"Since their introduction into land-based birds in 1988, H9N2 avian influenza A viruses have caused multiple human infections and become endemic in domestic poultry in Eurasia," said the study published in the journal of the Public Library of Science.
"This particular influenza subtype has been evolving and acquiring characteristics that raise concerns that it may become more transmissible among humans."
The team from the University of Maryland studied the transmission of this type of bird flu virus, known as H9N2, among ferrets to see how it spreads.
Examining five types of H9N2 viruses isolated from five types of poultry from 1988 to 2003, they discovered the virus seems not to spread in the air.
Instead an amino acid known as Leu226, found on the surface of the virus, was found to be important to the spread of the disease. They also found that ferrets were more susceptible to H9N2 viruses which had already infected humans.
"Although no aerosol transmission was observed, the virus replicated in multiple respiratory tissues and induced clinical signs similar to those observed with the parental human H3N2 virus," the report said.
"Our results suggest that the establishment and prevalence of H9N2 viruses in poultry pose a significant threat for humans."
The H5N1 strain of bird flu has killed more than 230 people worldwide since late 2003 through contact with infected birds, with about half of the cases in Indonesia.
Health experts fear the strain could mutate into a form easily transmitted from person to person, leading to a pandemic.