Genetic studies show that polar bears have “A relatively naïve immune system,” according to research wildlife biologist Todd Atwood, who heads the U.S. Geological Survey – Alaska Science Center Polar Bear Research Program. When polar bears are forced ashore they face new threats from disease.
Polar bears, marine predators known for traversing arctic sea ice to hunt seals, rarely interact with terrestrial species such as caribou. Atwood outlined “From a disease perspective, the sea ice acts as a structural barrier to the vectoring of certain disease agents.” Polar bears’ naïve immune systems haven’t encountered a wide variety of diseases. “They can be particularly vulnerable to novel infectious agents because they don’t have that genetic diversity for their immune system to recognize these different infectious agents and mount an immune response.”
Changing habitat = new risks
Polar bears face new disease threats. The polar bears’ world is changing. In 2008 polar bears were listed as threatened under the Endangered Species Act, which afforded the bears new protections. It is worth noting that, according to the National Wildlife Federation, polar bears were the first vertebrate species to be listed by the U.S. Endangered Species Act as threatened by extinction primarily due to global warming.
Polar bears travel arctic pack ice to hunt seals, their primary prey; ongoing depletion of sea ice extent and thickness causes complications for the species. “A consequence of climate change is that we are losing that sea ice habitat that served as a structural barrier and bears are responding in ways that are probably exposing them to new risk factors,” Atwood said.
Atwood called sea ice conditions volatile and highly dynamic. “The big concern is that that period of time when quality sea ice habitat is unavailable to polar bears is getting longer and longer.” When there’s no ice along Alaska’s north shore, individuals from the southern Beaufort Sea polar bear population have two choices: either swim to the distant pack ice or remain and scavenge along the coast. Unusual amounts of time spent scavenging brings polar bears into contact with species and disease threats their immune system isn’t necessarily equipped for. Atwood stated “We’re concerned that we’re seeing this climate-mediated increase of onshore occurrence and that then is exposing bears to novel risk factors that they haven’t been historically exposed to.”
Alaska called a crossroads
When polar bears feed alongside birds at sites where the remains of subsistence-harvested bowhead whales are aggregated on Alaska’s northern coast, they may be encountering risks from distant regions of the world. Alaska serves as a destination for millions of migratory birds that travel to the state from all around the world. Traveling animals as well as increasing amounts of human tourism and shipping opportunities can carry pathogens and toxins to the Arctic environment. “A number of ecological epidemiologists have made the prediction that we will see range expansion of parasites and infectious agents northward. And we’re seeing some of the early examples of that with the increased toxoplasma [parasite] exposure,” in polar bears, Atwood reported.
In the last 4 or 5 years scientists have noted polar bear exposure to infectious agents already to known to be present in the Arctic: brucella, phocine distemper and canine distemper. What’s more, “We’ve also seen exposure to infectious agents previously undocumented in polar bears.” One of those is Coxiella burnetii, a bacterial pathogen that causes disease in animals and humans.
The World Health Organization reports: “Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi; the diseases can be spread, directly or indirectly, from one person to another. Zoonotic diseases are infectious diseases of animals that can cause disease when transmitted to humans.”
Atwood: “Coxiella is a bacterium that is the causative agent of the disease called Q fever― a zoonotic disease that can move back and forth between humans and wildlife,” Atwood illuminated. Coxiella is highly pathogenic, which means it’s capable of causing disease and spreads easily. “It was weaponized by the U.S. bioweapons program in the 1950’s because of its high pathogenicity.” Atwood said “It causes like your typical flu-like disease in people. And we don’t know what type of disease it causes in wildlife but we do know that it can interfere with reproduction.”
Reproductive interference is one more pressure on a species already listed as threatened.
Polar bear exposure to Coxiella burnetii was documented in 2013 in the southern Beaufort Sea population. Atwood described: the previous northernmost known case was in northern fur seals in the Pribolof islands, “So it’s made a pretty big jump in range from that animal into our neck of the Arctic. We don’t know what the population level consequences of that are, we don’t know how they are being exposed.” To help answer those questions the researchers accessed the USGS archive of polar bear blood samples taken during routine captures since the mid-eighties. “We started looking back in time to see where we can locate the first instance of exposure and we’re far from having that done. But we have found a case as early as 2007.” Atwood’s team looks to understand “Is increased exposure a result of the changing behaviors bears exhibit because of loss of sea ice habitat? And also does it have population level implications?”
In order to keep tabs on polar bear health and on the status of disease in the Arctic, Atwood noted “We need to start really ramping up our monitoring capabilities now. If we are not monitoring, we’re not going to be able to detect an increase trend or prevalence or, more importantly, detect things that are occurring that historically have not been here.”
The status of polar bear health has implications for human populations as well. Polar bears are apex predators situated at the top of the food chain; accordingly, they are impacted by the health and status of the organisms below them in the food chain. Humans in the Arctic, especially those following a subsistence lifestyle, are similarly apex predators. Polar bears are called sentinels of ecosystem health– their status gives us insight into the status and well-being of other species, including humans.
USGS – Alaska Science Center page on Wildlife Disease and Environmental Health in Alaska
Laura Nielsen 2015
Frontier Scientists: presenting scientific discovery in the Arctic and beyond