Alaska polar bears are suffering from an illness that causes fur loss and skin lesions, according to scientists at the U.S. Geological Survey. The symptoms are similar to an illness that killed more than 60 seals last summer, though so far no polar bears have died from the disease.

Researchers checked nearly 40 polar bears over the past two weeks and found 12 affected with the illness. Despite the symptoms, the polar bears appeared otherwise healthy and were exhibiting normal behavior, according to the USGS. Researchers first spotted a sick bear on March 21 and have been collecting blood and tissue samples from the affected bears. They will continue to do so until May, when sea ice makes research in the area dangerous.

Scientists are worried the already threatened polar bear population could decline further if  the disease proves deadly. A similar sickness was seen in 1998 and 1999, when 10 polar bears were found with pronounced hair loss. None died from the illness.

We took biopsies in [1999] and couldn't establish a causative agent for the hair loss then, Tony DeGange, chief of the biology office at the USGS Science Center in Anchorage, told the Washington Times. But now we have this unexplained mortality event going on with seals. And they haven't been successful in figuring out what caused the seal deaths. Is it just a matter of coincidence or is it related? We don't know.

More than 100 ringed seals and Pacific walruses washed up on the shores of Alaska, Canada and Russia last summer. They showed fur loss and skin lesions accompanied by labored breathing and fatigue. The majority of the animals eventually died, according to the National Oceanic and Atmospheric Administration.

Despite extensive testing for a wide variety of well-known infectious agents, the cause(s) of the observed condition in walruses and ice seals remains unknown, the USGS said in a statement. Advanced testing techniques for unidentified infectious agents is continuing as well as further testing for potential causes including man-made and natural biotoxins, radiation, contaminants, auto-immune diseases, nutritional, hormonal and environmental factors.

Even though scientists are investigating radiation as a possible cause, they stressed it's unlikely the illness is being caused by contamination from Japan's Fukushima nuclear power plant, which was damaged last year in an earthquake.

While no polar bears have died yet, the appearance of a disease spreading through their population is disturbing, Andrew Wetzler, director of the Land and Wildlife Program for the National Resources Defense Council, said in a blog post.

At the very least, it should serve as a caution flag to scientists and government officials, he wrote. With polar bears already vulnerable to the effects of climate change, and now showing signs of a new emergent illness, this is not the time to be playing Russian roulette with the Beaufort Sea population either by rushing ahead with offshore oil exploration and drilling or by continuing Canada's commercial trade in polar bear skins.

The State of Alaska Division of Public Health said there's no evidence of the disease spreading to humans.