One year after the swine flu pandemic began, scientists detected the very same virus (H1N1) in free-ranging northern elephant seals off the central California coast, according to a study at UC Davis. This is the first report of that flu strain in any marine mammal.

The scientists detected H1N1 infection in two female northern elephant seals, and antibodies to the virus in an additional 28 elephant seals. Neither of the infected seals appeared to be ill. Researchers now believe marine mammals may be asymptomatic carriers of some diseases.

"This shows influenza viruses can move among species," said lead author Tracey Goldstein, an associate professor with the UC Davis One Health Institute and Wildlife Health Center.

Originating in pigs, H1N1 jumped to humans in 2009 and then spread worldwide as a pandemic. The goal of the research in seals conducted at UC Davis is to understand how viruses emerge and move among animals and people. The scientists have been studying flu viruses in wild birds and mammals since 2007 as part of the Centers of Excellence in Influenza Research and Surveillance program funded by National Institutes of Health.

Between 2009 and 2011, the team of scientists tested nasal swabs from more than 900 marine mammals from 10 different species off the Pacific Coast from Alaska to California.

"We thought we might find influenza viruses, which have been found before in marine mammals, but we did not expect to find pandemic H1N1," said Goldstein. "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?"

When elephant seals are at sea, they spend most of their time foraging in the northeast Pacific Ocean off the continental shelf, which makes direct contact with humans unlikely, the report said. Yet, the seals had been satellite tagged and tracked, so where they had been and when they arrived on the coast are clear.

The first seal traveled from California on Feb. 11 to southeast Alaska to forage off the continental shelf, returning to Point Piedras Blancas near San Simeon, Calif., on April 24. The second seal left Ano Nuevo State Reserve in San Mateo County, Calif., on Feb. 8, traveling to the northeast Pacific and returning on May 5. Infections in both seals were detected within days of their return to land. Researchers believe exposure likely occurred before the two seals reached land, either while at sea or upon entering the near-shore environment. Exposure may have occurred through contact with aquatic birds, which have been thought to be reservoirs for other influenza viruses in the past.

Infrequently, antibodies against influenza virus strains that circulated worldwide in humans have been detected in seals, the researchers said, indicating that exposure to these human-adapted viruses may be sporadic and infection self-limiting in marine mammals. During the winter of 1979 to 1980, H7N7 was isolated in harbor seals dying with severe viral pneumonia off the New England coast. In 1982 to 1983, H4N5 was documented and most recently in 2011, scientists isolated H3N8 in marine mammals.

"The study of influenza virus infections in unusual hosts, such as elephant seals, is likely to provide us with clues to understand the ability of influenza virus to jump from one host to another and initiate pandemics," said Adolfo Garcia-Sastre, professor of microbiology and director of the Global Health and Emerging Pathogens Institute at the Icahn School of Medicine. He is among the researchers who sequenced the virus isolates and characterized their phenotypic properties.

The World Health Organization now considers the H1N1 strain from 2009 to be under control, taking on the behavior of a seasonal virus.

Published May 15 in the journal PLOS ONE, the study matters to the people who handle marine mammals, such as veterinarians and animal rescue and rehabilitation workers, Goldstein said. Additionally, the findings are a reminder of the importance of wearing personal protective gear, both to prevent workers' exposure to diseases, as well as to prevent the transmission of human diseases to animals.

Source: T. Goldstein, I. Mena, S.J. Anthony, R. Medina, P.W. Robinson, D. J. Greig, et al. Pandemic H1N1 Influenza Isolated from Free-Ranging Northern Elephant Seals in 2010 off the Central California Coast. PLOS ONE. 2013.