Climate change is a threat to polar bear’s survival. Now they have a new deadly challenge facing them: bird flu. It was recently confirmed that a polar bear from northern Alaska has died from the disease.

The current strain of H5N1 influenza has affected a far wider range of species than any previously recorded strain. This has included several mammal species, such as foxes, otters, mink, sea lions and seals (including, for the first time, seals in Antarctica). Cases have been detected in humans, too.

However, while some cases in mammals have been associated with large numbers of animal deaths, the few cases in humans have, so far, shown only mild symptoms or have been asymptomatic.

So, why are there such differences between species, and what are the implications of this polar bear’s death for the wider polar bear population, as well as other large mammals and humans?

Influenza viruses are highly adaptable. Their relatively simple genetic code not only changes at random via mutation in the same way as truly living organisms, but also via reassortment. This is where closely related viruses that infect the same host cell exchange genetic material to produce novel genomes. This can lead to greater adaptation for invasion, survival and replication within that host species. This is probably how the current H5N1 strain has come to affect such a variety of bird species, with devastating effects for some populations.

Normally, large numbers of deaths associated with a disease are considered to be caused by the spread of a disease between individuals within the population. However, very specific genetic changes are needed for avian influenza viruses to become adapted to mammalian hosts. These changes have not yet been detected in the current strain of H5N1. Although individual-to-individual transmission of the current strain of H5N1 is rare, the virus is still a significant threat to wildlife and potentially humans.