As the climate warms, polar bears are facing more germs

Polar bears are facing more and more challenges as the climate warms. Most of these problems relate to the bears’ waning wintery habitats. But increasingly, they also may be infected with germs and parasites, a new study finds.

Compared to a few decades ago, polar bears living near Alaska are now more frequently exposed to five different pathogens. Researchers shared this finding October 23 in PLOS ONE. 

“Warming … just allows pathogens to persist in environments they couldn’t persist in before,” says Karyn Rode. She is a wildlife biologist for the U.S. Geological Survey and an author of the new study. She works at the Alaska Science Center in Anchorage.

Such changes in the Arctic are poorly understood. This region is undergoing rapid climate change. To see its effects, Rode was part of a team that looked at polar-bear immune systems.

The Chukchi Sea population of polar bears was a perfect fit. These animals live in the waters that lie between Alaska and Russia. The bears have lost much of their sea-ice habitat. And that has led many bears to spend long periods each summer on land. There, they have come into contact with people and their garbage — possible sources of pathogens. The Chukchi population also ranges farther south than many other polar bears.

“If there are pathogens moving northward into the range of polar bears, then [the Chukchi Sea] would be a place we would expect to detect that,” Rode says.

Searching for polar bear pathogens

The researchers screened blood serum and feces from 232 Chukchi bears between 2008 and 2017. The scientists looked for antibodies in blood against a range of pathogens. These included bacteria, viruses and parasites. If the blood has antibodies aimed at fighting a specific pathogen, it suggests the bear’s immune system has already faced that pathogen in the past. 

The team then compared this analysis to a similar one of 115 bears that were surveyed earlier — between 1987 and 1994.

The share of polar bears exposed to some of the pathogens has at least doubled since the 1990s, the new study finds. These germs were the parasite Neospora caninum and the two bacteria behind the diseases brucellosis and tularemia.

More bears in the more recent survey also had antibodies against canine distemper virus. It causes a fatal disease in dogs and certain other mammals. And the share of bears who’d encountered the parasite Toxoplasma gondii increased sevenfold. It went up from about 2 percent to 14 percent. Though T. gondii completes its life cycle in the guts of cats, it can infect many animal species.

Rode’s team also compared ratios of chemical markers in the bears’ hair. These markers all related to what the bears eat. Individual bears varied in the prey they ate most. But their specific diets were linked to their pathogen exposure, the new study finds.

“[Polar bears] are probably not the only species that has higher exposure to these pathogens,” Rode says. “It’s within the food chain that this has increased.”

In recent years, ringed seals have died off in large numbers from an unknown disease, Rode notes. Ringed seals are a key prey of the polar bears. This finding helped alert researchers to look at pathogen levels in polar bears.

Andy Dobson is a wildlife disease ecologist at Princeton University in New Jersey. He says the findings are interesting. But they are inconclusive, he adds. That’s because the samples were taken from two different areas within the domain of this bear population.

Still, the movement of pathogens likely affects the whole food chain — even humans. People eat some polar bears as part of a subsistence diet, Rode notes. More work is needed to know if there’s now an increased risk of these germs infecting people.