WASHINGTON (Reuters) - Countries around the world may be preparing for a possible H5N1 bird flu pandemic, but another strain called H9N2 also poses a threat to humanity, researchers reported on Tuesday.

Tests on the H9N2 strain of the virus show it is capable of infecting and spreading with very few changes, a team from the University of Maryland, St. Jude´s Children´s Research Hospital in Memphis, and elsewhere reported.

"Our results suggest that the establishment and prevalence of H9N2 viruses in poultry pose a significant threat for humans," the researchers wrote in the Public Library of Science journal PLoS ONE.

Most influenza experts agree that a pandemic -- a deadly global epidemic -- of some kind of flu is inevitable.

No one can predict what kind but the chief suspect is the H5N1 bird flu virus, which has infected 385 people and killed 243 of them since 2003. It is entrenched in birds now in some areas and has killed or forced the slaughter of 300 million.

Just a few mutations could turn it into a virus that people catch and transmit easily. But flu experts caution H5N1 is not the only virus with this potential.

H9N2, a virus seen mostly in birds, has infected at least four children in Hong Kong, causing mild illness, and is found in birds, pigs and other animals in Europe and Asia.

Maryland´s Daniel Perez and colleagues tinkered with the virus and tested it in ferrets, animals whose biology is very close to humans when it comes to flu.

A single mutation made H9N2 more virulent and pathogenic, and also helped it transmit more easily from one ferret to another, they reported in their study, available on the Internet at http://dx.plos.org/10.1371/journal.pone.0002923.

They also mixed H9N2 with an H3N2 virus, a type of influenza virus that causes seasonal flu in people. Scientists believe that if a human or animal is infected with two strains of flu at the same time, this "reassortment" can happen in nature.

The reassorted virus was easier for the ferrets to catch and transmit.

One reassuring finding -- neither of the lab-engineered viruses could be transmitted in the air, via aerosol. This might make them somewhat less transmissible, although people pick up flu from surfaces touched by an infected person.

"Although no aerosol transmission was observed, the virus replicated in multiple respiratory tissues and induced clinical signs similar to those observed with the ... human H3N2 virus," the researchers wrote.

There are hundreds of strains of avian influenza viruses, but only four -- H5N1, H7N3, H7N7, and H9N2 -- are known to have caused human infections, according to the World Health Organization.

(Reporting by Maggie Fox, editing by Will Dunham)