warming four times faster than the rest of the world. The milder polar conditions are likely allowing pathogens to flourish and spread in environments where they couldn’t historically survive. In addition, as the Arctic’s soil and permafrost thaw, they could be releasing trapped microbes, “zombie viruses” and bacteria, including anthrax.

“The polar bears are a good indicator for what’s happening in the ecosystem,” says study co-author Karyn Rode, a wildlife biologist with the U.S. Geological Survey, to the Washington Post’s Kasha Patel.

For the study, scientists focused on a population of polar bears living in the Chukchi Sea between Alaska and Russia. This group of polar bears, like others, is spending more time on land each summer, because Arctic sea ice is melting earlier in the spring and forming later come fall. Without access to their typical food sources, which they need sea ice to reach, the bears are now eating more garbage and human food—both possible sources of pathogens.

Researchers collected blood and fecal samples from 232 Chukchi Sea bears between 2008 and 2017. They tested the samples for antibodies, or protective proteins that are present when the body’s immune system has fought a specific pathogen. Then, they compared their results to those from 115 historical samples taken between 1987 and 1994.

Antibodies to five pathogens were more prevalent in the 2008–2017 samples: Toxoplasma gondii, Neospora caninum, Francisella tularensis (which causes tularemia, also known as “rabbit fever”), Brucella abortus/suis and canine distemper virus.

One of the most surprising changes was the uptick in exposure to Neospora caninum, a deadly parasite that typically infects dogs and cows. In the 2008–2017 samples, roughly 65 percent of bears tested positive for the parasite—up from just about 14 percent of bears in the earlier period.

The number of bears that tested positive for Toxoplasma gondii increased from 2.2 percent to 14.1 percent, while the prevalence of Brucella abortus/suis went from 3.5 percent to 10.9 percent.

Pathogen exposure varied depending on the bears’ diet, as well as their sex. Females had higher rates of exposure to certain pathogens than males did, likely because they spend more time on land in dens to give birth and raise their cubs. Indeed, some of the pathogens detected are more prevalent on land.

Still, the variability based on diet suggests the bears are picking up most of the pathogens from their prey, which consists of seals, walruses and whales. Researchers didn’t test those species directly, but their findings suggest other animals are also facing increased exposure.

“[The bears] are probably not the only species that has higher exposure to these pathogens,” Rode tells Science News’ Jake Buehler. “It’s within the food chain that this has increased.”

Beyond food, other vectors of disease can arise in a warming Arctic. Ticks, for instance, are becoming more common in Alaska as frost gives way to forests, research suggests. And “warmer temperatures accelerate the growth rates of pathogens and vectors, such as ticks and mosquitoes,” Khaled Abass, a toxicologist at Finland’s University of Oulu who was not involved with the study, tells the Washington Post.

Is the increased exposure to pathogens hurting the bears? The researchers can’t answer that question, but they suspect the higher prevalence of parasites, viruses and bacteria might make them more vulnerable to other stressors.

“Bears in general are pretty robust to disease,” Rode tells BBC News’ Victoria Gill. “But I think what it just highlights is that things [in the Arctic] are changing.”

The findings also raise concerns for human health, since some groups living in the Arctic eat polar bears, and some of the pathogens can be transmitted to humans. But the scientists say future research will need to be conducted to explore these and other potential ripple effects.

Sarah Kuta | READ MORE

Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.