When it comes to swine flu, elephant seals can now be included in the list of carriers. According to a new UC Davis study, the marine mammals distinctive for their honking noses contracted the H1N1 virus back in 2010, just as the pandemic caused by the virus was winding down in humans.

Influenza virus is famous for its ability to cross species barriers, and this isn't the first time a marine mammal has been found to carry a human strain, said lead author Tracey Goldstein, an associate professor with the UC Davis One Health Institute and Wildlife Health Center. Yet, she said, researchers had never found a human pandemic strain in marine mammals before.

The study was published Wednesday in the online journal PLOS ONE.

In 2009, the H1N1 virus started a worldwide influenza outbreak and ultimately killed as many as half a million people worldwide. The highly contagious infection was a new version of the flu, cobbled together with pieces of human, swine and bird viruses. Although the virus has been found in a few other species -- ferrets, cats, dogs and a cheetah -- unlike other influenza viruses the risk of H1N1 infection between species isn't well-documented, according to the federal Centers for Disease Control.

"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," Goldstein said. "Where did this human one come from?"


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Spurred by their interest in tracking the movement of influenza viruses in marine mammals, UC Davis wildlife biologists swabbed the noses of 72 elephant seals over the course of two years at Año Nuevo State Reserve in San Mateo County and Piedras Blancas in San Simeon. The animals were sampled before and after their annual spring foraging trip to Alaskan waters.

The animals tested clean before departure, but two of them came back bearing the virus in 2010. A year later, 16 elephant seal pups had blood tests showing they had been exposed to the virus before. None of the animals showed clinical signs of illness.

"The data suggest the animals were exposed when they were at sea, or coming into the nearshore environment," Goldstein said. "So that suggests that direct contact with people was not how they got the virus. It's a big mystery."

The fact that the elephant seals could have contracted the virus at sea presents an odd conundrum for the researchers.

"Marine mammals do get infected with viruses, and we assume it's direct contact, because we don't know how these viruses survive in the ocean," Goldstein said.

The researchers also raised the possibility that seabirds may have passed the virus to elephant seals, since birds have a history of carrying these viruses. Given that the animals also passed through busy shipping areas, discharge from ships could have been another possible source, she said.

"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," said Stephanie Venn-Watson, director of translational research at the National Marine Mammal Foundation in San Diego.

Venn-Watson, a marine mammal veterinarian who also holds a degree in public health, wasn't shocked by the results. "I'm not terribly surprised about the finding," Venn-Watson said. "When we go looking for new infectious agents (in marine mammals), we tend to find them."

The presence of H1N1 in elephant seals is not a cause for extraordinary alarm, Goldstein said. "It's just a general reminder to people who work with animals to make sure that they protect themselves."

Contact Jessica Shugart at 408-920-5782. Follow her at Twitter.com/ JessicaShugart