This year's H7N9 bird flu virus in China has several features that point to pandemic potential, according to studies in animal models.

Since March, the H7N9 avian influenza virus has infected at least 132 and killed 37 people in China. Humans lack protective immune responses to it, which raises concerns about its potential to spark a worldwide outbreak or pandemic.

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Two bird flu virus strains, H7N9 and H5N1, infect humans. (Pichi Chuang/Reuters)

The H5N1 avian flu is the only other bird flu virus that infects humans.

To learn more about how the virus spreads and infects, researchers in the U.S. and Japan analyzed two samples of the virus from patients in China and studied its ability to infect and replicate in ferrets — standard models for studying flu in mammals because like mammals, they infect one another through coughs and sneezes.

The two Chinese strains of H7N9 that were studied are known Anhui/1 and Shanghai/1.

"The robust replicative ability in mice, ferrets and nonhuman primates and the limited transmissibility in ferrets of Anhui/1 suggest that A(H7N9) viruses have pandemic potential," Yoshihiro Kawaoka of the University of Wisconsin-Madison and the University of Tokyo and his co-authors concluded in Wednesday's issue of the journal Nature.

Kawaoka's ferret studies of H5N1 were controversial, in part because the mutated virus spread easily among ferrets. Publication of the research into how the bird flu could spread through the air to mammals was voluntarily suspended over biosecurity fears.

In the study, the Anhui/1 strain was transmitted to one of three pairs of ferrets.

Antiviral drugs were effective against the H7N9 strains, the researchers found.

A second flu paper published on Tuesday in the journal mBio also looked at H7N9's human pandemic potential.

In the 94-year history of studying H7 flu viruses in animals, the viruses have never caused human pandemics. But the history isn't reassuring when several usual features of the viruses are  considered, scientists from the U.S. National Institute of Allergy and Infectious Diseases said.

Itching, swelling

Other H7 viruses have caused large-scale epidemics among poultry in New York, Canada, Mexico, the Netherlands, and Italy. They have infected mammals including humans, study authors David Morens, Jeffery Taubenberger and Anthony Fauci said.

In humans, H7 viruses tend to infect conjunctival cells which can lead to signs and symptoms in the eyes such as itching, swelling and tearing that in theory could promote spread between people.

Other H7 viruses are able to infect mammals such as horses and pigs.

Swine can be a breeding ground for flu viruses.

"The history of H7 viruses constitutes a set of complex observations that may or may not be relevant to the emergence of an H7N9 pandemic, and we thus find it challenging to fit them into calculations of potential human risk," they wrote.

"Regardless of whether the current H7N9 outbreak dies out or proceeds to pandemic potential, we have a unique opportunity to learn more of influenza's many secrets and thereby enhance our ability to  prevent and control and important disease that seems destined to appear  again and again, in multiple guises, far into the foreseeable future."