Polar Bears

Climate change brings disease threat for polar bears

New Scientist,

13 May 2013

With its habitats shrinking and food supplies dwindling, the fate of the polar bear looks grim in the face of climate change . Now comes news that the iconic Arctic mammal may face another potentially devastating threat: it may be particularly vulnerable to new pathogens moving northwards as a result of warming.

Diana Weber, who works at both the New College of Florida, Sarasota, and the American Museum of Natural History in New York, led a team that sequenced DNA from 98 polar bears in Canada. They looked specifically for genes coding the major histocompatibility complex (MHC) – a molecule found on the surface of cells that acts as a crucial component of the immune systems of most vertebrates.

The MHC molecules register the presence of pathogens by binding to them. This alerts the body's immune cells to recognise the foreign invaders and mobilise. Because the MHC molecule's binding site is coded for by highly variable genes, it is able to detect a wide range of pathogens.

Not in the polar bear, though. Weber and colleagues found the polar bears had especially low diversity in their MHC genes. The researchers suggest it may be an adaptation to life in the Arctic, which is relatively free of bugs compared with lower latitudes. Previous work in Atlantic salmon has shown that the diversity of genes coding for the MHC binding site is lower in animals that typically live in lower temperature conditions.

If adaptation for survival in the Arctic environment has led to a less versatile immune system, then Arctic species such as the polar bear may be at risk from an influx of pathogens as global temperatures rise, the researchers warn.

New diseases seen

"There are a number of diseases now observed in Arctic animals [that were] not previously seen or not as prevalent," says Weber. "Low diversity, especially in the immune system, should be a concern."

"We know that regional shifts in the Arctic's climate might – directly or indirectly – change host-pathogen relationships as well as impact the efficiency of host immune responses," says Karina Acevedo-Whitehouse, who works at the Autonomous University of Queretaro, Mexico, and the Institute of Zoology in London.

She says it is hard to make predictions because no one knows which pathogens might extend their ranges northwards. But a low diversity at the MHC could "plausibly increase the chance of a disease-driven [polar bear] population decline".

The study is important as it "makes us aware of this danger", says Axel Janke, an evolutionary geneticist at the Biodiversity and Climate Research Centre in Frankfurt, Germany, whose recent work reveals that genetic diversity among polar bears is low.

However, he notes that low immune diversity does not necessarily translate into disease susceptibility. The cheetah is a textbook case, he says. "We've known for 30 years that the cheetah has a low variability at some immune genes, but they thrive in a pathogen-laden environment."

Polar bears are likely to have survived periods of warming before. This time the warming is more rapid, though, and comes with the added challenge of human-driven habitat destruction, illegal hunting and pollution.

"All this adds up to a very real threat to polar bear survival," says Janke