An avian influenza virus in the wild may be just a small step away from spreading effectively between mammals, a controversial U.S study report warns.

The study by Yoshihiro Kawaoka at the University of Wisconsin-Madison has been published online by the journal Nature after months of debate, spurred by fears the findings could be used in bio-terrorism, over whether to release the findings publicly.

Avian H5N1, or bird flu, can be fatal in humans, but is not easily transmitted between people so cases are considered rare.

To find out if H5N1 could evolve easy transmissible between humans, Kawaoka and his team mutated genes in the virus and discovered the hybrid virus could spread between ferrets in separate cages after acquiring just four mutations, Nature reported Wednesday.

Some H5N1 strains in the Middle East can already recognize human receptors, so Kawaoka's work suggests they could be just one stabilizing mutation away from being able to spread between humans, Nature reported.

The research has created debate about whether the benefits of knowing about these potentially dangerous mutations outweighed the risks of publishing them openly, and the U.S. National Science Advisory Board for Biosecurity recommended in December that researcher papers on the subject be censored before publication, citing concerns the strains could be used by bioterrorists.

But in March, after consulting with international flu experts and health-agency representatives, the reversed itself, saying revised versions of the research papers should be published in full. (UPI)