In May 2013, scientists working at the University of California, Davis, announced a startling finding: The H1N1 influenza virus (the so-called “swine flu”), which caused a minor pandemic among humans in 2009, has now been detected among marine mammals. These researchers found the virus in just one species, the elephant seal, but their results raise the possibility that H1N1 is circulating freely amongst other marine mammals, too.
Which species the influenza virus circulates in is of vital importance to medical researchers. Although the H1N1 pandemic of 2009 was eventually contained without devastating consequences, influenza experts continue to worry about the possibility of an extremely deadly pandemic occurring at some point in the future. Beginning in 1918, the so-called “Spanish flu” pandemic killed 18 to 20 million people. Influenza is known to circulate regularly in a few non-human species, especially birds and pigs, but sometimes crops up in other species as well. Biologists have been attempting to figure out which ones, and especially how it makes the jump, in order to better predict whether another deadly pandemic is likely to happen.
That search is what led the UC Davis team, led by associate professor Tracey Goldstein, to look at marine mammals. They spent several years taking “nasal swabs” from Pacific seals at various sites from California to Alaska. “We thought we might find influenza viruses, which have been found before in marine mammals,” says Goldstein.
However, her team also found something they didn’t expect at all: the same H1N1 variant which caused the human pandemic of 2009. H1N1 is believed to have passed to humans in Mexico from pigs, although researchers actually identified a mixture of genes in it, some of them from avian influenza, so the actual H1N1 (2009) virus is a hybrid type. It then subsided, at least in humans, partly because of a hastily organized vaccination campaign and because of careful quarantining of severely ill people in hospitals around the world. However, it could have spread back to birds or pigs in rural areas, the same as it jumped to humans in the first place.
Goldstein’s team discovered something much more perplexing: They identified two elephant seals in California which tested positive for H1N1. Even more strangely, she says, the seals were negative for H1N1 when they were first tagged and tested in 2009. It was only in 2010, when they returned to the coast after months foraging for food farther out in the Pacific, that two adult females were tested again and found to be positive for H1N1. As unlikely as the infection jumping from humans to seals on the coast might seem, it would be even less likely to happen in the open ocean. One possibility is that H1N1 is now circulating freely, but at low levels, in the seal population of the Pacific.
The new research also suggests that seals, and perhaps other marine mammals as well, carry flu viruses without developing any visible illness, making them what the article calls “asymptomatic carriers.” By 2011, the researchers were no longer finding adult seals with H1N1, but they did test a number of seal pups (babies), and some of them carried antibodies to H1N1, meaning their mothers had been exposed to the virus before and their immune systems had developed defences against it. The authors say further research is now ongoing about how common H1N1 is among Pacific seals, and how they contracted it in the first place.
Goldstein’s article, which was co-authored by Ignacio Mena and eight other authors, appears in the online journal PLOS One. A summary for non-experts has also been made available online by UC Davis.