Experts and researchers working with marine mammals are now being advised to wear protective suits and gear as the dreaded influenza A H1N1 subtype virus has mutated and had been detected present and active in elephant seals off the coast of central California.

Although this isn't the first time a marine mammal has been found to carry a human strain, researchers still had never found a human pandemic strain in marine mammals before, Tracey Goldstein, an associate professor with the UC Davis One Health Institute and Wildlife Health Center who led the team that made the discovery, said.

"We thought we might find influenza viruses, which have been found before in marine mammals, but we did not expect to find pandemic H1N1," Dr Goldstein said. "This shows influenza viruses can move among species."

Scientists are baffled as to how the seals got the infection.

"H1N1 was circulating in humans in 2009. The seals on land in early 2010 tested negative before they went to sea, but when they returned from sea in spring 2010, they tested positive. So the question is where did it come from?" Dr Goldstein said.

"Most of the other viruses that we see in marine mammals tend to be more linked with bird influenzas, and I think that was the surprise."

When at sea, elephant seals spend most of their time hunting in the northeast Pacific Ocean, located off the continental shelf. This meant that direct contact with humans is highly unlikely.

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Yet, nasal swabs tests proved the dreaded influenza A H1N1 virus is actively present and has infected at least two elephant seals living off the Californian coast. Moreover, tests further showed that antibodies to the virus were present in 28 other elephant seals, signifying more widespread exposure.

Researchers suspected the seals got infected with the H1N1 virus before they reached land, either while at sea or upon entering the near-shore environment. They also theorised the seals may have gotten the H1N1 from ships dumping human waste in the ocean waters.

The elephant seals may have also acquired the virus from seabirds, since birds have a history of carrying viruses.

"Marine mammals are great sentinels, and this reminds us that infectious diseases don't just stop at the land and sea border, but can enter the ocean and come back to remind us of their presence," Stephanie Venn-Watson, director of translational research at the National Marine Mammal Foundation in San Diego, told the Daily Democrat.

The infected seals demonstrated no flu like symptoms. Researchers are guessing northern elephant seals may have an innate immunity to H1N1.