Humans are not alone in having to worry about flu season; scientists say sea otters that live in the waters off Washington state contracted the same virus in the wake of a 2009 human pandemic.

Although scientists say an exact source of the exposure and its timing can't be pinned down, the finding of the pandemic 2009 H1N1 virus in otters suggests human flu viruses can infect the sea-dwelling mammals.

"Our study shows that sea otters may be a newly identified animal host of influenza viruses," says Hon Ip, scientist and co-author of the study conducted by the U.S. Geological Survey and Centers for Disease Control and Prevention.

In almost three-quarters of sea otters examined in the 2011 study, blood samples showed the existence of antibodies for the H1N1 flu virus, says Ip.

While none of the otters showed symptoms of being ill, the evidence of antibodies confirms a previous exposure to influenza in the animals.

"We are unsure how these animals became infected," says CDC scientist Zhunan Li, the study's lead author. "This population of sea otters lives in a relatively remote environment and rarely comes into contact with humans."

The only other known H1N1 virus infection in marine mammals was discovered in a 2010 study of elephant seals living in ocean water off central California.

As with the sea otters, scientists say they don't know how the infection occurs or how the seals were exposed.

While the study constitutes the first time the virus has been found in otters, the H1N1 virus has been previously detected in other animals, including pigs, chickens, ducks, horses and the elephant seals.

The H1N1 strain of the flu virus has moved across the globe since it was first detected in 2009, becoming the predominant variant of flu virus circulating in the 2013-2014 flu season, the researchers said.

"Our new study identifies sea otters as another marine mammal species that is susceptible to influenza viruses and highlights the complex interspecies transmission of flu viruses in the marine environment," USGS researcher LeAnn White says.

Institutions taking part in the sea otter study and sampling included several USGS wildlife, science and ecology research centers, along with the Seattle Aquarium and the Monterey Bay Aquarium in California.

Research into zoonotic diseases -- illnesses that can pass between human and animals -- can provide early warnings to public health institutions and wildlife communities and to the public at large.