The animal connection to human flu

By Amy Cox
CNN

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(CNN) -- People aren't the only ones who can get hit with the flu. The influenza virus can infect a range of animals, including chickens, pigs, whales, dogs, horses, ferrets and seals.

By studying birds and other animals carrying the flu, researchers hope to discover clues about how the illness in humans operates and how to best treat an infection, especially as worries over bird flu in people rise.

"The biggest thing is the comparisons we can make -- what characteristics make it better for jumping from bird to human or what makes a more virulent form," said Richard Webby, an influenza expert at St. Jude Children's Research Hospital in Memphis, Tennessee, who is working on a vaccine against bird flu.

In addition to researching the flu in mice, scientists also look at the virus in an unlikely subject -- the ferret, a furry and elongated animal that can catch the flu from humans.

"The ferret is probably the most commonly talked about as being the best model," said David Swayne, an animal influenza expert with the U.S. Department of Agriculture. "By studying the ferret, you can see how the infection goes about in the ferret and draw comparisons on how it may be similar to humans. You can study vaccines, anti-viral drugs and how effectively they work. ... [The mouse model is] helpful, but it's not as predictive."

The animal with the most potential for study, Swayne said, is the common pig, because the infections in pigs and humans for different strains of influenza are very similar. Plus, humans have been infected with the pig strain of the flu before.

In 1976, initial reports labeled a swine flu found in people in New Jersey the "killer flu," spurring 40 million to get vaccinated.

But the virus did not spread beyond the area and the epidemic never materialized.

Mysteries remain

To curb possible spread of animal viruses or potential outbreaks from forming, surveillance is key, said Swayne.

"A country like the United States that is developed very well economically has a very developed veterinary infrastructure. On the agriculture side, too [there are government agencies] to monitor where we have influenza and how frequent it is," he said.

"When you get to the poorer countries, you can't do as much surveillance, so that becomes an issue in how much they really know about influenza in their country and animals and what's the potential for it to be transferred over to humans."

Fundamental behaviors of the influenza virus are still a great unknown. Unlike many organisms, influenza genes constantly change in subtle ways. Abrupt changes that enable a virus to jump species and then spread in those new animals are little understood.

"We haven't moved that far along," Webby said. "We don't understand what makes a virus leap. ... Things we have learned are more of the subtleties -- we are finding out more about slight differences in human and avian flu, for example, how they bond to different parts of the molecule."

In another perplexing case of species jumping, researchers in September 2005 traced a dog influenza outbreak in New York state back to a highly contagious horse virus, the first time an equine flu virus has been found to jump species, according to a study in Science magazine. The evidence "raises the possibility that dogs may provide a new source for transmission of novel influenza A viruses to humans," the researchers wrote in the report.

In the quest to better understand where viruses have come from and where they're going, Swayne and his colleagues use molecular epidemiology, a kind of DNA detection on animal flu. Viruses from all over the world, including wild bird viruses, are sent to them so they can sequence the genetic material.

They then can compare that gene sequence to other influenza viruses, Swayne said, "and like a family tree, we generate this comparison to see how closely related some of these viruses are. The closer they are related, they have a common origin."

Keeping tabs on that common origin and other animal flus may help with an early warning of possible strains that may emerge in people, because animals may help prepare the virus to adapt to a human host, according to the European Scientific Working Group on Influenza.

Swayne offers some optimism on the fight to prevent new animal strains, especially the bird flu virus H5N1, from entering the United States or traveling around the world.

In a joint project with the University of Alaska, he and co-workers from the U.S. Department of Agriculture have done surveillance since 1998 in western Alaska, an area that is a potential shared breeding ground between Asian birds and North American birds.

"After analyzing 12,000 samples over eight years, we have not found any H5N1 viruses. That's good news," he said. But, he cautions, the finding doesn't mean "from here to the next year that there won't be an H5N1, it just means we'll have to keep vigilant."

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