Ferreting Out the Swine Flu Mystery

Jon is a staff writer for Science.

Now a popular pet, ferrets have also become the animal model of choice for many influenza studies, as they can easily be infected with the virus and have similar respiratory tracts to humans'. But a report last October of an outbreak of influenza in a ferret colony has led to questions about whether this animal model can help sort out a critical question about the current swine flu outbreak: What allows this particular virus to transmit so well between humans?

No Few studies have ever tested swine flu viruses on ferrets, and little information exists about natural influenza outbreaks in the species. But at the annual conference of the American Association of Veterinary Laboratory Diagnosticians last October, researchers from Iowa State University reported (see page 101 of the conference proceedings) that an outbreak of influenza in a ferret colony on an Iowa farm infected 8% of about 1000 of the minklike animals.

The researchers, led by virologist Kyoung-Jin Yoon, discovered that the outbreak was caused by an H1N1 swine influenza virus. The ferrets lived about 0.4 kilometers from a swine farm.

The current human outbreak is also caused by an H1N1 virus of swine origin. Shortly after his team typed the strain causing the human outbreak, Ruben Donis, a virologist who heads the molecular virology and vaccines branch of the U.S. Centers for Disease Control and Prevention (see interview with ScienceInsider), noticed the ferret study. He immediately wondered whether the swine H1N1 spreading through ferrets might be the same as the one causing the human outbreak. “I thought, ah, a smoking gun,” he says.

Yoon made the same connection when he learned details about the virus response for the human outbreak. But a more detailed comparison of the genetic sequence of the two H1N1s clarified that they differed substantially. “I was relieved,” says Yoon, who worried that the ferrets, like swine, might have been a “mixing vessel” of influenza viruses from different species.

Still, Donis says the Iowa study is “very, very relevant,” as it indicates that the ferret “is not going to work for us as a model” to help tease out how a swine H1N1 mutated to become transmissible between humans. Until the current outbreak, the transmission of swine flu from human to human was virtually unheard of. If ferrets mimicked humans and swine strains did not move between them,  researchers could make mutations in the viruses to determine the genetic changes that controlled the transmission. But now it appears that the receptors on the surface of ferret cells that allow influenza inside must be too different from those of humans to help solve this riddle.

Score another point for the flu.

Follow our full coverage on the swine flu outbreak here.

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